change: Add From<Address> implementation for Mailbox (#879)

from-address: It's a simple implementation - it uses the address as the address and uses `None` for the name parameter.

.editorconfig

@@ -0,0 +1,8 @@
+root = true
+
+[*]
+insert_final_newline = true
+
+[*.rs]
+indent_size = 4
+indent_style = space

.github/workflows/test.yml

@@ -13,7 +13,7 @@ env:
 
 jobs:
   rustfmt:
-    name: rustfmt / stable
+    name: rustfmt / nightly-2022-11-12
     runs-on: ubuntu-latest
 
     steps:
@@ -22,7 +22,7 @@ jobs:
 
       - name: Install rust
         run: |
-          rustup update --no-self-update stable
+          rustup default nightly-2022-11-12
           rustup component add rustfmt
 
       - name: cargo fmt
@@ -52,17 +52,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
 
-      - name: Setup cache
-        uses: actions/cache@v2
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-check
-
       - name: Install rust
         run: rustup update --no-self-update stable
+
+      - name: Setup cache
+        uses: Swatinem/rust-cache@v2
       
       - name: Install cargo hack
         run: cargo install cargo-hack --debug
@@ -81,27 +75,21 @@ jobs:
             rust: stable
           - name: beta
             rust: beta
-          - name: 1.52.1
+          - name: 1.60.0
-            rust: 1.52.1
+            rust: 1.60.0
 
     steps:
       - name: Checkout
         uses: actions/checkout@v2
 
-      - name: Setup cache
-        uses: actions/cache@v2
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-test-${{ matrix.rust }}
-
       - name: Install rust
         run: |
           rustup default ${{ matrix.rust }}
           rustup update --no-self-update ${{ matrix.rust }}
 
+      - name: Setup cache
+        uses: Swatinem/rust-cache@v2
+
       - name: Install postfix  
         run: |
           DEBIAN_FRONTEND=noninteractive sudo apt-get update
@@ -110,15 +98,35 @@ jobs:
       - name: Run SMTP server
         run: smtp-sink 2525 1000&
 
+      - name: Install coredns
+        run: |
+          wget -q https://github.com/coredns/coredns/releases/download/v1.8.6/coredns_1.8.6_linux_amd64.tgz
+          tar xzf coredns_1.8.6_linux_amd64.tgz
+
+      - name: Start coredns
+        run: |
+          sudo ./coredns -conf testdata/coredns.conf &
+          sudo systemctl stop systemd-resolved
+          echo "nameserver 127.0.0.54" | sudo tee /etc/resolv.conf
+
+      - name: Install dkimverify
+        run: sudo apt -y install python3-dkim
+
+      - name: Work around early dependencies MSRV bump
+        run: cargo update -p async-global-executor --precise 2.0.4
+
       - name: Test with no default features
         run: cargo test --no-default-features
 
       - name: Test with default features
         run: cargo test
 
-      - name: Test with all features
+      - name: Test with all features (-native-tls)
-        run: cargo test --all-features
+        run: cargo test --no-default-features --features async-std,async-std1,async-std1-rustls-tls,async-trait,base64,boring,boring-tls,builder,dkim,ed25519-dalek,email-encoding,fastrand,file-transport,file-transport-envelope,futures-io,futures-rustls,futures-util,hostname,httpdate,mime,mime03,nom,once_cell,pool,quoted_printable,rsa,rustls,rustls-native-certs,rustls-pemfile,rustls-tls,sendmail-transport,serde,serde_json,sha2,smtp-transport,socket2,tokio1,tokio1-boring-tls,tokio1-rustls-tls,tokio1_boring,tokio1_crate,tokio1_rustls,tracing,uuid,webpki-roots
   
+      - name: Test with all features (-boring-tls)
+        run: cargo test --no-default-features --features async-std,async-std1,async-std1-rustls-tls,async-trait,base64,builder,dkim,ed25519-dalek,email-encoding,fastrand,file-transport,file-transport-envelope,futures-io,futures-rustls,futures-util,hostname,httpdate,mime,mime03,native-tls,nom,once_cell,pool,quoted_printable,rsa,rustls,rustls-native-certs,rustls-pemfile,rustls-tls,sendmail-transport,serde,serde_json,sha2,smtp-transport,socket2,tokio1,tokio1-native-tls,tokio1-rustls-tls,tokio1_crate,tokio1_native_tls_crate,tokio1_rustls,tracing,uuid,webpki-roots
+
 #  coverage:
 #    name: Coverage
 #    runs-on: ubuntu-latest

CHANGELOG.md

@@ -1,11 +1,109 @@
+<a name="v0.10.4"></a>
+### v0.10.4 (2023-04-02)
+
+#### Misc
+
+* Bumped rustls to 0.21 and all related dependencies ([#867])
+
+[#867]: https://github.com/lettre/lettre/pull/867
+
+<a name="v0.10.3"></a>
+### v0.10.3 (2023-02-20)
+
+#### Announcements
+
+It was found that what had been used until now as a basic lettre 0.10
+`MessageBuilder::body` example failed to mention that for maximum
+compatibility with various email clients a `Content-Type` header
+should always be present in the message.
+
+##### Before
+
+```rust
+Message::builder()
+  // [...] some headers skipped for brevity
+  .body(String::from("A plaintext or html body"))?
+```
+
+##### Patch
+
+```diff
+ Message::builder()
+   // [...] some headers skipped for brevity
++  .header(ContentType::TEXT_PLAIN) // or `TEXT_HTML` if the body is html
+   .body(String::from("A plaintext or html body"))?
+```
+
+#### Features
+
+* Add support for rustls-native-certs when using rustls ([#843])
+
+[#843]: https://github.com/lettre/lettre/pull/843
+
+<a name="v0.10.2"></a>
+### v0.10.2 (2023-01-29)
+
+#### Upgrade notes
+
+* MSRV is now 1.60 ([#828])
+
+#### Features
+
+* Allow providing a custom `tokio` stream for `AsyncSmtpTransport` ([#805])
+* Return whole SMTP error message ([#821])
+
+#### Bug fixes
+
+* Mailbox displays wrongly when containing a comma and a non-ascii char in its name ([#827])
+* Require `quoted_printable` ^0.4.6 in order to fix encoding of tabs and spaces at the end of line ([#837])
+
+#### Misc
+
+* Increase tracing ([#848])
+* Bump `idna` to 0.3 ([#816])
+* Update `base64` to 0.21 ([#840] and [#851])
+* Update `rsa` to 0.8 ([#829] and [#852])
+
+[#805]: https://github.com/lettre/lettre/pull/805
+[#816]: https://github.com/lettre/lettre/pull/816
+[#821]: https://github.com/lettre/lettre/pull/821
+[#827]: https://github.com/lettre/lettre/pull/827
+[#828]: https://github.com/lettre/lettre/pull/828
+[#829]: https://github.com/lettre/lettre/pull/829
+[#837]: https://github.com/lettre/lettre/pull/837
+[#840]: https://github.com/lettre/lettre/pull/840
+[#848]: https://github.com/lettre/lettre/pull/848
+[#851]: https://github.com/lettre/lettre/pull/851
+[#852]: https://github.com/lettre/lettre/pull/852
+
+<a name="v0.10.1"></a>
+### v0.10.1 (2022-07-20)
+
+#### Features
+
+* Add `boring-tls` support for `SmtpTransport` and `AsyncSmtpTransport`. The latter is only supported with the tokio runtime. ([#797]) ([#798])
+* Make the minimum TLS version configurable. ([#799]) ([#800])
+
+#### Bug Fixes
+
+* Ensure connections are closed on abort. ([#801])
+* Fix SMTP dot stuffing. ([#803])
+
+[#797]: https://github.com/lettre/lettre/pull/797
+[#798]: https://github.com/lettre/lettre/pull/798
+[#799]: https://github.com/lettre/lettre/pull/799
+[#800]: https://github.com/lettre/lettre/pull/800
+[#801]: https://github.com/lettre/lettre/pull/801
+[#803]: https://github.com/lettre/lettre/pull/803
+
 <a name="v0.10.0"></a>
-### v0.10.0 (unreleased)
+### v0.10.0 (2022-06-29)
 
 #### Upgrade notes
 
 Several breaking changes were made between 0.9 and 0.10, but changes should be straightforward:
 
-* MSRV is now 1.52.1
+* MSRV is now 1.56.0
 * The `lettre_email` crate has been merged into `lettre`. To migrate, replace `lettre_email` with `lettre::message`
   and make sure to enable the `builder` feature (it's enabled by default).
 * `SendableEmail` has been renamed to `Email` and `EmailBuilder::build()` produces it directly. To migrate,
@@ -29,6 +127,7 @@ Several breaking changes were made between 0.9 and 0.10, but changes should be s
 * Refactor `TlsParameters` implementation to not expose the internal TLS library
 * `FileTransport` writes emails into `.eml` instead of `.json`
 * When the hostname feature is disabled or hostname cannot be fetched, `127.0.0.1` is used instead of `localhost` as EHLO parameter (for better RFC compliance and mail server compatibility)
+* The `sendmail` and `file` transports aren't enabled by default anymore.
 * The `new` method of `ClientId` is deprecated
 * Rename `serde-impls` feature to `serde`
 * The `SendmailTransport` now uses the `sendmail` command in current `PATH` by default instead of

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "lettre"
 # remember to update html_root_url and README.md (Cargo.toml example and deps.rs badge)
-version = "0.10.0-rc.4"
+version = "0.10.4"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -10,7 +10,8 @@ license = "MIT"
 authors = ["Alexis Mousset <contact@amousset.me>", "Paolo Barbolini <paolo@paolo565.org>"]
 categories = ["email", "network-programming"]
 keywords = ["email", "smtp", "mailer", "message", "sendmail"]
-edition = "2018"
+edition = "2021"
+rust-version = "1.60"
 
 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -18,32 +19,35 @@ is-it-maintained-open-issues = { repository = "lettre/lettre" }
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-idna = "0.2"
+idna = "0.3"
-once_cell = "1"
+once_cell = { version = "1", optional = true }
 tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # feature
 
 # builder
 httpdate = { version = "1", optional = true }
 mime = { version = "0.3.4", optional = true }
 fastrand = { version = "1.4", optional = true }
-quoted_printable = { version = "0.4", optional = true }
+quoted_printable = { version = "0.4.6", optional = true }
-base64 = { version = "0.13", optional = true }
+base64 = { version = "0.21", optional = true }
-regex = { version = "1", default-features = false, features = ["std", "unicode-case"] }
+email-encoding = { version = "0.2", optional = true }
 
 # file transport
-uuid = { version = "0.8", features = ["v4"], optional = true }
+uuid = { version = "1", features = ["v4"], optional = true }
 serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
 
-# smtp
+# smtp-transport
 nom = { version = "7", optional = true }
 hostname = { version = "0.3", optional = true } # feature
+socket2 = { version = "0.4.4", optional = true }
 
 ## tls
 native-tls = { version = "0.2", optional = true } # feature
-rustls = { version = "0.20", features = ["dangerous_configuration"], optional = true }
+rustls = { version = "0.21", features = ["dangerous_configuration"], optional = true }
-rustls-pemfile = { version = "0.2.1", optional = true }
+rustls-pemfile = { version = "1", optional = true }
-webpki-roots = { version = "0.22", optional = true }
+rustls-native-certs = { version = "0.6.2", optional = true }
+webpki-roots = { version = "0.23", optional = true }
+boring = { version = "2.0.0", optional = true }
 
 # async
 futures-io = { version = "0.3.7", optional = true }
@@ -51,24 +55,35 @@ futures-util = { version = "0.3.7", default-features = false, features = ["io"],
 async-trait = { version = "0.1", optional = true }
 
 ## async-std
-async-std = { version = "1.8", optional = true, features = ["unstable"] }
+async-std = { version = "1.8", optional = true }
 #async-native-tls = { version = "0.3.3", optional = true }
-futures-rustls = { version = "0.22", optional = true }
+futures-rustls = { version = "0.24", optional = true }
 
 ## tokio
-tokio1_crate = { package = "tokio", version = "1", features = ["fs", "process", "time", "net", "io-util"], optional = true }
+tokio1_crate = { package = "tokio", version = "1", optional = true }
 tokio1_native_tls_crate = { package = "tokio-native-tls", version = "0.3", optional = true }
-tokio1_rustls = { package = "tokio-rustls", version = "0.23", optional = true }
+tokio1_rustls = { package = "tokio-rustls", version = "0.24", optional = true }
+tokio1_boring = { package = "tokio-boring", version = "2.1.4", optional = true }
+
+## dkim
+sha2 = { version = "0.10", optional = true, features = ["oid"] }
+rsa = { version = "0.8", optional = true }
+ed25519-dalek = { version = "1.0.1", optional = true }
+
+# email formats
+email_address = { version = "0.2.1", default-features = false }
 
 [dev-dependencies]
-criterion = "0.3"
+pretty_assertions = "1"
-tracing-subscriber = "0.2.10"
+criterion = "0.4"
+tracing = { version = "0.1.16", default-features = false, features = ["std"] }
+tracing-subscriber = "0.3"
 glob = "0.3"
 walkdir = "2"
 tokio1_crate = { package = "tokio", version = "1", features = ["macros", "rt-multi-thread"] }
 async-std = { version = "1.8", features = ["attributes"] }
 serde_json = "1"
-maud = "0.22.1"
+maud = "0.24"
 
 [[bench]]
 harness = false
@@ -76,19 +91,21 @@ name = "transport_smtp"
 
 [features]
 default = ["smtp-transport", "pool", "native-tls", "hostname", "builder"]
-builder = ["httpdate", "mime", "base64", "fastrand", "quoted_printable"]
+builder = ["httpdate", "mime", "fastrand", "quoted_printable", "email-encoding"]
 mime03 = ["mime"]
 
 # transports
-file-transport = ["uuid"]
+file-transport = ["uuid", "tokio1_crate?/fs", "tokio1_crate?/io-util"]
 file-transport-envelope = ["serde", "serde_json", "file-transport"]
-sendmail-transport = []
+sendmail-transport = ["tokio1_crate?/process", "tokio1_crate?/io-util", "async-std?/unstable"]
-smtp-transport = ["base64", "nom"]
+smtp-transport = ["base64", "nom", "socket2", "once_cell", "tokio1_crate?/rt", "tokio1_crate?/time", "tokio1_crate?/net"]
 
 pool = ["futures-util"]
 
 rustls-tls = ["webpki-roots", "rustls", "rustls-pemfile"]
 
+boring-tls = ["boring"]
+
 # async
 async-std1 = ["async-std", "async-trait", "futures-io", "futures-util"]
 #async-std1-native-tls = ["async-std1", "native-tls", "async-native-tls"]
@@ -96,11 +113,18 @@ async-std1-rustls-tls = ["async-std1", "rustls-tls", "futures-rustls"]
 tokio1 = ["tokio1_crate", "async-trait", "futures-io", "futures-util"]
 tokio1-native-tls = ["tokio1", "native-tls", "tokio1_native_tls_crate"]
 tokio1-rustls-tls = ["tokio1", "rustls-tls", "tokio1_rustls"]
+tokio1-boring-tls = ["tokio1", "boring-tls", "tokio1_boring"]
+
+dkim = ["base64", "sha2", "rsa", "ed25519-dalek"]
 
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs", "--cfg", "lettre_ignore_tls_mismatch"]
 
+[[example]]
+name = "autoconfigure"
+required-features = ["smtp-transport", "native-tls"]
+
 [[example]]
 name = "basic_html"
 required-features = ["file-transport", "builder"]

LICENSE

@@ -1,5 +1,5 @@
-Copyright (c) 2014-2020 Alexis Mousset <contact@amousset.me>
+Copyright (c) 2014-2022 Alexis Mousset <contact@amousset.me>
-Copyright (c) 2019-2020 Paolo Barbolini <paolo@paolo565.org>
+Copyright (c) 2019-2022 Paolo Barbolini <paolo@paolo565.org>
 Copyright (c) 2018 K. <kayo@illumium.org>
 
 Permission is hereby granted, free of charge, to any

README.md

@@ -28,27 +28,14 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0-rc.4">
+  <a href="https://deps.rs/crate/lettre/0.10.4">
-    <img src="https://deps.rs/crate/lettre/0.10.0-rc.4/status.svg"
+    <img src="https://deps.rs/crate/lettre/0.10.4/status.svg"
       alt="dependency status" />
   </a>
 </div>
 
 ---
 
-**NOTE**: this readme refers to the 0.10 version of lettre, which is
-in release candidate state. Use the [`v0.9.x`](https://github.com/lettre/lettre/tree/v0.9.x)
-branch for the previous stable release.
-
-0.10 is already widely used and is already thought to be more reliable than 0.9, so it should generally be used
-for new projects.
-
-We'd love to hear your feedback about 0.10 design and APIs before final release!
-Start a [discussion](https://github.com/lettre/lettre/discussions) in the repository, whether for
-feedback or if you need help or advice using or upgrading lettre 0.10.
-
----
-
 ## Features
 
 Lettre provides the following features:
@@ -63,18 +50,24 @@ Lettre does not provide (for now):
 
 * Email parsing
 
+## Supported Rust Versions
+
+Lettre supports all Rust versions released in the last 6 months. At the time of writing
+the minimum supported Rust version is 1.60, but this could change at any time either from
+one of our dependencies bumping their MSRV or by a new patch release of lettre.
+
 ## Example
 
-This library requires Rust 1.52.1 or newer.
+This library requires Rust 1.60 or newer.
 To use this library, add the following to your `Cargo.toml`:
 
-
 ```toml
 [dependencies]
-lettre = "0.10.0-rc.4"
+lettre = "0.10"
 ```
 
 ```rust,no_run
+use lettre::message::header::ContentType;
 use lettre::transport::smtp::authentication::Credentials;
 use lettre::{Message, SmtpTransport, Transport};
 
@@ -83,10 +76,11 @@ let email = Message::builder()
     .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
     .to("Hei <hei@domain.tld>".parse().unwrap())
     .subject("Happy new year")
+    .header(ContentType::TEXT_PLAIN)
     .body(String::from("Be happy!"))
     .unwrap();
 
-let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
 // Open a remote connection to gmail
 let mailer = SmtpTransport::relay("smtp.gmail.com")
@@ -101,13 +95,32 @@ match mailer.send(&email) {
 }
 ```
 
+## Not sure of which connect options to use?
+
+Clone the lettre git repository and run the following command (replacing `SMTP_HOST` with your SMTP server's hostname)
+
+```shell
+cargo run --example autoconfigure SMTP_HOST
+```
+
 ## Testing
 
 The `lettre` tests require an open mail server listening locally on port 2525 and the `sendmail` command. If you have python installed 
-such a server can be launched with `python -m smtpd -n -c DebuggingServer localhost:2525`
+such a server can be launched with `python -m smtpd -n -c DebuggingServer 127.0.0.1:2525`
 
 Alternatively only unit tests can be run by doing `cargo test --lib`.
 
+## Troubleshooting
+
+These are general steps to be followed when troubleshooting SMTP related issues.
+
+- Ensure basic connectivity, ensure requisite ports are open and daemons are listening.
+- Confirm that your service provider allows traffic on the ports being used for mail transfer.
+- Check SMTP relay authentication and configuration.
+- Validate your DNS records. (DMARC, SPF, DKIM, MX)
+- Verify your SSL/TLS certificates are setup properly.
+- Investigate if filtering, formatting, or filesize limits are causing messages to be lost, delayed, or blocked by relays or remote hosts.
+
 ## Code of conduct
 
 Anyone who interacts with Lettre in any space, including but not limited to

examples/asyncstd1_smtp_starttls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
-    AsyncTransport, Message,
+    AsyncStd1Executor, AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -12,10 +12,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
@@ -27,6 +28,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/asyncstd1_smtp_tls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
-    AsyncTransport, Message,
+    AsyncStd1Executor, AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -12,10 +12,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
@@ -27,6 +28,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/autoconfigure.rs

@@ -0,0 +1,93 @@
+use std::{env, process, time::Duration};
+
+use lettre::SmtpTransport;
+
+fn main() {
+    tracing_subscriber::fmt::init();
+
+    let smtp_host = match env::args().nth(1) {
+        Some(smtp_host) => smtp_host,
+        None => {
+            println!("Please provide the SMTP host as the first argument to this command");
+            process::exit(1);
+        }
+    };
+
+    // TLS wrapped connection
+    {
+        tracing::info!(
+            "Trying to establish a TLS wrapped connection to {}",
+            smtp_host
+        );
+
+        let transport = SmtpTransport::relay(&smtp_host)
+            .expect("build SmtpTransport::relay")
+            .timeout(Some(Duration::from_secs(10)))
+            .build();
+        match transport.test_connection() {
+            Ok(true) => {
+                tracing::info!("Successfully connected to {} via a TLS wrapped connection (SmtpTransport::relay). This is the fastest option available for connecting to an SMTP server", smtp_host);
+            }
+            Ok(false) => {
+                tracing::error!("Couldn't connect to {} via a TLS wrapped connection. No more information is available", smtp_host);
+            }
+            Err(err) => {
+                tracing::error!(err = %err, "Couldn't connect to {} via a TLS wrapped connection", smtp_host);
+            }
+        }
+    }
+
+    println!();
+
+    // Plaintext connection which MUST then successfully upgrade to TLS via STARTTLS
+    {
+        tracing::info!("Trying to establish a plaintext connection to {} and then updating it via the SMTP STARTTLS extension", smtp_host);
+
+        let transport = SmtpTransport::starttls_relay(&smtp_host)
+            .expect("build SmtpTransport::starttls_relay")
+            .timeout(Some(Duration::from_secs(10)))
+            .build();
+        match transport.test_connection() {
+            Ok(true) => {
+                tracing::info!("Successfully connected to {} via a plaintext connection which then got upgraded to TLS via the SMTP STARTTLS extension (SmtpTransport::starttls_relay). This is the second best option after the previous TLS wrapped option", smtp_host);
+            }
+            Ok(false) => {
+                tracing::error!(
+                    "Couldn't connect to {} via STARTTLS. No more information is available",
+                    smtp_host
+                );
+            }
+            Err(err) => {
+                tracing::error!(err = %err, "Couldn't connect to {} via STARTTLS", smtp_host);
+            }
+        }
+    }
+
+    println!();
+
+    // Plaintext connection (very insecure)
+    {
+        tracing::info!(
+            "Trying to establish a plaintext connection to {}",
+            smtp_host
+        );
+
+        let transport = SmtpTransport::builder_dangerous(&smtp_host)
+            .timeout(Some(Duration::from_secs(10)))
+            .build();
+        match transport.test_connection() {
+            Ok(true) => {
+                tracing::info!("Successfully connected to {} via a plaintext connection. This option is very insecure and shouldn't be used on the public internet (SmtpTransport::builder_dangerous)", smtp_host);
+            }
+            Ok(false) => {
+                tracing::error!(
+                    "Couldn't connect to {} via a plaintext connection. No more information is available",
+                    smtp_host
+                );
+            }
+            Err(err) => {
+                tracing::error!(err = %err, "Couldn't connect to {} via a plaintext connection", smtp_host);
+            }
+        }
+    }
+}

examples/smtp.rs

@@ -1,4 +1,4 @@
-use lettre::{Message, SmtpTransport, Transport};
+use lettre::{message::header::ContentType, Message, SmtpTransport, Transport};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,6 +8,7 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
@@ -17,6 +18,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_selfsigned.rs

@@ -1,6 +1,7 @@
 use std::fs;
 
 use lettre::{
+    message::header::ContentType,
     transport::smtp::{
         authentication::Credentials,
         client::{Certificate, Tls, TlsParameters},
@@ -16,18 +17,19 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
     // Use a custom certificate stored on disk to securely verify the server's certificate
     let pem_cert = fs::read("certificate.pem").unwrap();
     let cert = Certificate::from_pem(&pem_cert).unwrap();
-    let tls = TlsParameters::builder("smtp.server.com".to_string())
+    let tls = TlsParameters::builder("smtp.server.com".to_owned())
         .add_root_certificate(cert)
         .build()
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to the smtp server
     let mailer = SmtpTransport::builder_dangerous("smtp.server.com")
@@ -39,6 +41,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_starttls.rs

@@ -1,4 +1,7 @@
-use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
+use lettre::{
+    message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+    SmtpTransport, Transport,
+};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,10 +11,11 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer = SmtpTransport::starttls_relay("smtp.gmail.com")
@@ -22,6 +26,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_tls.rs

@@ -1,4 +1,7 @@
-use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
+use lettre::{
+    message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+    SmtpTransport, Transport,
+};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,10 +11,11 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer = SmtpTransport::relay("smtp.gmail.com")
@@ -22,6 +26,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/tokio1_smtp_starttls.rs

@@ -1,12 +1,11 @@
 // This line is only to make it compile from lettre's examples folder,
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
-use tokio1_crate as tokio;
-
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
-    Tokio1Executor,
+    AsyncTransport, Message, Tokio1Executor,
 };
+use tokio1_crate as tokio;
 
 #[tokio::main]
 async fn main() {
@@ -17,10 +16,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer: AsyncSmtpTransport<Tokio1Executor> =
@@ -32,6 +32,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/tokio1_smtp_tls.rs

@@ -1,12 +1,11 @@
 // This line is only to make it compile from lettre's examples folder,
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
-use tokio1_crate as tokio;
-
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
-    Tokio1Executor,
+    AsyncTransport, Message, Tokio1Executor,
 };
+use tokio1_crate as tokio;
 
 #[tokio::main]
 async fn main() {
@@ -17,10 +16,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer: AsyncSmtpTransport<Tokio1Executor> =
@@ -32,6 +32,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

rustfmt.toml

@@ -0,0 +1,3 @@
+format_code_in_doc_comments = true
+imports_granularity = "Crate"
+group_imports = "StdExternalCrate"

src/address/envelope.rs

@@ -1,6 +1,3 @@
-#[cfg(feature = "builder")]
-use std::convert::TryFrom;
-
 use super::Address;
 #[cfg(feature = "builder")]
 use crate::message::header::{self, Headers};

src/address/types.rs

@@ -1,10 +1,6 @@
 //! Representation of an email address
 
-use idna::domain_to_ascii;
-use once_cell::sync::Lazy;
-use regex::Regex;
 use std::{
-    convert::{TryFrom, TryInto},
     error::Error,
     ffi::OsStr,
     fmt::{Display, Formatter, Result as FmtResult},
@@ -12,6 +8,9 @@ use std::{
     str::FromStr,
 };
 
+use email_address::EmailAddress;
+use idna::domain_to_ascii;
+
 /// Represents an email address with a user and a domain name.
 ///
 /// This type contains email in canonical form (_user@domain.tld_).
@@ -55,20 +54,6 @@ pub struct Address {
     at_start: usize,
 }
 
-// Regex from the specs
-// https://html.spec.whatwg.org/multipage/forms.html#valid-e-mail-address
-// It will mark esoteric email addresses like quoted string as invalid
-static USER_RE: Lazy<Regex> =
-    Lazy::new(|| Regex::new(r"^(?i)[a-z0-9.!#$%&'*+/=?^_`{|}~-]+\z").unwrap());
-static DOMAIN_RE: Lazy<Regex> = Lazy::new(|| {
-    Regex::new(
-        r"(?i)^[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?(?:.[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?)*$",
-    )
-    .unwrap()
-});
-// literal form, ipv4 or ipv6 address (SMTP 4.1.3)
-static LITERAL_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(?i)\[([A-f0-9:\.]+)\]\z").unwrap());
-
 impl Address {
     /// Creates a new email address from a user and domain.
     ///
@@ -126,7 +111,7 @@ impl Address {
     }
 
     pub(super) fn check_user(user: &str) -> Result<(), AddressError> {
-        if USER_RE.is_match(user) {
+        if EmailAddress::is_valid_local_part(user) {
             Ok(())
         } else {
             Err(AddressError::InvalidUser)
@@ -142,16 +127,19 @@ impl Address {
     }
 
     fn check_domain_ascii(domain: &str) -> Result<(), AddressError> {
-        if DOMAIN_RE.is_match(domain) {
+        // Domain
+        if EmailAddress::is_valid_domain(domain) {
             return Ok(());
         }
 
-        if let Some(caps) = LITERAL_RE.captures(domain) {
+        // IP
-            if let Some(cap) = caps.get(1) {
+        let ip = domain
-                if cap.as_str().parse::<IpAddr>().is_ok() {
+            .strip_prefix('[')
-                    return Ok(());
+            .and_then(|ip| ip.strip_suffix(']'))
-                }
+            .unwrap_or(domain);
-            }
+
+        if ip.parse::<IpAddr>().is_ok() {
+            return Ok(());
         }
 
         Err(AddressError::InvalidDomain)
@@ -196,7 +184,7 @@ where
         let domain = domain.as_ref();
         Address::check_domain(domain)?;
 
-        let serialized = format!("{}@{}", user, domain);
+        let serialized = format!("{user}@{domain}");
         Ok(Address {
             serialized,
             at_start: user.len(),
@@ -238,7 +226,7 @@ fn check_address(val: &str) -> Result<usize, AddressError> {
     Ok(user.len())
 }
 
-#[derive(Debug, PartialEq, Clone, Copy)]
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
 /// Errors in email addresses parsing
 pub enum AddressError {
     /// Missing domain or user
@@ -269,7 +257,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn parse_address() {
+    fn ascii_address() {
         let addr_str = "something@example.com";
         let addr = Address::from_str(addr_str).unwrap();
         let addr2 = Address::new("something", "example.com").unwrap();
@@ -279,4 +267,36 @@ mod tests {
         assert_eq!(addr2.user(), "something");
         assert_eq!(addr2.domain(), "example.com");
     }
+
+    #[test]
+    fn ascii_address_ipv4() {
+        let addr_str = "something@1.1.1.1";
+        let addr = Address::from_str(addr_str).unwrap();
+        let addr2 = Address::new("something", "1.1.1.1").unwrap();
+        assert_eq!(addr, addr2);
+        assert_eq!(addr.user(), "something");
+        assert_eq!(addr.domain(), "1.1.1.1");
+        assert_eq!(addr2.user(), "something");
+        assert_eq!(addr2.domain(), "1.1.1.1");
+    }
+
+    #[test]
+    fn ascii_address_ipv6() {
+        let addr_str = "something@[2606:4700:4700::1111]";
+        let addr = Address::from_str(addr_str).unwrap();
+        let addr2 = Address::new("something", "[2606:4700:4700::1111]").unwrap();
+        assert_eq!(addr, addr2);
+        assert_eq!(addr.user(), "something");
+        assert_eq!(addr.domain(), "[2606:4700:4700::1111]");
+        assert_eq!(addr2.user(), "something");
+        assert_eq!(addr2.domain(), "[2606:4700:4700::1111]");
+    }
+
+    #[test]
+    fn check_parts() {
+        assert!(Address::check_user("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa").is_err());
+        assert!(
+            Address::check_domain("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.com").is_err()
+        );
+    }
 }

src/base64.rs

@@ -0,0 +1,12 @@
+use ::base64::{
+    engine::{general_purpose::STANDARD, Engine},
+    DecodeError,
+};
+
+pub(crate) fn encode<T: AsRef<[u8]>>(input: T) -> String {
+    STANDARD.encode(input)
+}
+
+pub(crate) fn decode<T: AsRef<[u8]>>(input: T) -> Result<Vec<u8>, DecodeError> {
+    STANDARD.decode(input)
+}

src/executor.rs

@@ -1,7 +1,3 @@
-use async_trait::async_trait;
-#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
-use futures_util::future::BoxFuture;
-
 use std::fmt::Debug;
 #[cfg(feature = "smtp-transport")]
 use std::future::Future;
@@ -12,6 +8,10 @@ use std::path::Path;
 #[cfg(feature = "smtp-transport")]
 use std::time::Duration;
 
+use async_trait::async_trait;
+#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
+use futures_util::future::BoxFuture;
+
 #[cfg(all(
     feature = "smtp-transport",
     any(feature = "tokio1", feature = "async-std1")
@@ -109,7 +109,6 @@ impl Executor for Tokio1Executor {
     #[cfg(feature = "smtp-transport")]
     type Sleep = tokio1_crate::time::Sleep;
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     fn spawn<F>(fut: F) -> Self::Handle
     where
@@ -119,13 +118,11 @@ impl Executor for Tokio1Executor {
         tokio1_crate::spawn(fut)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     fn sleep(duration: Duration) -> Self::Sleep {
         tokio1_crate::time::sleep(duration)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     async fn connect(
         hostname: &str,
@@ -146,6 +143,7 @@ impl Executor for Tokio1Executor {
             timeout,
             hello_name,
             tls_parameters,
+            None,
         )
         .await?;
 
@@ -165,13 +163,11 @@ impl Executor for Tokio1Executor {
         Ok(conn)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "file-transport-envelope")]
     async fn fs_read(path: &Path) -> IoResult<Vec<u8>> {
         tokio1_crate::fs::read(path).await
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "file-transport")]
     async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()> {
         tokio1_crate::fs::write(path, contents).await
@@ -209,7 +205,6 @@ impl Executor for AsyncStd1Executor {
     #[cfg(feature = "smtp-transport")]
     type Sleep = BoxFuture<'static, ()>;
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     fn spawn<F>(fut: F) -> Self::Handle
     where
@@ -219,14 +214,12 @@ impl Executor for AsyncStd1Executor {
         async_std::task::spawn(fut)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     fn sleep(duration: Duration) -> Self::Sleep {
         let fut = async move { async_std::task::sleep(duration).await };
         Box::pin(fut)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     async fn connect(
         hostname: &str,
@@ -266,13 +259,11 @@ impl Executor for AsyncStd1Executor {
         Ok(conn)
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "file-transport-envelope")]
     async fn fs_read(path: &Path) -> IoResult<Vec<u8>> {
         async_std::fs::read(path).await
     }
 
-    #[doc(hidden)]
     #[cfg(feature = "file-transport")]
     async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()> {
         async_std::fs::write(path, contents).await
@@ -288,15 +279,13 @@ impl SpawnHandle for async_std::task::JoinHandle<()> {
 }
 
 mod private {
-    use super::*;
-
     pub trait Sealed {}
 
     #[cfg(feature = "tokio1")]
-    impl Sealed for Tokio1Executor {}
+    impl Sealed for super::Tokio1Executor {}
 
     #[cfg(feature = "async-std1")]
-    impl Sealed for AsyncStd1Executor {}
+    impl Sealed for super::AsyncStd1Executor {}
 
     #[cfg(all(feature = "smtp-transport", feature = "tokio1"))]
     impl Sealed for tokio1_crate::task::JoinHandle<()> {}

src/lib.rs

@@ -6,7 +6,7 @@
 //! * Secure defaults
 //! * Async support
 //!
-//! Lettre requires Rust 1.52.1 or newer.
+//! Lettre requires Rust 1.60 or newer.
 //!
 //! ## Features
 //!
@@ -41,6 +41,15 @@
 //!
 //! NOTE: native-tls isn't supported with `async-std`
 //!
+//! #### SMTP over TLS via the boring crate (Boring TLS)
+//!
+//! _Secure SMTP connections using TLS from the `boring-tls` crate_
+//!
+//! * **boring-tls**: TLS support for the synchronous version of the API
+//! * **tokio1-boring-tls**: TLS support for the `tokio1` async version of the API
+//!
+//! NOTE: boring-tls isn't supported with `async-std`
+//!
 //! #### SMTP over TLS via the rustls crate
 //!
 //! _Secure SMTP connections using TLS from the `rustls-tls` crate_
@@ -85,6 +94,7 @@
 //! * **serde**: Serialization/Deserialization of entities
 //! * **tracing**: Logging using the `tracing` crate
 //! * **mime03**: Allow creating a [`ContentType`] from an existing [mime 0.3] `Mime` struct
+//! * **dkim**: Add support for signing email with DKIM
 //!
 //! [`SMTP`]: crate::transport::smtp
 //! [`sendmail`]: crate::transport::sendmail
@@ -97,8 +107,9 @@
 //! [Tokio 1.x]: https://docs.rs/tokio/1
 //! [async-std 1.x]: https://docs.rs/async-std/1
 //! [mime 0.3]: https://docs.rs/mime/0.3
+//! [DKIM]: https://datatracker.ietf.org/doc/html/rfc6376
 
-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0-rc.4")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.4")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -108,20 +119,42 @@
     trivial_numeric_casts,
     unstable_features,
     unused_import_braces,
-    rust_2018_idioms
+    rust_2018_idioms,
+    clippy::string_add,
+    clippy::string_add_assign,
+    clippy::clone_on_ref_ptr,
+    clippy::verbose_file_reads,
+    clippy::unnecessary_self_imports,
+    clippy::string_to_string,
+    clippy::mem_forget,
+    clippy::cast_lossless,
+    clippy::inefficient_to_string,
+    clippy::inline_always,
+    clippy::linkedlist,
+    clippy::macro_use_imports,
+    clippy::manual_assert,
+    clippy::unnecessary_join,
+    clippy::wildcard_imports,
+    clippy::str_to_string,
+    clippy::empty_structs_with_brackets,
+    clippy::zero_sized_map_values
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[cfg(not(lettre_ignore_tls_mismatch))]
 mod compiletime_checks {
+    #[cfg(all(feature = "native-tls", feature = "boring-tls"))]
+    compile_error!("feature \"native-tls\" and feature \"boring-tls\" cannot be enabled at the same time, otherwise
+    the executable will fail to link.");
+
     #[cfg(all(
         feature = "tokio1",
         feature = "native-tls",
         not(feature = "tokio1-native-tls")
     ))]
     compile_error!("Lettre is being built with the `tokio1` and the `native-tls` features, but the `tokio1-native-tls` feature hasn't been turned on.
-If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+    If you were trying to opt into `rustls-tls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
-If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 
     #[cfg(all(
         feature = "tokio1",
@@ -129,8 +162,17 @@ If you're building a library which depends on lettre import it without default f
         not(feature = "tokio1-rustls-tls")
     ))]
     compile_error!("Lettre is being built with the `tokio1` and the `rustls-tls` features, but the `tokio1-rustls-tls` feature hasn't been turned on.
-If you'd like to use native-tls make sure that the `rustls-tls` hasn't been enabled by mistake.
+    If you'd like to use `native-tls` make sure that the `rustls-tls` feature hasn't been enabled by mistake.
-If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
+
+    #[cfg(all(
+        feature = "tokio1",
+        feature = "boring-tls",
+        not(feature = "tokio1-boring-tls")
+    ))]
+    compile_error!("Lettre is being built with the `tokio1` and the `boring-tls` features, but the `tokio1-boring-tls` feature hasn't been turned on.
+    If you'd like to use `boring-tls` make sure that the `rustls-tls` feature hasn't been enabled by mistake.
+    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 
     /*
     #[cfg(all(
@@ -149,8 +191,8 @@ If you're building a library which depends on lettre import it without default f
     ))]
     compile_error!("Lettre is being built with the `async-std1` and the `native-tls` features, but the async-std integration doesn't support native-tls yet.
 If you'd like to work on the issue please take a look at https://github.com/lettre/lettre/issues/576.
-If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+If you were trying to opt into `rustls-tls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
-If you're building a library which depends on lettre import lettre without default features and enable just the features you need.");
+Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 
     #[cfg(all(
         feature = "async-std1",
@@ -158,11 +200,13 @@ If you're building a library which depends on lettre import lettre without defau
         not(feature = "async-std1-rustls-tls")
     ))]
     compile_error!("Lettre is being built with the `async-std1` and the `rustls-tls` features, but the `async-std1-rustls-tls` feature hasn't been turned on.
-If you'd like to use native-tls make sure that the `rustls-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+If you'd like to use `native-tls` make sure that the `rustls-tls` hasn't been enabled by mistake.
-If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 }
 
 pub mod address;
+#[cfg(any(feature = "smtp-transport", feature = "dkim"))]
+mod base64;
 pub mod error;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 mod executor;
@@ -171,6 +215,8 @@ mod executor;
 pub mod message;
 pub mod transport;
 
+use std::error::Error as StdError;
+
 #[cfg(feature = "async-std1")]
 pub use self::executor::AsyncStd1Executor;
 #[cfg(all(any(feature = "tokio1", feature = "async-std1")))]
@@ -207,21 +253,17 @@ pub use crate::transport::sendmail::SendmailTransport;
     any(feature = "tokio1", feature = "async-std1")
 ))]
 pub use crate::transport::smtp::AsyncSmtpTransport;
+#[cfg(feature = "smtp-transport")]
+pub use crate::transport::smtp::SmtpTransport;
 #[doc(inline)]
 pub use crate::transport::Transport;
 use crate::{address::Envelope, error::Error};
 
-#[cfg(feature = "smtp-transport")]
-pub use crate::transport::smtp::SmtpTransport;
-use std::error::Error as StdError;
-
 pub(crate) type BoxError = Box<dyn StdError + Send + Sync>;
 
 #[cfg(test)]
 #[cfg(feature = "builder")]
 mod test {
-    use std::convert::TryFrom;
-
     use super::*;
     use crate::message::{header, header::Headers, Mailbox, Mailboxes};
 

src/message/attachment.rs

@@ -29,7 +29,7 @@ impl Attachment {
     /// # use std::error::Error;
     /// use std::fs;
     ///
-    /// use lettre::message::{Attachment, header::ContentType};
+    /// use lettre::message::{header::ContentType, Attachment};
     ///
     /// # fn main() -> Result<(), Box<dyn Error>> {
     /// let filename = String::from("invoice.pdf");
@@ -64,7 +64,7 @@ impl Attachment {
     /// # use std::error::Error;
     /// use std::fs;
     ///
-    /// use lettre::message::{Attachment, header::ContentType};
+    /// use lettre::message::{header::ContentType, Attachment};
     ///
     /// # fn main() -> Result<(), Box<dyn Error>> {
     /// let content_id = String::from("123");
@@ -96,7 +96,7 @@ impl Attachment {
                 builder.header(header::ContentDisposition::attachment(&filename))
             }
             Disposition::Inline(content_id) => builder
-                .header(header::ContentId::from(format!("<{}>", content_id)))
+                .header(header::ContentId::from(format!("<{content_id}>")))
                 .header(header::ContentDisposition::inline()),
         };
         builder = builder.header(content_type);

src/message/body.rs

@@ -1,8 +1,4 @@
-use std::{
+use std::{mem, ops::Deref};
-    io::{self, Write},
-    mem,
-    ops::Deref,
-};
 
 use crate::message::header::ContentTransferEncoding;
 
@@ -41,7 +37,7 @@ impl Body {
     pub fn new<B: Into<MaybeString>>(buf: B) -> Self {
         let mut buf: MaybeString = buf.into();
 
-        let encoding = buf.encoding();
+        let encoding = buf.encoding(false);
         buf.encode_crlf();
         Self::new_impl(buf.into(), encoding)
     }
@@ -61,7 +57,22 @@ impl Body {
     ) -> Result<Self, Vec<u8>> {
         let mut buf: MaybeString = buf.into();
 
-        if !buf.is_encoding_ok(encoding) {
+        let best_encoding = buf.encoding(true);
+        let ok = match (encoding, best_encoding) {
+            (ContentTransferEncoding::SevenBit, ContentTransferEncoding::SevenBit) => true,
+            (
+                ContentTransferEncoding::EightBit,
+                ContentTransferEncoding::SevenBit | ContentTransferEncoding::EightBit,
+            ) => true,
+            (ContentTransferEncoding::SevenBit | ContentTransferEncoding::EightBit, _) => false,
+            (
+                ContentTransferEncoding::QuotedPrintable
+                | ContentTransferEncoding::Base64
+                | ContentTransferEncoding::Binary,
+                _,
+            ) => true,
+        };
+        if !ok {
             return Err(buf.into());
         }
 
@@ -91,36 +102,13 @@ impl Body {
                 Self::dangerous_pre_encoded(encoded, ContentTransferEncoding::QuotedPrintable)
             }
             ContentTransferEncoding::Base64 => {
-                let base64_len = buf.len() * 4 / 3 + 4;
+                let len = email_encoding::body::base64::encoded_len(buf.len());
-                let base64_endings_len = base64_len + base64_len / LINE_MAX_LENGTH;
 
-                let mut out = Vec::with_capacity(base64_endings_len);
+                let mut out = String::with_capacity(len);
-                {
+                email_encoding::body::base64::encode(&buf, &mut out)
-                    let writer = LineWrappingWriter::new(&mut out, LINE_MAX_LENGTH);
+                    .expect("encode body as base64");
-                    let mut writer = base64::write::EncoderWriter::new(writer, base64::STANDARD);
 
-                    // TODO: use writer.write_all(self.as_ref()).expect("base64 encoding never fails");
+                Self::dangerous_pre_encoded(out.into_bytes(), ContentTransferEncoding::Base64)
-
-                    // modified Write::write_all to work around base64 crate bug
-                    // TODO: remove once https://github.com/marshallpierce/rust-base64/issues/148 is fixed
-                    {
-                        let mut buf: &[u8] = buf.as_ref();
-                        while !buf.is_empty() {
-                            match writer.write(buf) {
-                                Ok(0) => {
-                                    // ignore 0 writes
-                                }
-                                Ok(n) => {
-                                    buf = &buf[n..];
-                                }
-                                Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => {}
-                                Err(e) => panic!("base64 encoding never fails: {}", e),
-                            }
-                        }
-                    }
-                }
-
-                Self::dangerous_pre_encoded(out, ContentTransferEncoding::Base64)
             }
         }
     }
@@ -153,21 +141,20 @@ impl Body {
 impl MaybeString {
     /// Suggests the best `Content-Transfer-Encoding` to be used for this `MaybeString`
     ///
-    /// If the `MaybeString` was created from a `String` composed only of US-ASCII
+    /// The `binary` encoding is never returned
-    /// characters, with no lines longer than 1000 characters, then 7bit
+    fn encoding(&self, supports_utf8: bool) -> ContentTransferEncoding {
-    /// encoding will be used, else quoted-printable will be chosen.
+        use email_encoding::body::Encoding;
-    ///
+
-    /// If the `MaybeString` was instead created from a `Vec<u8>`, base64 encoding is always
+        let output = match self {
-    /// chosen.
+            Self::String(s) => Encoding::choose(s.as_str(), supports_utf8),
-    ///
+            Self::Binary(b) => Encoding::choose(b.as_slice(), supports_utf8),
-    /// `8bit` and `binary` encodings are never returned, as they may not be
+        };
-    /// supported by all SMTP servers.
+
-    pub fn encoding(&self) -> ContentTransferEncoding {
+        match output {
-        match &self {
+            Encoding::SevenBit => ContentTransferEncoding::SevenBit,
-            Self::String(s) if is_7bit_encoded(s.as_ref()) => ContentTransferEncoding::SevenBit,
+            Encoding::EightBit => ContentTransferEncoding::EightBit,
-            // TODO: consider when base64 would be a better option because of output size
+            Encoding::QuotedPrintable => ContentTransferEncoding::QuotedPrintable,
-            Self::String(_) => ContentTransferEncoding::QuotedPrintable,
+            Encoding::Base64 => ContentTransferEncoding::Base64,
-            Self::Binary(_) => ContentTransferEncoding::Base64,
         }
     }
 
@@ -178,18 +165,6 @@ impl MaybeString {
             Self::Binary(_) => {}
         }
     }
-
-    /// Returns `true` if using `encoding` to encode this `MaybeString`
-    /// would result into an invalid encoded body.
-    fn is_encoding_ok(&self, encoding: ContentTransferEncoding) -> bool {
-        match encoding {
-            ContentTransferEncoding::SevenBit => is_7bit_encoded(self),
-            ContentTransferEncoding::EightBit => is_8bit_encoded(self),
-            ContentTransferEncoding::Binary
-            | ContentTransferEncoding::QuotedPrintable
-            | ContentTransferEncoding::Base64 => true,
-        }
-    }
 }
 
 /// A trait for something that takes an encoded [`Body`].
@@ -273,73 +248,6 @@ impl Deref for MaybeString {
     }
 }
 
-/// Checks whether it contains only US-ASCII characters,
-/// and no lines are longer than 1000 characters including the `\n` character.
-///
-/// Most efficient content encoding available
-fn is_7bit_encoded(buf: &[u8]) -> bool {
-    buf.is_ascii() && !contains_too_long_lines(buf)
-}
-
-/// Checks that no lines are longer than 1000 characters,
-/// including the `\n` character.
-/// NOTE: 8bit isn't supported by all SMTP servers.
-fn is_8bit_encoded(buf: &[u8]) -> bool {
-    !contains_too_long_lines(buf)
-}
-
-/// Checks if there are lines that are longer than 1000 characters,
-/// including the `\n` character.
-fn contains_too_long_lines(buf: &[u8]) -> bool {
-    buf.len() > 1000 && buf.split(|&b| b == b'\n').any(|line| line.len() > 999)
-}
-
-const LINE_SEPARATOR: &[u8] = b"\r\n";
-const LINE_MAX_LENGTH: usize = 78 - LINE_SEPARATOR.len();
-
-/// A `Write`r that inserts a line separator `\r\n` every `max_line_length` bytes.
-struct LineWrappingWriter<'a, W> {
-    writer: &'a mut W,
-    current_line_length: usize,
-    max_line_length: usize,
-}
-
-impl<'a, W> LineWrappingWriter<'a, W> {
-    pub fn new(writer: &'a mut W, max_line_length: usize) -> Self {
-        Self {
-            writer,
-            current_line_length: 0,
-            max_line_length,
-        }
-    }
-}
-
-impl<'a, W> Write for LineWrappingWriter<'a, W>
-where
-    W: Write,
-{
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        let remaining_line_len = self.max_line_length - self.current_line_length;
-        let write_len = std::cmp::min(buf.len(), remaining_line_len);
-
-        self.writer.write_all(&buf[..write_len])?;
-
-        if remaining_line_len == write_len {
-            self.writer.write_all(LINE_SEPARATOR)?;
-
-            self.current_line_length = 0;
-        } else {
-            self.current_line_length += write_len;
-        }
-
-        Ok(write_len)
-    }
-
-    fn flush(&mut self) -> io::Result<()> {
-        self.writer.flush()
-    }
-}
-
 /// In place conversion to CRLF line endings
 fn in_place_crlf_line_endings(string: &mut String) {
     let indices = find_all_lf_char_indices(string);
@@ -377,6 +285,8 @@ fn find_all_lf_char_indices(s: &str) -> Vec<usize> {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::{in_place_crlf_line_endings, Body, ContentTransferEncoding};
 
     #[test]
@@ -509,13 +419,10 @@ mod test {
 
     #[test]
     fn quoted_printable_detect() {
-        let encoded = Body::new(String::from("Привет, мир!"));
+        let encoded = Body::new(String::from("Questo messaggio è corto"));
 
         assert_eq!(encoded.encoding(), ContentTransferEncoding::QuotedPrintable);
-        assert_eq!(
+        assert_eq!(encoded.as_ref(), b"Questo messaggio =C3=A8 corto");
-            encoded.as_ref(),
-            b"=D0=9F=D1=80=D0=B8=D0=B2=D0=B5=D1=82, =D0=BC=D0=B8=D1=80!".as_ref()
-        );
     }
 
     #[test]
@@ -547,14 +454,17 @@ mod test {
 
     #[test]
     fn quoted_printable_encode_line_wrap() {
-        let encoded = Body::new(String::from("Текст письма в уникоде"));
+        let encoded = Body::new(String::from(
+            "Se lo standard 📬 fosse stato più semplice avremmo finito molto prima.",
+        ));
 
         assert_eq!(encoded.encoding(), ContentTransferEncoding::QuotedPrintable);
+        println!("{}", std::str::from_utf8(encoded.as_ref()).unwrap());
         assert_eq!(
             encoded.as_ref(),
             concat!(
-                "=D0=A2=D0=B5=D0=BA=D1=81=D1=82 =D0=BF=D0=B8=D1=81=D1=8C=D0=BC=D0=B0 =D0=B2 =\r\n",
+                "Se lo standard =F0=9F=93=AC fosse stato pi=C3=B9 semplice avremmo finito mo=\r\n",
-                "=D1=83=D0=BD=D0=B8=D0=BA=D0=BE=D0=B4=D0=B5"
+                "lto prima."
             )
             .as_bytes()
         );
@@ -562,21 +472,25 @@ mod test {
 
     #[test]
     fn base64_detect() {
-        let input = Body::new(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+        let input = Body::new(vec![0; 80]);
         let encoding = input.encoding();
         assert_eq!(encoding, ContentTransferEncoding::Base64);
     }
 
     #[test]
     fn base64_encode_bytes() {
-        let encoded = Body::new_with_encoding(
+        let encoded =
-            vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
+            Body::new_with_encoding(vec![0; 80], ContentTransferEncoding::Base64).unwrap();
-            ContentTransferEncoding::Base64,
-        )
-        .unwrap();
 
         assert_eq!(encoded.encoding(), ContentTransferEncoding::Base64);
-        assert_eq!(encoded.as_ref(), b"AAECAwQFBgcICQ==");
+        assert_eq!(
+            encoded.as_ref(),
+            concat!(
+                "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\r\n",
+                "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="
+            )
+            .as_bytes()
+        );
     }
 
     #[test]

src/message/dkim.rs

@@ -0,0 +1,614 @@
+use std::{
+    borrow::Cow,
+    error::Error as StdError,
+    fmt::{self, Display},
+    iter::IntoIterator,
+    time::SystemTime,
+};
+
+use ed25519_dalek::Signer;
+use rsa::{pkcs1::DecodeRsaPrivateKey, pkcs1v15::Pkcs1v15Sign, RsaPrivateKey};
+use sha2::{Digest, Sha256};
+
+use crate::message::{
+    header::{HeaderName, HeaderValue},
+    Headers, Message,
+};
+
+/// Describe Dkim Canonicalization to apply to either body or headers
+#[derive(Copy, Clone, Debug)]
+pub enum DkimCanonicalizationType {
+    Simple,
+    Relaxed,
+}
+
+impl Display for DkimCanonicalizationType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            DkimCanonicalizationType::Simple => "simple",
+            DkimCanonicalizationType::Relaxed => "relaxed",
+        })
+    }
+}
+
+/// Describe Canonicalization to be applied before signing
+#[derive(Copy, Clone, Debug)]
+pub struct DkimCanonicalization {
+    pub header: DkimCanonicalizationType,
+    pub body: DkimCanonicalizationType,
+}
+
+impl Default for DkimCanonicalization {
+    fn default() -> Self {
+        DkimCanonicalization {
+            header: DkimCanonicalizationType::Simple,
+            body: DkimCanonicalizationType::Relaxed,
+        }
+    }
+}
+
+/// Format canonicalization to be shown in Dkim header
+impl Display for DkimCanonicalization {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}/{}", self.header, self.body)
+    }
+}
+
+/// Describe the algorithm used for signing the message
+#[derive(Copy, Clone, Debug)]
+pub enum DkimSigningAlgorithm {
+    Rsa,
+    Ed25519,
+}
+
+impl Display for DkimSigningAlgorithm {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            DkimSigningAlgorithm::Rsa => "rsa",
+            DkimSigningAlgorithm::Ed25519 => "ed25519",
+        })
+    }
+}
+
+/// Describe DkimSigning key error
+#[derive(Debug)]
+pub struct DkimSigningKeyError(InnerDkimSigningKeyError);
+
+#[derive(Debug)]
+enum InnerDkimSigningKeyError {
+    Base64(base64::DecodeError),
+    Rsa(rsa::pkcs1::Error),
+    Ed25519(ed25519_dalek::ed25519::Error),
+}
+
+impl Display for DkimSigningKeyError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match &self.0 {
+            InnerDkimSigningKeyError::Base64(_err) => "base64 decode error",
+            InnerDkimSigningKeyError::Rsa(_err) => "rsa decode error",
+            InnerDkimSigningKeyError::Ed25519(_err) => "ed25519 decode error",
+        })
+    }
+}
+
+impl StdError for DkimSigningKeyError {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        Some(match &self.0 {
+            InnerDkimSigningKeyError::Base64(err) => err,
+            InnerDkimSigningKeyError::Rsa(err) => err,
+            InnerDkimSigningKeyError::Ed25519(err) => err,
+        })
+    }
+}
+
+/// Describe a signing key to be carried by DkimConfig struct
+#[derive(Debug)]
+pub struct DkimSigningKey(InnerDkimSigningKey);
+
+#[derive(Debug)]
+enum InnerDkimSigningKey {
+    Rsa(RsaPrivateKey),
+    Ed25519(ed25519_dalek::Keypair),
+}
+
+impl DkimSigningKey {
+    pub fn new(
+        private_key: &str,
+        algorithm: DkimSigningAlgorithm,
+    ) -> Result<DkimSigningKey, DkimSigningKeyError> {
+        Ok(Self(match algorithm {
+            DkimSigningAlgorithm::Rsa => InnerDkimSigningKey::Rsa(
+                RsaPrivateKey::from_pkcs1_pem(private_key)
+                    .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Rsa(err)))?,
+            ),
+            DkimSigningAlgorithm::Ed25519 => InnerDkimSigningKey::Ed25519(
+                ed25519_dalek::Keypair::from_bytes(
+                    &crate::base64::decode(private_key).map_err(|err| {
+                        DkimSigningKeyError(InnerDkimSigningKeyError::Base64(err))
+                    })?,
+                )
+                .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Ed25519(err)))?,
+            ),
+        }))
+    }
+    fn get_signing_algorithm(&self) -> DkimSigningAlgorithm {
+        match self.0 {
+            InnerDkimSigningKey::Rsa(_) => DkimSigningAlgorithm::Rsa,
+            InnerDkimSigningKey::Ed25519(_) => DkimSigningAlgorithm::Ed25519,
+        }
+    }
+}
+
+/// A struct to describe Dkim configuration applied when signing a message
+/// selector: the name of the key publied in DNS
+/// domain: the domain for which we sign the message
+/// private_key: private key in PKCS1 string format
+/// headers: a list of headers name to be included in the signature. Signing of more than one
+/// header with same name is not supported
+/// canonicalization: the canonicalization to be applied on the message
+/// pub signing_algorithm: the signing algorithm to be used when signing
+#[derive(Debug)]
+pub struct DkimConfig {
+    selector: String,
+    domain: String,
+    private_key: DkimSigningKey,
+    headers: Vec<HeaderName>,
+    canonicalization: DkimCanonicalization,
+}
+
+impl DkimConfig {
+    /// Create a default signature configuration with a set of headers and "simple/relaxed"
+    /// canonicalization
+    pub fn default_config(
+        selector: String,
+        domain: String,
+        private_key: DkimSigningKey,
+    ) -> DkimConfig {
+        DkimConfig {
+            selector,
+            domain,
+            private_key,
+            headers: vec![
+                HeaderName::new_from_ascii_str("From"),
+                HeaderName::new_from_ascii_str("Subject"),
+                HeaderName::new_from_ascii_str("To"),
+                HeaderName::new_from_ascii_str("Date"),
+            ],
+            canonicalization: DkimCanonicalization {
+                header: DkimCanonicalizationType::Simple,
+                body: DkimCanonicalizationType::Relaxed,
+            },
+        }
+    }
+
+    /// Create a DkimConfig
+    pub fn new(
+        selector: String,
+        domain: String,
+        private_key: DkimSigningKey,
+        headers: Vec<HeaderName>,
+        canonicalization: DkimCanonicalization,
+    ) -> DkimConfig {
+        DkimConfig {
+            selector,
+            domain,
+            private_key,
+            headers,
+            canonicalization,
+        }
+    }
+}
+
+/// Create a Headers struct with a Dkim-Signature Header created from given parameters
+fn dkim_header_format(
+    config: &DkimConfig,
+    timestamp: u64,
+    headers_list: &str,
+    body_hash: &str,
+    signature: &str,
+) -> Headers {
+    let mut headers = Headers::new();
+    let header_name =
+        dkim_canonicalize_header_tag("DKIM-Signature", config.canonicalization.header);
+    let header_name = HeaderName::new_from_ascii(header_name.into()).unwrap();
+    headers.insert_raw(HeaderValue::new(header_name, format!("v=1; a={signing_algorithm}-sha256; d={domain}; s={selector}; c={canon}; q=dns/txt; t={timestamp}; h={headers_list}; bh={body_hash}; b={signature}",domain=config.domain, selector=config.selector,canon=config.canonicalization,timestamp=timestamp,headers_list=headers_list,body_hash=body_hash,signature=signature,signing_algorithm=config.private_key.get_signing_algorithm())));
+    headers
+}
+
+/// Canonicalize the body of an email
+fn dkim_canonicalize_body(
+    mut body: &[u8],
+    canonicalization: DkimCanonicalizationType,
+) -> Cow<'_, [u8]> {
+    match canonicalization {
+        DkimCanonicalizationType::Simple => {
+            // Remove empty lines at end
+            while body.ends_with(b"\r\n\r\n") {
+                body = &body[..body.len() - 2];
+            }
+            Cow::Borrowed(body)
+        }
+        DkimCanonicalizationType::Relaxed => {
+            let mut out = Vec::with_capacity(body.len());
+            loop {
+                match body {
+                    [b' ' | b'\t', b'\r', b'\n', ..] => {}
+                    [b' ' | b'\t', b' ' | b'\t', ..] => {}
+                    [b' ' | b'\t', ..] => out.push(b' '),
+                    [c, ..] => out.push(*c),
+                    [] => break,
+                }
+                body = &body[1..];
+            }
+            // Remove empty lines at end
+            while out.ends_with(b"\r\n\r\n") {
+                out.truncate(out.len() - 2);
+            }
+            Cow::Owned(out)
+        }
+    }
+}
+
+fn dkim_canonicalize_headers_relaxed(headers: &str) -> String {
+    let mut r = String::with_capacity(headers.len());
+
+    fn skip_whitespace(h: &str) -> &str {
+        match h.as_bytes().first() {
+            Some(b' ' | b'\t') => skip_whitespace(&h[1..]),
+            _ => h,
+        }
+    }
+
+    fn name(h: &str, out: &mut String) {
+        if let Some(name_end) = h.bytes().position(|c| c == b':') {
+            let (name, rest) = h.split_at(name_end + 1);
+            *out += name;
+            // Space after header colon is stripped.
+            value(skip_whitespace(rest), out);
+        } else {
+            // This should never happen.
+            *out += h;
+        }
+    }
+
+    fn value(h: &str, out: &mut String) {
+        match h.as_bytes() {
+            // Continuation lines.
+            [b'\r', b'\n', b' ' | b'\t', ..] => {
+                out.push(' ');
+                value(skip_whitespace(&h[2..]), out);
+            }
+            // End of header.
+            [b'\r', b'\n', ..] => {
+                *out += "\r\n";
+                name(&h[2..], out)
+            }
+            // Sequential whitespace.
+            [b' ' | b'\t', b' ' | b'\t' | b'\r', ..] => value(&h[1..], out),
+            // All whitespace becomes spaces.
+            [b'\t', ..] => {
+                out.push(' ');
+                value(&h[1..], out)
+            }
+            [_, ..] => {
+                let mut chars = h.chars();
+                out.push(chars.next().unwrap());
+                value(chars.as_str(), out)
+            }
+            [] => {}
+        }
+    }
+
+    name(headers, &mut r);
+
+    r
+}
+
+/// Canonicalize header tag
+fn dkim_canonicalize_header_tag(
+    name: &str,
+    canonicalization: DkimCanonicalizationType,
+) -> Cow<'_, str> {
+    match canonicalization {
+        DkimCanonicalizationType::Simple => Cow::Borrowed(name),
+        DkimCanonicalizationType::Relaxed => Cow::Owned(name.to_lowercase()),
+    }
+}
+
+/// Canonicalize signed headers passed as headers_list among mail_headers using canonicalization
+fn dkim_canonicalize_headers<'a>(
+    headers_list: impl IntoIterator<Item = &'a str>,
+    mail_headers: &Headers,
+    canonicalization: DkimCanonicalizationType,
+) -> String {
+    let mut covered_headers = Headers::new();
+    for name in headers_list {
+        if let Some(h) = mail_headers.find_header(name) {
+            let name = dkim_canonicalize_header_tag(name, canonicalization);
+            covered_headers.insert_raw(HeaderValue::dangerous_new_pre_encoded(
+                HeaderName::new_from_ascii(name.into()).unwrap(),
+                h.get_raw().into(),
+                h.get_encoded().into(),
+            ));
+        }
+    }
+
+    let serialized = covered_headers.to_string();
+
+    match canonicalization {
+        DkimCanonicalizationType::Simple => serialized,
+        DkimCanonicalizationType::Relaxed => dkim_canonicalize_headers_relaxed(&serialized),
+    }
+}
+
+/// Sign with Dkim a message by adding Dkim-Signture header created with configuration expressed by
+/// dkim_config
+
+pub fn dkim_sign(message: &mut Message, dkim_config: &DkimConfig) {
+    dkim_sign_fixed_time(message, dkim_config, SystemTime::now())
+}
+
+fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timestamp: SystemTime) {
+    let timestamp = timestamp
+        .duration_since(SystemTime::UNIX_EPOCH)
+        .unwrap()
+        .as_secs();
+    let headers = message.headers();
+    let body_hash = Sha256::digest(dkim_canonicalize_body(
+        &message.body_raw(),
+        dkim_config.canonicalization.body,
+    ));
+    let bh = crate::base64::encode(body_hash);
+    let mut signed_headers_list =
+        dkim_config
+            .headers
+            .iter()
+            .fold(String::new(), |mut list, header| {
+                if !list.is_empty() {
+                    list.push(':');
+                }
+
+                list.push_str(header);
+                list
+            });
+    if let DkimCanonicalizationType::Relaxed = dkim_config.canonicalization.header {
+        signed_headers_list.make_ascii_lowercase();
+    }
+    let dkim_header = dkim_header_format(dkim_config, timestamp, &signed_headers_list, &bh, "");
+    let signed_headers = dkim_canonicalize_headers(
+        dkim_config.headers.iter().map(|h| h.as_ref()),
+        headers,
+        dkim_config.canonicalization.header,
+    );
+    let canonicalized_dkim_header = dkim_canonicalize_headers(
+        ["DKIM-Signature"],
+        &dkim_header,
+        dkim_config.canonicalization.header,
+    );
+    let mut hashed_headers = Sha256::new();
+    hashed_headers.update(signed_headers.as_bytes());
+    hashed_headers.update(canonicalized_dkim_header.trim_end().as_bytes());
+    let hashed_headers = hashed_headers.finalize();
+    let signature = match &dkim_config.private_key.0 {
+        InnerDkimSigningKey::Rsa(private_key) => crate::base64::encode(
+            private_key
+                .sign(Pkcs1v15Sign::new::<Sha256>(), &hashed_headers)
+                .unwrap(),
+        ),
+        InnerDkimSigningKey::Ed25519(private_key) => {
+            crate::base64::encode(private_key.sign(&hashed_headers).to_bytes())
+        }
+    };
+    let dkim_header = dkim_header_format(
+        dkim_config,
+        timestamp,
+        &signed_headers_list,
+        &bh,
+        &signature,
+    );
+    message.headers.insert_raw(HeaderValue::new(
+        HeaderName::new_from_ascii_str("DKIM-Signature"),
+        dkim_header.get_raw("DKIM-Signature").unwrap().to_owned(),
+    ));
+}
+
+#[cfg(test)]
+mod test {
+    use pretty_assertions::assert_eq;
+
+    use super::{
+        super::{
+            header::{HeaderName, HeaderValue},
+            Header, Message,
+        },
+        dkim_canonicalize_body, dkim_canonicalize_headers, dkim_sign_fixed_time,
+        DkimCanonicalization, DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm,
+        DkimSigningKey,
+    };
+    use crate::StdError;
+
+    const KEY_RSA: &str = "-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----";
+
+    #[derive(Clone)]
+    struct TestHeader(String);
+
+    impl Header for TestHeader {
+        fn name() -> HeaderName {
+            HeaderName::new_from_ascii_str("Test")
+        }
+
+        fn parse(s: &str) -> Result<Self, Box<dyn StdError + Send + Sync>> {
+            Ok(Self(s.into()))
+        }
+
+        fn display(&self) -> HeaderValue {
+            HeaderValue::new(Self::name(), self.0.clone())
+        }
+    }
+
+    fn test_message() -> Message {
+        Message::builder()
+            .from("Test O'Leary <test+ezrz@example.net>".parse().unwrap())
+            .to("Test2 <test2@example.org>".parse().unwrap())
+            .date(std::time::UNIX_EPOCH)
+            .header(TestHeader("test  test very very long with spaces and extra spaces   \twill be folded to several lines ".to_owned()))
+            .subject("Test with utf-8 ë")
+            .body("test\r\n\r\ntest   \ttest\r\n\r\n\r\n".to_owned()).unwrap()
+    }
+
+    #[test]
+    fn test_headers_simple_canonicalize() {
+        let message = test_message();
+        dbg!(message.headers.to_string());
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Simple), "From: =?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\nTest: test  test very very long with spaces and extra spaces   \twill be\r\n folded to several lines \r\n")
+    }
+
+    #[test]
+    fn test_headers_relaxed_canonicalize() {
+        let message = test_message();
+        dbg!(message.headers.to_string());
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Relaxed),"from:=?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\ntest:test test very very long with spaces and extra spaces will be folded to several lines\r\n")
+    }
+
+    #[test]
+    fn test_body_simple_canonicalize() {
+        let body = b" C \r\nD \t E\r\n\r\n\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Simple).into_owned(),
+            b" C \r\nD \t E\r\n"
+        );
+    }
+
+    #[test]
+    fn test_body_relaxed_canonicalize() {
+        let body = b" C \r\nD \t E\r\n\tF\r\n\t\r\n\r\n\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Relaxed).into_owned(),
+            b" C\r\nD E\r\n F\r\n"
+        );
+    }
+
+    #[test]
+    fn test_signature_rsa_simple() {
+        let mut message = test_message();
+        let signing_key = DkimSigningKey::new(KEY_RSA, DkimSigningAlgorithm::Rsa).unwrap();
+        dkim_sign_fixed_time(
+            &mut message,
+            &DkimConfig::new(
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
+                signing_key,
+                vec![
+                    HeaderName::new_from_ascii_str("Date"),
+                    HeaderName::new_from_ascii_str("From"),
+                    HeaderName::new_from_ascii_str("Subject"),
+                    HeaderName::new_from_ascii_str("To"),
+                ],
+                DkimCanonicalization {
+                    header: DkimCanonicalizationType::Simple,
+                    body: DkimCanonicalizationType::Simple,
+                },
+            ),
+            std::time::UNIX_EPOCH,
+        );
+        let signed = message.formatted();
+        let signed = std::str::from_utf8(&signed).unwrap();
+        assert_eq!(
+            signed,
+            std::concat!(
+                "From: =?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\n",
+                "To: Test2 <test2@example.org>\r\n",
+                "Date: Thu, 01 Jan 1970 00:00:00 +0000\r\n",
+                "Test: test  test very very long with spaces and extra spaces   \twill be\r\n",
+                " folded to several lines \r\n",
+                "Subject: Test with utf-8 =?utf-8?b?w6s=?=\r\n",
+                "Content-Transfer-Encoding: 7bit\r\n",
+                "DKIM-Signature: v=1; a=rsa-sha256; d=example.org; s=dkimtest;\r\n",
+                " c=simple/simple; q=dns/txt; t=0; h=Date:From:Subject:To;\r\n",
+                " bh=f3Zksdcjqa/xRBwdyFzIXWCcgP7XTgxjCgYsXOMKQl4=;\r\n",
+                " b=NhoIMMAALoSgu5lKAR0+MUQunOWnU7wpF9ORUFtpxq9sGZDo9AX43AMhFemyM5W204jpFwMU6pm7AMR1nOYBdSYye4yUALtvT2nqbJBwSh7JeYu+z22t1RFKp7qQR1il8aSrkbZuNMFHYuSEwW76QtKwcNqP4bQOzS9CzgQp0ABu8qwYPBr/EypykPTfqjtyN+ywrfdqjjGOzTpRGolH0hc3CrAETNjjHbNBgKgucXmXTN7hMRdzqWjeFPxizXwouwNAavFClPG0l33gXVArFWn+CkgA84G/s4zuJiF7QPZR87Pu4pw/vIlSXxH4a42W3tT19v9iBTH7X7ldYegtmQ==\r\n",
+                "\r\n",
+                "test\r\n",
+                "\r\n",
+                "test   \ttest\r\n",
+                "\r\n",
+                "\r\n",
+            )
+        );
+    }
+
+    #[test]
+    fn test_signature_rsa_relaxed() {
+        let mut message = test_message();
+        let signing_key = DkimSigningKey::new(KEY_RSA, DkimSigningAlgorithm::Rsa).unwrap();
+        dkim_sign_fixed_time(
+            &mut message,
+            &DkimConfig::new(
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
+                signing_key,
+                vec![
+                    HeaderName::new_from_ascii_str("Date"),
+                    HeaderName::new_from_ascii_str("From"),
+                    HeaderName::new_from_ascii_str("Subject"),
+                    HeaderName::new_from_ascii_str("To"),
+                ],
+                DkimCanonicalization {
+                    header: DkimCanonicalizationType::Relaxed,
+                    body: DkimCanonicalizationType::Relaxed,
+                },
+            ),
+            std::time::UNIX_EPOCH,
+        );
+        let signed = message.formatted();
+        let signed = std::str::from_utf8(&signed).unwrap();
+        println!("{signed}");
+        assert_eq!(
+            signed,
+            std::concat!(
+                "From: =?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\n",
+                "To: Test2 <test2@example.org>\r\n",
+                "Date: Thu, 01 Jan 1970 00:00:00 +0000\r\n",
+                "Test: test  test very very long with spaces and extra spaces   \twill be\r\n",
+                " folded to several lines \r\n","Subject: Test with utf-8 =?utf-8?b?w6s=?=\r\n",
+                "Content-Transfer-Encoding: 7bit\r\n",
+                "DKIM-Signature: v=1; a=rsa-sha256; d=example.org; s=dkimtest;\r\n",
+                " c=relaxed/relaxed; q=dns/txt; t=0; h=date:from:subject:to;\r\n",
+                " bh=qN8je6qJgWFGSnN2MycC/XKPbN6BOrMJyAX2h4m19Ss=;\r\n",
+                " b=YaVfmH8dbGEywoLJ4uhbvYqDyQG1UGKFH3PE7zXGgk+YFxUgkwWjoA3aQupDNQtfTjfUsNe0dnrjyZP+ylnESpZBpbCIf5/n3FEh6j3RQthqNbQblcfH/U8mazTuRbVjYBbTZQDaQCMPTz+8D+ZQfXo2oq6dGzTuGvmuYft0CVsq/BIp/EkhZHqiphDeVJSHD4iKW8+L2XwEWThoY92xOYc1G0TtBwz2UJgtiHX2YulH/kRBHeK3dKn9RTNVL3VZ+9ZrnFwIhET9TPGtU2I+q0EMSWF9H9bTrASMgW/U+E0VM2btqJlrTU6rQ7wlQeHdwecLnzXcyhCUInF1+veMNw==\r\n",
+                "\r\n",
+                "test\r\n",
+                "\r\n",
+                "test   \ttest\r\n",
+                "\r\n",
+                "\r\n",
+            )
+        );
+    }
+}

src/message/header/content.rs

@@ -3,7 +3,7 @@ use std::{
     str::FromStr,
 };
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Transfer-Encoding` of the body
@@ -35,8 +35,9 @@ impl Header for ContentTransferEncoding {
         Ok(s.parse()?)
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
-        self.to_string()
+        let val = self.to_string();
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -74,8 +75,10 @@ impl Default for ContentTransferEncoding {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::ContentTransferEncoding;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_transfer_encoding() {
@@ -94,20 +97,20 @@ mod test {
     fn parse_content_transfer_encoding() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "7bit".to_string(),
+            "7bit".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentTransferEncoding>(),
             Some(ContentTransferEncoding::SevenBit)
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "base64".to_string(),
+            "base64".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentTransferEncoding>(),

src/message/header/content_disposition.rs

@@ -1,29 +1,57 @@
-use super::{Header, HeaderName};
+use std::fmt::Write;
+
+use email_encoding::headers::EmailWriter;
+
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Disposition` of an attachment
 ///
 /// Defined in [RFC2183](https://tools.ietf.org/html/rfc2183)
 #[derive(Debug, Clone, PartialEq)]
-pub struct ContentDisposition(String);
+pub struct ContentDisposition(HeaderValue);
 
 impl ContentDisposition {
     /// An attachment which should be displayed inline into the message
     pub fn inline() -> Self {
-        Self("inline".into())
+        Self(HeaderValue::dangerous_new_pre_encoded(
+            Self::name(),
+            "inline".to_owned(),
+            "inline".to_owned(),
+        ))
     }
 
     /// An attachment which should be displayed inline into the message, but that also
     /// species the filename in case it were to be downloaded
     pub fn inline_with_name(file_name: &str) -> Self {
-        debug_assert!(!file_name.contains('"'), "file_name shouldn't contain '\"'");
+        Self::with_name("inline", file_name)
-        Self(format!("inline; filename=\"{}\"", file_name))
     }
 
     /// An attachment which is separate from the body of the message, and can be downloaded separately
     pub fn attachment(file_name: &str) -> Self {
-        debug_assert!(!file_name.contains('"'), "file_name shouldn't contain '\"'");
+        Self::with_name("attachment", file_name)
-        Self(format!("attachment; filename=\"{}\"", file_name))
+    }
+
+    fn with_name(kind: &str, file_name: &str) -> Self {
+        let raw_value = format!("{kind}; filename=\"{file_name}\"");
+
+        let mut encoded_value = String::new();
+        let line_len = "Content-Disposition: ".len();
+        {
+            let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false, false);
+            w.write_str(kind).expect("writing `kind` returned an error");
+            w.write_char(';').expect("writing `;` returned an error");
+            w.optional_breakpoint();
+
+            email_encoding::headers::rfc2231::encode("filename", file_name, &mut w)
+                .expect("some Write implementation returned an error");
+        }
+
+        Self(HeaderValue::dangerous_new_pre_encoded(
+            Self::name(),
+            raw_value,
+            encoded_value,
+        ))
     }
 }
 
@@ -33,18 +61,28 @@ impl Header for ContentDisposition {
     }
 
     fn parse(s: &str) -> Result<Self, BoxError> {
-        Ok(Self(s.into()))
+        match (s.split_once(';'), s) {
+            (_, "inline") => Ok(Self::inline()),
+            (Some((kind @ ("inline" | "attachment"), file_name)), _) => file_name
+                .split_once(" filename=\"")
+                .and_then(|(_, file_name)| file_name.strip_suffix('"'))
+                .map(|file_name| Self::with_name(kind, file_name))
+                .ok_or_else(|| "Unsupported ContentDisposition value".into()),
+            _ => Err("Unsupported ContentDisposition value".into()),
+        }
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
         self.0.clone()
     }
 }
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::ContentDisposition;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_disposition() {
@@ -52,12 +90,12 @@ mod test {
 
         headers.set(ContentDisposition::inline());
 
-        assert_eq!(format!("{}", headers), "Content-Disposition: inline\r\n");
+        assert_eq!(format!("{headers}"), "Content-Disposition: inline\r\n");
 
         headers.set(ContentDisposition::attachment("something.txt"));
 
         assert_eq!(
-            format!("{}", headers),
+            format!("{headers}"),
             "Content-Disposition: attachment; filename=\"something.txt\"\r\n"
         );
     }
@@ -66,20 +104,20 @@ mod test {
     fn parse_content_disposition() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
-            "inline".to_string(),
+            "inline".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentDisposition>(),
             Some(ContentDisposition::inline())
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
-            "attachment; filename=\"something.txt\"".to_string(),
+            "attachment; filename=\"something.txt\"".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentDisposition>(),

src/message/header/content_type.rs

@@ -6,7 +6,7 @@ use std::{
 
 use mime::Mime;
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Type` of the body
@@ -54,8 +54,8 @@ impl Header for ContentType {
         Ok(Self(s.parse()?))
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
-        self.0.to_string()
+        HeaderValue::new(Self::name(), self.0.to_string())
     }
 }
 
@@ -94,11 +94,13 @@ impl Display for ContentTypeErr {
 // -- Serialization and Deserialization --
 #[cfg(feature = "serde")]
 mod serde {
-    use serde::de::{self, Deserialize, Deserializer, Visitor};
-    use serde::ser::{Serialize, Serializer};
-
     use std::fmt;
 
+    use serde::{
+        de::{self, Deserialize, Deserializer, Visitor},
+        ser::{Serialize, Serializer},
+    };
+
     use super::ContentType;
 
     impl Serialize for ContentType {
@@ -133,8 +135,7 @@ mod serde {
                     match ContentType::parse(mime) {
                         Ok(content_type) => Ok(content_type),
                         Err(_) => Err(E::custom(format!(
-                            "Couldn't parse the following MIME-Type: {}",
+                            "Couldn't parse the following MIME-Type: {mime}"
-                            mime
                         ))),
                     }
                 }
@@ -147,8 +148,10 @@ mod serde {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::ContentType;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_type() {
@@ -173,17 +176,17 @@ mod test {
     fn parse_content_type() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
-            "text/plain; charset=utf-8".to_string(),
+            "text/plain; charset=utf-8".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_PLAIN));
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
-            "text/html; charset=utf-8".to_string(),
+            "text/html; charset=utf-8".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_HTML));
     }

src/message/header/date.rs

@@ -2,13 +2,13 @@ use std::time::SystemTime;
 
 use httpdate::HttpDate;
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// Message `Date` header
 ///
 /// Defined in [RFC2822](https://tools.ietf.org/html/rfc2822#section-3.3)
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Date(HttpDate);
 
 impl Date {
@@ -32,29 +32,29 @@ impl Header for Date {
 
     fn parse(s: &str) -> Result<Self, BoxError> {
         let mut s = String::from(s);
-        if s.ends_with(" -0000") {
+        if s.ends_with("+0000") {
             // The httpdate crate expects the `Date` to end in ` GMT`, but email
-            // uses `-0000`, so we crudely fix this issue here.
+            // uses `+0000` to indicate UTC, so we crudely fix this issue here.
 
-            s.truncate(s.len() - "-0000".len());
+            s.truncate(s.len() - "+0000".len());
             s.push_str("GMT");
         }
 
         Ok(Self(s.parse::<HttpDate>()?))
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
-        let mut s = self.0.to_string();
+        let mut val = self.0.to_string();
-        if s.ends_with(" GMT") {
+        if val.ends_with(" GMT") {
             // The httpdate crate always appends ` GMT` to the end of the string,
             // but this is considered an obsolete date format for email
             // https://tools.ietf.org/html/rfc2822#appendix-A.6.2,
-            // so we replace `GMT` with `-0000`
+            // so we replace `GMT` with `+0000`
-            s.truncate(s.len() - "GMT".len());
+            val.truncate(val.len() - "GMT".len());
-            s.push_str("-0000");
+            val.push_str("+0000");
         }
 
-        s
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -74,8 +74,10 @@ impl From<Date> for SystemTime {
 mod test {
     use std::time::{Duration, SystemTime};
 
+    use pretty_assertions::assert_eq;
+
     use super::Date;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_date() {
@@ -88,7 +90,7 @@ mod test {
 
         assert_eq!(
             headers.to_string(),
-            "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n".to_string()
+            "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n".to_owned()
         );
 
         // Tue, 15 Nov 1994 08:12:32 GMT
@@ -98,7 +100,7 @@ mod test {
 
         assert_eq!(
             headers.to_string(),
-            "Date: Tue, 15 Nov 1994 08:12:32 -0000\r\n"
+            "Date: Tue, 15 Nov 1994 08:12:32 +0000\r\n"
         );
     }
 
@@ -106,10 +108,10 @@ mod test {
     fn parse_date() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
-            "Tue, 15 Nov 1994 08:12:31 -0000".to_string(),
+            "Tue, 15 Nov 1994 08:12:31 +0000".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Date>(),
@@ -118,10 +120,10 @@ mod test {
             ))
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
-            "Tue, 15 Nov 1994 08:12:32 -0000".to_string(),
+            "Tue, 15 Nov 1994 08:12:32 +0000".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Date>(),

src/message/header/mailbox.rs

@@ -1,4 +1,6 @@
-use super::{Header, HeaderName};
+use email_encoding::headers::EmailWriter;
+
+use super::{Header, HeaderName, HeaderValue};
 use crate::{
     message::mailbox::{Mailbox, Mailboxes},
     BoxError,
@@ -12,7 +14,7 @@ pub trait MailboxesHeader {
 macro_rules! mailbox_header {
     ($(#[$doc:meta])*($type_name: ident, $header_name: expr)) => {
         $(#[$doc])*
-        #[derive(Debug, Clone, PartialEq)]
+        #[derive(Debug, Clone, PartialEq, Eq)]
         pub struct $type_name(Mailbox);
 
         impl Header for $type_name {
@@ -25,8 +27,15 @@ macro_rules! mailbox_header {
                 Ok(Self(mailbox))
             }
 
-            fn display(&self) -> String {
+            fn display(&self) -> HeaderValue {
-                self.0.to_string()
+                let mut encoded_value = String::new();
+                let line_len = $header_name.len() + ": ".len();
+                {
+                    let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false, false);
+                    self.0.encode(&mut w).expect("writing `Mailbox` returned an error");
+                }
+
+                HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
         }
 
@@ -49,7 +58,7 @@ macro_rules! mailbox_header {
 macro_rules! mailboxes_header {
     ($(#[$doc:meta])*($type_name: ident, $header_name: expr)) => {
         $(#[$doc])*
-        #[derive(Debug, Clone, PartialEq)]
+        #[derive(Debug, Clone, PartialEq, Eq)]
         pub struct $type_name(pub(crate) Mailboxes);
 
         impl MailboxesHeader for $type_name {
@@ -68,8 +77,15 @@ macro_rules! mailboxes_header {
                 Ok(Self(mailbox))
             }
 
-            fn display(&self) -> String {
+            fn display(&self) -> HeaderValue {
-                self.0.to_string()
+                let mut encoded_value = String::new();
+                let line_len = $header_name.len() + ": ".len();
+                {
+                    let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false, false);
+                    self.0.encode(&mut w).expect("writing `Mailboxes` returned an error");
+                }
+
+                HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
         }
 
@@ -160,8 +176,10 @@ mailboxes_header! {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::{From, Mailbox, Mailboxes};
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_single_without_name() {
@@ -175,12 +193,12 @@ mod test {
 
     #[test]
     fn format_single_with_name() {
-        let from = Mailboxes::new().with("K. <kayo@example.com>".parse().unwrap());
+        let from = Mailboxes::new().with("Kayo <kayo@example.com>".parse().unwrap());
 
         let mut headers = Headers::new();
         headers.set(From(from));
 
-        assert_eq!(headers.to_string(), "From: K. <kayo@example.com>\r\n");
+        assert_eq!(headers.to_string(), "From: Kayo <kayo@example.com>\r\n");
     }
 
     #[test]
@@ -201,7 +219,7 @@ mod test {
     #[test]
     fn format_multi_with_name() {
         let from = vec![
-            "K. <kayo@example.com>".parse().unwrap(),
+            "Kayo <kayo@example.com>".parse().unwrap(),
             "Pony P. <pony@domain.tld>".parse().unwrap(),
         ];
 
@@ -210,7 +228,7 @@ mod test {
 
         assert_eq!(
             headers.to_string(),
-            "From: K. <kayo@example.com>, Pony P. <pony@domain.tld>\r\n"
+            "From: Kayo <kayo@example.com>, \"Pony P.\" <pony@domain.tld>\r\n"
         );
     }
 
@@ -232,10 +250,10 @@ mod test {
         let from = vec!["kayo@example.com".parse().unwrap()].into();
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "kayo@example.com".to_string(),
+            "kayo@example.com".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
     }
@@ -245,10 +263,10 @@ mod test {
         let from = vec!["K. <kayo@example.com>".parse().unwrap()].into();
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "K. <kayo@example.com>".to_string(),
+            "K. <kayo@example.com>".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
     }
@@ -261,10 +279,10 @@ mod test {
         ];
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "kayo@example.com, pony@domain.tld".to_string(),
+            "kayo@example.com, pony@domain.tld".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
     }
@@ -277,11 +295,38 @@ mod test {
         ];
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "K. <kayo@example.com>, Pony P. <pony@domain.tld>".to_string(),
+            "K. <kayo@example.com>, Pony P. <pony@domain.tld>".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
     }
+
+    #[test]
+    fn parse_multi_with_name_containing_comma() {
+        let from: Vec<Mailbox> = vec![
+            "Test, test <1@example.com>".parse().unwrap(),
+            "Test2, test2 <2@example.com>".parse().unwrap(),
+        ];
+
+        let mut headers = Headers::new();
+        headers.insert_raw(HeaderValue::new(
+            HeaderName::new_from_ascii_str("From"),
+            "Test, test <1@example.com>, Test2, test2 <2@example.com>".to_owned(),
+        ));
+
+        assert_eq!(headers.get::<From>(), Some(From(from.into())));
+    }
+
+    #[test]
+    fn parse_multi_with_name_containing_comma_last_broken() {
+        let mut headers = Headers::new();
+        headers.insert_raw(HeaderValue::new(
+            HeaderName::new_from_ascii_str("From"),
+            "Test, test <1@example.com>, Test2, test2".to_owned(),
+        ));
+
+        assert_eq!(headers.get::<From>(), None);
+    }
 }

src/message/header/mod.rs

@@ -3,10 +3,12 @@
 use std::{
     borrow::Cow,
     error::Error,
-    fmt::{self, Display, Formatter},
+    fmt::{self, Display, Formatter, Write},
     ops::Deref,
 };
 
+use email_encoding::headers::EmailWriter;
+
 pub use self::{
     content::*,
     content_disposition::ContentDisposition,
@@ -34,13 +36,13 @@ pub trait Header: Clone {
 
     fn parse(s: &str) -> Result<Self, BoxError>;
 
-    fn display(&self) -> String;
+    fn display(&self) -> HeaderValue;
 }
 
 /// A set of email headers
 #[derive(Debug, Clone, Default)]
 pub struct Headers {
-    headers: Vec<(HeaderName, String)>,
+    headers: Vec<HeaderValue>,
 }
 
 impl Headers {
@@ -68,13 +70,14 @@ impl Headers {
     ///
     /// Returns `None` if `Header` isn't present in `Headers`.
     pub fn get<H: Header>(&self) -> Option<H> {
-        self.get_raw(&H::name()).and_then(|raw| H::parse(raw).ok())
+        self.get_raw(&H::name())
+            .and_then(|raw_value| H::parse(raw_value).ok())
     }
 
     /// Sets `Header` into `Headers`, overriding `Header` if it
     /// was already present in `Headers`
     pub fn set<H: Header>(&mut self, header: H) {
-        self.insert_raw(H::name(), header.display());
+        self.insert_raw(header.display());
     }
 
     /// Remove `Header` from `Headers`, returning it
@@ -82,7 +85,7 @@ impl Headers {
     /// Returns `None` if `Header` isn't in `Headers`.
     pub fn remove<H: Header>(&mut self) -> Option<H> {
         self.remove_raw(&H::name())
-            .and_then(|(_name, raw)| H::parse(&raw).ok())
+            .and_then(|value| H::parse(&value.raw_value).ok())
     }
 
     /// Clears `Headers`, removing all headers from it
@@ -97,62 +100,46 @@ impl Headers {
     ///
     /// Returns `None` if `name` isn't present in `Headers`.
     pub fn get_raw(&self, name: &str) -> Option<&str> {
-        self.find_header(name).map(|(_name, value)| value)
+        self.find_header(name).map(|value| value.raw_value.as_str())
     }
 
     /// Inserts a raw header into `Headers`, overriding `value` if it
     /// was already present in `Headers`.
-    pub fn insert_raw(&mut self, name: HeaderName, value: String) {
+    pub fn insert_raw(&mut self, value: HeaderValue) {
-        match self.find_header_mut(&name) {
+        match self.find_header_mut(&value.name) {
-            Some((_, current_value)) => {
+            Some(current_value) => {
                 *current_value = value;
             }
             None => {
-                self.headers.push((name, value));
+                self.headers.push(value);
             }
         }
     }
 
-    /// Appends a raw header into `Headers`
-    ///
-    /// If a header with a name of `name` is already present,
-    /// appends `, ` + `value` to it's current value.
-    pub fn append_raw(&mut self, name: HeaderName, value: String) {
-        match self.find_header_mut(&name) {
-            Some((_name, prev_value)) => {
-                prev_value.push_str(", ");
-                prev_value.push_str(&value);
-            }
-            None => self.headers.push((name, value)),
-        }
-    }
-
     /// Remove a raw header from `Headers`, returning it
     ///
     /// Returns `None` if `name` isn't present in `Headers`.
-    pub fn remove_raw(&mut self, name: &str) -> Option<(HeaderName, String)> {
+    pub fn remove_raw(&mut self, name: &str) -> Option<HeaderValue> {
         self.find_header_index(name).map(|i| self.headers.remove(i))
     }
 
-    fn find_header(&self, name: &str) -> Option<(&HeaderName, &str)> {
+    pub(crate) fn find_header(&self, name: &str) -> Option<&HeaderValue> {
         self.headers
             .iter()
-            .find(|&(name_, _value)| name.eq_ignore_ascii_case(name_))
+            .find(|value| name.eq_ignore_ascii_case(&value.name))
-            .map(|t| (&t.0, t.1.as_str()))
     }
 
-    fn find_header_mut(&mut self, name: &str) -> Option<(&HeaderName, &mut String)> {
+    fn find_header_mut(&mut self, name: &str) -> Option<&mut HeaderValue> {
         self.headers
             .iter_mut()
-            .find(|(name_, _value)| name.eq_ignore_ascii_case(name_))
+            .find(|value| name.eq_ignore_ascii_case(&value.name))
-            .map(|t| (&t.0, &mut t.1))
     }
 
     fn find_header_index(&self, name: &str) -> Option<usize> {
         self.headers
             .iter()
             .enumerate()
-            .find(|&(_i, (name_, _value))| name.eq_ignore_ascii_case(name_))
+            .find(|(_i, value)| name.eq_ignore_ascii_case(&value.name))
             .map(|(i, _)| i)
     }
 }
@@ -160,10 +147,10 @@ impl Headers {
 impl Display for Headers {
     /// Formats `Headers`, ready to put them into an email
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        for (name, value) in &self.headers {
+        for value in &self.headers {
-            Display::fmt(name, f)?;
+            f.write_str(&value.name)?;
             f.write_str(": ")?;
-            HeaderValueEncoder::encode(name, value, f)?;
+            f.write_str(&value.encoded_value)?;
             f.write_str("\r\n")?;
         }
 
@@ -290,55 +277,86 @@ impl PartialEq<HeaderName> for &str {
     }
 }
 
-const ENCODING_START_PREFIX: &str = "=?utf-8?b?";
+/// A safe for use header value
-const ENCODING_END_SUFFIX: &str = "?=";
+#[derive(Debug, Clone, PartialEq)]
-const MAX_LINE_LEN: usize = 76;
+pub struct HeaderValue {
+    name: HeaderName,
+    raw_value: String,
+    encoded_value: String,
+}
+
+impl HeaderValue {
+    /// Construct a new `HeaderValue` and encode it
+    ///
+    /// Takes the header `name` and the `raw_value` and encodes
+    /// it via `RFC2047` and line folds it.
+    ///
+    /// [`RFC2047`]: https://datatracker.ietf.org/doc/html/rfc2047
+    pub fn new(name: HeaderName, raw_value: String) -> Self {
+        let mut encoded_value = String::with_capacity(raw_value.len());
+        HeaderValueEncoder::encode(&name, &raw_value, &mut encoded_value).unwrap();
+
+        Self {
+            name,
+            raw_value,
+            encoded_value,
+        }
+    }
+
+    /// Construct a new `HeaderValue` using a pre-encoded header value
+    ///
+    /// This method is _extremely_ dangerous as it opens up
+    /// the encoder to header injection attacks, but is sometimes
+    /// acceptable for use if `encoded_value` contains only ascii
+    /// printable characters and is already line folded.
+    ///
+    /// When in doubt use [`HeaderValue::new`].
+    pub fn dangerous_new_pre_encoded(
+        name: HeaderName,
+        raw_value: String,
+        encoded_value: String,
+    ) -> Self {
+        Self {
+            name,
+            raw_value,
+            encoded_value,
+        }
+    }
+
+    #[cfg(feature = "dkim")]
+    pub(crate) fn get_raw(&self) -> &str {
+        &self.raw_value
+    }
+
+    #[cfg(feature = "dkim")]
+    pub(crate) fn get_encoded(&self) -> &str {
+        &self.encoded_value
+    }
+}
 
 /// [RFC 1522](https://tools.ietf.org/html/rfc1522) header value encoder
-struct HeaderValueEncoder {
+struct HeaderValueEncoder<'a> {
-    line_len: usize,
+    writer: EmailWriter<'a>,
     encode_buf: String,
 }
 
-impl HeaderValueEncoder {
+impl<'a> HeaderValueEncoder<'a> {
-    fn encode(name: &str, value: &str, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    fn encode(name: &str, value: &'a str, f: &'a mut impl fmt::Write) -> fmt::Result {
-        let (words_iter, encoder) = Self::new(name, value);
+        let encoder = Self::new(name, f);
-        encoder.format(words_iter, f)
+        encoder.format(value.split_inclusive(' '))
     }
 
-    fn new<'a>(name: &str, value: &'a str) -> (WordsPlusFillIterator<'a>, Self) {
+    fn new(name: &str, writer: &'a mut dyn Write) -> Self {
-        (
+        let line_len = name.len() + ": ".len();
-            WordsPlusFillIterator { s: value },
+        let writer = EmailWriter::new(writer, line_len, 0, false, false);
-            Self {
+
-                line_len: name.len() + ": ".len(),
+        Self {
-                encode_buf: String::new(),
+            writer,
-            },
+            encode_buf: String::new(),
-        )
+        }
     }
 
-    fn format(
+    fn format(mut self, words_iter: impl Iterator<Item = &'a str>) -> fmt::Result {
-        mut self,
-        words_iter: WordsPlusFillIterator<'_>,
-        f: &mut fmt::Formatter<'_>,
-    ) -> fmt::Result {
-        /// Estimate if an encoded string of `len` would fix in an empty line
-        fn would_fit_new_line(len: usize) -> bool {
-            len < (MAX_LINE_LEN - " ".len())
-        }
-
-        /// Estimate how long a string of `len` would be after base64 encoding plus
-        /// adding the encoding prefix and suffix to it
-        fn base64_len(len: usize) -> usize {
-            ENCODING_START_PREFIX.len() + (len * 4 / 3 + 4) + ENCODING_END_SUFFIX.len()
-        }
-
-        /// Estimate how many more bytes we can fit in the current line
-        fn available_len_to_max_encode_len(len: usize) -> usize {
-            len.saturating_sub(
-                ENCODING_START_PREFIX.len() + (len * 3 / 4 + 4) + ENCODING_END_SUFFIX.len(),
-            )
-        }
-
         for next_word in words_iter {
             let allowed = allowed_str(next_word);
 
@@ -346,207 +364,54 @@ impl HeaderValueEncoder {
                 // This word only contains allowed characters
 
                 // the next word is allowed, but we may have accumulated some words to encode
-                self.flush_encode_buf(f, true)?;
+                self.flush_encode_buf()?;
 
-                if next_word.len() > self.remaining_line_len() {
+                self.writer.folding().write_str(next_word)?;
-                    // not enough space left on this line to encode word
-
-                    if self.something_written_to_this_line() && would_fit_new_line(next_word.len())
-                    {
-                        // word doesn't fit this line, but something had already been written to it,
-                        // and word would fit the next line, so go to a new line
-                        // so go to new line
-                        self.new_line(f)?;
-                    } else {
-                        // word neither fits this line and the next one, cut it
-                        // in the middle and make it fit
-
-                        let mut next_word = next_word;
-
-                        while !next_word.is_empty() {
-                            if self.remaining_line_len() == 0 {
-                                self.new_line(f)?;
-                            }
-
-                            let len = self.remaining_line_len().min(next_word.len());
-                            let first_part = &next_word[..len];
-                            next_word = &next_word[len..];
-
-                            f.write_str(first_part)?;
-                            self.line_len += first_part.len();
-                        }
-
-                        continue;
-                    }
-                }
-
-                // word fits, write it!
-                f.write_str(next_word)?;
-                self.line_len += next_word.len();
             } else {
                 // This word contains unallowed characters
-
+                self.encode_buf.push_str(next_word);
-                if self.remaining_line_len() >= base64_len(self.encode_buf.len() + next_word.len())
-                {
-                    // next_word fits
-                    self.encode_buf.push_str(next_word);
-                    continue;
-                }
-
-                // next_word doesn't fit this line
-
-                if would_fit_new_line(base64_len(next_word.len())) {
-                    // ...but it would fit the next one
-
-                    self.flush_encode_buf(f, false)?;
-                    self.new_line(f)?;
-
-                    self.encode_buf.push_str(next_word);
-                    continue;
-                }
-
-                // ...and also wouldn't fit the next one.
-                // chop it up into pieces
-
-                let mut next_word = next_word;
-
-                while !next_word.is_empty() {
-                    let mut len = available_len_to_max_encode_len(self.remaining_line_len())
-                        .min(next_word.len());
-
-                    if len == 0 {
-                        self.flush_encode_buf(f, false)?;
-                        self.new_line(f)?;
-                    }
-
-                    // avoid slicing on a char boundary
-                    while !next_word.is_char_boundary(len) {
-                        len += 1;
-                    }
-                    let first_part = &next_word[..len];
-                    next_word = &next_word[len..];
-
-                    self.encode_buf.push_str(first_part);
-                }
             }
         }
 
-        self.flush_encode_buf(f, false)?;
+        self.flush_encode_buf()?;
 
         Ok(())
     }
 
-    /// Returns the number of bytes left for the current line
+    fn flush_encode_buf(&mut self) -> fmt::Result {
-    fn remaining_line_len(&self) -> usize {
-        MAX_LINE_LEN - self.line_len
-    }
-
-    /// Returns true if something has been written to the current line
-    fn something_written_to_this_line(&self) -> bool {
-        self.line_len > 1
-    }
-
-    fn flush_encode_buf(
-        &mut self,
-        f: &mut fmt::Formatter<'_>,
-        switching_to_allowed: bool,
-    ) -> fmt::Result {
-        use std::fmt::Write;
-
         if self.encode_buf.is_empty() {
             // nothing to encode
             return Ok(());
         }
 
-        let mut write_after = None;
+        let prefix = self.encode_buf.trim_end_matches(' ');
+        email_encoding::headers::rfc2047::encode(prefix, &mut self.writer)?;
 
-        if switching_to_allowed {
+        // TODO: add a better API for doing this in email-encoding
-            // If the next word only contains allowed characters, and the string to encode
+        let spaces = self.encode_buf.len() - prefix.len();
-            // ends with a space, take the space out of the part to encode
+        for _ in 0..spaces {
-
+            self.writer.space();
-            let last_char = self.encode_buf.pop().expect("self.encode_buf isn't empty");
-            if is_space_like(last_char) {
-                write_after = Some(last_char);
-            } else {
-                self.encode_buf.push(last_char);
-            }
-        }
-
-        f.write_str(ENCODING_START_PREFIX)?;
-        let encoded = base64::display::Base64Display::with_config(
-            self.encode_buf.as_bytes(),
-            base64::STANDARD,
-        );
-        Display::fmt(&encoded, f)?;
-        f.write_str(ENCODING_END_SUFFIX)?;
-
-        self.line_len += ENCODING_START_PREFIX.len();
-        self.line_len += self.encode_buf.len() * 4 / 3 + 4;
-        self.line_len += ENCODING_END_SUFFIX.len();
-
-        if let Some(write_after) = write_after {
-            f.write_char(write_after)?;
-            self.line_len += 1;
         }
 
         self.encode_buf.clear();
         Ok(())
     }
-
-    fn new_line(&mut self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str("\r\n ")?;
-        self.line_len = 1;
-
-        Ok(())
-    }
-}
-
-/// Iterator yielding a string split space by space, but including all space
-/// characters between it and the next word
-struct WordsPlusFillIterator<'a> {
-    s: &'a str,
-}
-
-impl<'a> Iterator for WordsPlusFillIterator<'a> {
-    type Item = &'a str;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if self.s.is_empty() {
-            return None;
-        }
-
-        let next_word = self
-            .s
-            .char_indices()
-            .skip(1)
-            .skip_while(|&(_i, c)| !is_space_like(c))
-            .find(|&(_i, c)| !is_space_like(c))
-            .map(|(i, _)| i);
-
-        let word = &self.s[..next_word.unwrap_or_else(|| self.s.len())];
-        self.s = &self.s[word.len()..];
-        Some(word)
-    }
-}
-
-const fn is_space_like(c: char) -> bool {
-    c == ',' || c == ' '
 }
 
 fn allowed_str(s: &str) -> bool {
-    s.chars().all(allowed_char)
+    s.bytes().all(allowed_char)
 }
 
-const fn allowed_char(c: char) -> bool {
+const fn allowed_char(c: u8) -> bool {
-    c >= 1 as char && c <= 9 as char
+    c >= 1 && c <= 9 || c == 11 || c == 12 || c >= 14 && c <= 127
-        || c == 11 as char
-        || c == 12 as char
-        || c >= 14 as char && c <= 127 as char
 }
 
 #[cfg(test)]
 mod tests {
-    use super::{HeaderName, Headers};
+    use pretty_assertions::assert_eq;
+
+    use super::{HeaderName, HeaderValue, Headers, To};
+    use crate::message::Mailboxes;
 
     #[test]
     fn valid_headername() {
@@ -607,10 +472,10 @@ mod tests {
     #[test]
     fn format_ascii() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "John Doe <example@example.com>, Jean Dupont <jean@example.com>".to_string(),
+            "John Doe <example@example.com>, Jean Dupont <jean@example.com>".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -621,16 +486,16 @@ mod tests {
     #[test]
     fn format_ascii_with_folding() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand <jemand@example.com>, Jean Dupont <jean@example.com>".to_string(),
+            "Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand <jemand@example.com>, Jean Dupont <jean@example.com>".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "To: Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith \r\n",
+                "To: Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith\r\n",
-                " <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand \r\n",
+                " <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand\r\n",
                 " <jemand@example.com>, Jean Dupont <jean@example.com>\r\n"
             )
         );
@@ -639,16 +504,16 @@ mod tests {
     #[test]
     fn format_ascii_with_folding_long_line() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
+            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_owned()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "Subject: Hello! This is lettre, and this \r\n ",
+                "Subject: Hello! This is lettre, and this\r\n",
-                "IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I \r\n",
+                " IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I\r\n",
                 " guess that's it!\r\n"
             )
         );
@@ -658,15 +523,17 @@ mod tests {
     fn format_ascii_with_folding_very_long_line() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know".to_string()
+            "Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know".to_owned()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "Subject: Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoY\r\n",
+                "Subject: Hello!\r\n",
-                " ouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know\r\n",
+                " IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut!\r\n",
+                " I don't know\r\n",
             )
         );
     }
@@ -674,28 +541,24 @@ mod tests {
     #[test]
     fn format_ascii_with_folding_giant_word() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_string()
+            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_owned()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
-            concat!(
+            "Subject: 1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz\r\n",
-                "Subject: 1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijkl\r\n",
-                " mnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdef\r\n",
-                " ghijklmnopqrstuvwxyz\r\n",
-            )
         );
     }
 
     #[test]
     fn format_special() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "Seán <sean@example.com>".to_string(),
+            "Seán <sean@example.com>".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -706,10 +569,10 @@ mod tests {
     #[test]
     fn format_special_emoji() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "🌎 <world@example.com>".to_string(),
+            "🌎 <world@example.com>".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -720,19 +583,46 @@ mod tests {
     #[test]
     fn format_special_with_folding() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        let to = To::from(Mailboxes::from_iter([
-            HeaderName::new_from_ascii_str("To"),
+            "🌍 <world@example.com>".parse().unwrap(),
-            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_string(),
+            "🦆 Everywhere <ducks@example.com>".parse().unwrap(),
-        );
+            "Иванов Иван Иванович <ivanov@example.com>".parse().unwrap(),
+            "Jānis Bērziņš <janis@example.com>".parse().unwrap(),
+            "Seán Ó Rudaí <sean@example.com>".parse().unwrap(),
+        ]));
+        headers.set(to);
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?= \r\n",
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhiBFdmVyeXdo?=\r\n",
-                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyIA==?=\r\n",
+                " =?utf-8?b?ZXJl?= <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LI=?=\r\n",
-                " =?utf-8?b?0JjQstCw0L0g0JjQstCw0L3QvtCy0LjRhw==?= <ivanov@example.com>, \r\n",
+                " =?utf-8?b?0LDQvSDQmNCy0LDQvdC+0LLQuNGH?= <ivanov@example.com>,\r\n",
-                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, \r\n",
+                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, =?utf-8?b?U2U=?=\r\n",
-                " =?utf-8?b?U2XDoW4gw5MgUnVkYcOt?= <sean@example.com>\r\n"
+                " =?utf-8?b?w6FuIMOTIFJ1ZGHDrQ==?= <sean@example.com>\r\n",
+            )
+        );
+    }
+
+    #[test]
+    fn format_special_with_folding_raw() {
+        let mut headers = Headers::new();
+        headers.insert_raw(HeaderValue::new(
+            HeaderName::new_from_ascii_str("To"),
+            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_owned(),
+        ));
+
+        // TODO: fix the fact that the encoder doesn't know that
+        // the space between the name and the address should be
+        // removed when wrapping.
+        assert_eq!(
+            headers.to_string(),
+            concat!(
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?=\r\n",
+                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9?=\r\n",
+                " =?utf-8?b?INCY0LLQsNC90L7QstC40Yc=?= <ivanov@example.com>,\r\n",
+                "  =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>,\r\n",
+                "  =?utf-8?b?U2XDoW4gw5MgUnVkYcOt?= <sean@example.com>\r\n",
             )
         );
     }
@@ -741,23 +631,31 @@ mod tests {
     fn format_slice_on_char_boundary_bug() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_string(),
+            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_owned(),)
         );
 
         assert_eq!(
             headers.to_string(),
-            "Subject: =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz?=\r\n"
+            concat!(
+                "Subject: =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz?=\r\n",
+                " =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbM=?=\r\n",
+                " =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbM=?=\r\n",
+                " =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbM=?=\r\n",
+                " =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+ls/CfpbM=?=\r\n",
+                " =?utf-8?b?8J+ls/CfpbPwn6Wz8J+ls/CfpbPwn6Wz8J+lsw==?=\r\n"
+            )
         );
     }
 
     #[test]
     fn format_bad_stuff() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! \r\n This is \" bad \0. 👋".to_string(),
+            "Hello! \r\n This is \" bad \0. 👋".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -769,33 +667,40 @@ mod tests {
     fn format_everything() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
+            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_owned()
+            )
         );
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_string(),
+            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_owned(),
+            )
         );
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Someone <somewhere@example.com>".to_string(),
+            "Someone <somewhere@example.com>".to_owned(),
-        );
+        ));
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "quoted-printable".to_string(),
+            "quoted-printable".to_owned(),
-        );
+        ));
 
+        // TODO: fix the fact that the encoder doesn't know that
+        // the space between the name and the address should be
+        // removed when wrapping.
         assert_eq!(
             headers.to_string(),
             concat!(
-                "Subject: Hello! This is lettre, and this \r\n",
+                "Subject: Hello! This is lettre, and this\r\n",
-                " IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I \r\n",
+                " IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I\r\n",
                 " guess that's it!\r\n",
-                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?= \r\n",
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?=\r\n",
-                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyIA==?=\r\n",
+                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9?=\r\n",
-                " =?utf-8?b?0JjQstCw0L0g0JjQstCw0L3QvtCy0LjRhw==?= <ivanov@example.com>, \r\n",
+                " =?utf-8?b?INCY0LLQsNC90L7QstC40Yc=?= <ivanov@example.com>,\r\n",
-                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, \r\n",
+                "  =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>,\r\n",
-                " =?utf-8?b?U2XDoW4gw5MgUnVkYcOt?= <sean@example.com>\r\n",
+                "  =?utf-8?b?U2XDoW4gw5MgUnVkYcOt?= <sean@example.com>\r\n",
                 "From: Someone <somewhere@example.com>\r\n",
                 "Content-Transfer-Encoding: quoted-printable\r\n",
             )
@@ -805,16 +710,16 @@ mod tests {
     #[test]
     fn issue_653() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "＋仮名 :a;go; ;;;;;s;;;;;;;;;;;;;;;;fffeinmjgggggggggｆっ".to_string(),
+            "＋仮名 :a;go; ;;;;;s;;;;;;;;;;;;;;;;fffeinmjgggggggggｆっ".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go; \r\n",
+                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go; =?utf-8?b?Ozs7OztzOzs7Ozs7Ozs7?=\r\n",
-                " =?utf-8?b?Ozs7OztzOzs7Ozs7Ozs7Ozs7Ozs7O2ZmZmVpbm1qZ2dnZ2dnZ2dn772G44Gj?=\r\n"
+                " =?utf-8?b?Ozs7Ozs7O2ZmZmVpbm1qZ2dnZ2dnZ2dn772G44Gj?=\r\n",
             )
         );
     }

src/message/header/special.rs

@@ -1,10 +1,10 @@
 use crate::{
-    message::header::{Header, HeaderName},
+    message::header::{Header, HeaderName, HeaderValue},
     BoxError,
 };
 
 /// Message format version, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-4)
-#[derive(Debug, Copy, Clone, PartialEq)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct MimeVersion {
     major: u8,
     minor: u8,
@@ -16,15 +16,18 @@ pub struct MimeVersion {
 pub const MIME_VERSION_1_0: MimeVersion = MimeVersion::new(1, 0);
 
 impl MimeVersion {
+    /// Build a new `MimeVersion` header
     pub const fn new(major: u8, minor: u8) -> Self {
         MimeVersion { major, minor }
     }
 
+    /// Get the `major` value of this `MimeVersion` header.
     #[inline]
     pub const fn major(self) -> u8 {
         self.major
     }
 
+    /// Get the `minor` value of this `MimeVersion` header.
     #[inline]
     pub const fn minor(self) -> u8 {
         self.minor
@@ -50,8 +53,9 @@ impl Header for MimeVersion {
         Ok(MimeVersion::new(major, minor))
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
-        format!("{}.{}", self.major, self.minor)
+        let val = format!("{}.{}", self.major, self.minor);
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -63,8 +67,10 @@ impl Default for MimeVersion {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::{MimeVersion, MIME_VERSION_1_0};
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_mime_version() {
@@ -83,17 +89,17 @@ mod test {
     fn parse_mime_version() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
-            "1.0".to_string(),
+            "1.0".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MIME_VERSION_1_0));
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
-            "0.1".to_string(),
+            "0.1".to_owned(),
-        );
+        ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MimeVersion::new(0, 1)));
     }

src/message/header/textual.rs

@@ -1,10 +1,10 @@
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 macro_rules! text_header {
     ($(#[$attr:meta])* Header($type_name: ident, $header_name: expr )) => {
         $(#[$attr])*
-        #[derive(Debug, Clone, PartialEq)]
+        #[derive(Debug, Clone, PartialEq, Eq)]
         pub struct $type_name(String);
 
         impl Header for $type_name {
@@ -16,8 +16,8 @@ macro_rules! text_header {
                 Ok(Self(s.into()))
             }
 
-            fn display(&self) -> String {
+            fn display(&self) -> HeaderValue {
-                self.0.clone()
+             HeaderValue::new(Self::name(),   self.0.clone())
             }
         }
 
@@ -85,8 +85,10 @@ text_header! {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::Subject;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_ascii() {
@@ -107,13 +109,24 @@ mod test {
         );
     }
 
+    #[test]
+    fn format_utf8_word() {
+        let mut headers = Headers::new();
+        headers.set(Subject("Administratör".into()));
+
+        assert_eq!(
+            headers.to_string(),
+            "Subject: =?utf-8?b?QWRtaW5pc3RyYXTDtnI=?=\r\n"
+        );
+    }
+
     #[test]
     fn parse_ascii() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Sample subject".to_string(),
+            "Sample subject".to_owned(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Subject>(),

src/message/mailbox/serde.rs

@@ -1,17 +1,19 @@
-use crate::message::{Mailbox, Mailboxes};
+use std::fmt::{Formatter, Result as FmtResult};
+
 use serde::{
     de::{Deserializer, Error as DeError, MapAccess, SeqAccess, Visitor},
     ser::Serializer,
     Deserialize, Serialize,
 };
-use std::fmt::{Formatter, Result as FmtResult};
+
+use crate::message::{Mailbox, Mailboxes};
 
 impl Serialize for Mailbox {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 
@@ -109,7 +111,7 @@ impl Serialize for Mailboxes {
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 
@@ -152,9 +154,11 @@ impl<'de> Deserialize<'de> for Mailboxes {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+    use serde_json::from_str;
+
     use super::*;
     use crate::address::Address;
-    use serde_json::from_str;
 
     #[test]
     fn parse_address_string() {

src/message/mailbox/types.rs

@@ -1,11 +1,14 @@
-use crate::address::{Address, AddressError};
 use std::{
-    convert::TryFrom,
     fmt::{Display, Formatter, Result as FmtResult, Write},
+    mem,
     slice::Iter,
     str::FromStr,
 };
 
+use email_encoding::headers::EmailWriter;
+
+use crate::address::{Address, AddressError};
+
 /// Represents an email address with an optional name for the sender/recipient.
 ///
 /// This type contains email address and the sender/recipient name (_Some Name \<user@domain.tld\>_ or _withoutname@domain.tld_).
@@ -63,6 +66,22 @@ impl Mailbox {
     pub fn new(name: Option<String>, email: Address) -> Self {
         Mailbox { name, email }
     }
+
+    pub(crate) fn encode(&self, w: &mut EmailWriter<'_>) -> FmtResult {
+        if let Some(name) = &self.name {
+            email_encoding::headers::quoted_string::encode(name, w)?;
+            w.optional_breakpoint();
+            w.write_char('<')?;
+        }
+
+        w.write_str(self.email.as_ref())?;
+
+        if self.name.is_some() {
+            w.write_char('>')?;
+        }
+
+        Ok(())
+    }
 }
 
 impl Display for Mailbox {
@@ -70,7 +89,7 @@ impl Display for Mailbox {
         if let Some(ref name) = self.name {
             let name = name.trim();
             if !name.is_empty() {
-                f.write_str(name)?;
+                write_word(f, name)?;
                 f.write_str(" <")?;
                 self.email.fmt(f)?;
                 return f.write_char('>');
@@ -126,6 +145,12 @@ impl FromStr for Mailbox {
     }
 }
 
+impl From<Address> for Mailbox {
+    fn from(value: Address) -> Self {
+        Self::new(None, value)
+    }
+}
+
 /// Represents a sequence of [`Mailbox`] instances.
 ///
 /// This type contains a sequence of mailboxes (_Some Name \<user@domain.tld\>, Another Name \<other@domain.tld\>, withoutname@domain.tld, ..._).
@@ -250,6 +275,20 @@ impl Mailboxes {
     pub fn iter(&self) -> Iter<'_, Mailbox> {
         self.0.iter()
     }
+
+    pub(crate) fn encode(&self, w: &mut EmailWriter<'_>) -> FmtResult {
+        let mut first = true;
+        for mailbox in self.iter() {
+            if !mem::take(&mut first) {
+                w.write_char(',')?;
+                w.optional_breakpoint();
+            }
+
+            mailbox.encode(w)?;
+        }
+
+        Ok(())
+    }
 }
 
 impl Default for Mailboxes {
@@ -282,6 +321,18 @@ impl From<Mailboxes> for Vec<Mailbox> {
     }
 }
 
+impl FromIterator<Mailbox> for Mailboxes {
+    fn from_iter<T: IntoIterator<Item = Mailbox>>(iter: T) -> Self {
+        Self(Vec::from_iter(iter))
+    }
+}
+
+impl Extend<Mailbox> for Mailboxes {
+    fn extend<T: IntoIterator<Item = Mailbox>>(&mut self, iter: T) {
+        self.0.extend(iter);
+    }
+}
+
 impl IntoIterator for Mailboxes {
     type Item = Mailbox;
     type IntoIter = ::std::vec::IntoIter<Mailbox>;
@@ -291,14 +342,6 @@ impl IntoIterator for Mailboxes {
     }
 }
 
-impl Extend<Mailbox> for Mailboxes {
-    fn extend<T: IntoIterator<Item = Mailbox>>(&mut self, iter: T) {
-        for elem in iter {
-            self.0.push(elem);
-        }
-    }
-}
-
 impl Display for Mailboxes {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         let mut iter = self.iter();
@@ -319,19 +362,132 @@ impl Display for Mailboxes {
 impl FromStr for Mailboxes {
     type Err = AddressError;
 
-    fn from_str(src: &str) -> Result<Self, Self::Err> {
+    fn from_str(mut src: &str) -> Result<Self, Self::Err> {
-        src.split(',')
+        let mut mailboxes = Vec::new();
-            .map(|m| m.trim().parse())
+
-            .collect::<Result<Vec<_>, _>>()
+        if !src.is_empty() {
-            .map(Mailboxes)
+            // n-1 elements
+            let mut skip = 0;
+            while let Some(i) = src[skip..].find(',') {
+                let left = &src[..skip + i];
+
+                match left.trim().parse() {
+                    Ok(mailbox) => {
+                        mailboxes.push(mailbox);
+
+                        src = &src[left.len() + ",".len()..];
+                        skip = 0;
+                    }
+                    Err(AddressError::MissingParts) => {
+                        skip = left.len() + ",".len();
+                    }
+                    Err(err) => {
+                        return Err(err);
+                    }
+                }
+            }
+
+            // last element
+            let mailbox = src.trim().parse()?;
+            mailboxes.push(mailbox);
+        }
+
+        Ok(Mailboxes(mailboxes))
+    }
+}
+
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.6
+fn write_word(f: &mut Formatter<'_>, s: &str) -> FmtResult {
+    if s.as_bytes().iter().copied().all(is_valid_atom_char) {
+        f.write_str(s)
+    } else {
+        // Quoted string: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
+        f.write_char('"')?;
+        for c in s.chars() {
+            write_quoted_string_char(f, c)?;
+        }
+        f.write_char('"')?;
+
+        Ok(())
+    }
+}
+
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.4
+fn is_valid_atom_char(c: u8) -> bool {
+    matches!(c,
+		// Not really allowed but can be inserted between atoms.
+		b'\t' |
+		b' ' |
+
+		b'!' |
+		b'#' |
+		b'$' |
+		b'%' |
+		b'&' |
+		b'\'' |
+		b'*' |
+		b'+' |
+		b'-' |
+		b'/' |
+		b'0'..=b'8' |
+		b'=' |
+		b'?' |
+		b'A'..=b'Z' |
+		b'^' |
+		b'_' |
+		b'`' |
+		b'a'..=b'z' |
+		b'{' |
+		b'|' |
+		b'}' |
+		b'~' |
+
+		// Not techically allowed but will be escaped into allowed characters.
+		128..=255)
+}
+
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
+fn write_quoted_string_char(f: &mut Formatter<'_>, c: char) -> FmtResult {
+    match c {
+        // Can not be encoded.
+        '\n' | '\r' => Err(std::fmt::Error),
+
+        // Note, not qcontent but can be put before or after any qcontent.
+        '\t' | ' ' => f.write_char(c),
+
+        c if match c as u32 {
+            // NO-WS-CTL: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
+            1..=8 | 11 | 12 | 14..=31 | 127 |
+
+            // The rest of the US-ASCII except \ and "
+            33 |
+            35..=91 |
+            93..=126 |
+
+            // Non-ascii characters will be escaped separately later.
+            128.. => true,
+            _ => false,
+        } =>
+        {
+            f.write_char(c)
+        }
+
+        _ => {
+            // quoted-pair https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
+            f.write_char('\\')?;
+            f.write_char(c)
+        }
     }
 }
 
 #[cfg(test)]
 mod test {
-    use super::Mailbox;
     use std::convert::TryInto;
 
+    use pretty_assertions::assert_eq;
+
+    use super::Mailbox;
+
     #[test]
     fn mailbox_format_address_only() {
         assert_eq!(
@@ -350,7 +506,63 @@ mod test {
                 "{}",
                 Mailbox::new(Some("K.".into()), "kayo@example.com".parse().unwrap())
             ),
-            "K. <kayo@example.com>"
+            "\"K.\" <kayo@example.com>"
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_comma() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Last, First".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Last, First" <kayo@example.com>"#
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_comma_and_non_ascii() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Laşt, First".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Laşt, First" <kayo@example.com>"#
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_comma_and_quoted_non_ascii() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some(r#"Laşt, "First""#.into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Laşt, \"First\"" <kayo@example.com>"#
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_color() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Chris's Wiki :: blog".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Chris's Wiki :: blog" <kayo@example.com>"#
         );
     }
 
@@ -372,7 +584,7 @@ mod test {
                 "{}",
                 Mailbox::new(Some(" K. ".into()), "kayo@example.com".parse().unwrap())
             ),
-            "K. <kayo@example.com>"
+            "\"K.\" <kayo@example.com>"
         );
     }
 
@@ -414,7 +626,7 @@ mod test {
     #[test]
     fn parse_address_from_tuple() {
         assert_eq!(
-            ("K.".to_string(), "kayo@example.com".to_string()).try_into(),
+            ("K.".to_owned(), "kayo@example.com".to_owned()).try_into(),
             Ok(Mailbox::new(
                 Some("K.".into()),
                 "kayo@example.com".parse().unwrap()

src/message/mimebody.rs

@@ -1,11 +1,11 @@
-use std::io::Write;
+use std::{io::Write, iter::repeat_with};
+
+use mime::Mime;
 
 use crate::message::{
     header::{self, ContentTransferEncoding, ContentType, Header, Headers},
     EmailFormat, IntoBody,
 };
-use mime::Mime;
-use std::iter::repeat_with;
 
 /// MIME part variants
 #[derive(Debug, Clone)]
@@ -190,9 +190,9 @@ impl MultiPartKind {
             },
             boundary,
             match self {
-                Self::Encrypted { protocol } => format!("; protocol=\"{}\"", protocol),
+                Self::Encrypted { protocol } => format!("; protocol=\"{protocol}\""),
                 Self::Signed { protocol, micalg } =>
-                    format!("; protocol=\"{}\"; micalg=\"{}\"", protocol, micalg),
+                    format!("; protocol=\"{protocol}\"; micalg=\"{micalg}\""),
                 _ => String::new(),
             }
         )
@@ -398,6 +398,8 @@ impl EmailFormat for MultiPart {
 
 #[cfg(test)]
 mod test {
+    use pretty_assertions::assert_eq;
+
     use super::*;
     use crate::message::header;
 
@@ -477,7 +479,7 @@ mod test {
         assert_eq!(
             String::from_utf8(part.formatted()).unwrap(),
             concat!(
-                "Content-Type: multipart/mixed; \r\n",
+                "Content-Type: multipart/mixed;\r\n",
                 " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"\r\n",
                 "\r\n",
                 "--0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\r\n",
@@ -524,8 +526,8 @@ mod test {
         assert_eq!(
             String::from_utf8(part.formatted()).unwrap(),
             concat!(
-                "Content-Type: multipart/encrypted; \r\n",
+                "Content-Type: multipart/encrypted;\r\n",
-                " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"; \r\n",
+                " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\";\r\n",
                 " protocol=\"application/pgp-encrypted\"\r\n",
                 "\r\n",
                 "--0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\r\n",
@@ -580,8 +582,8 @@ mod test {
         assert_eq!(
             String::from_utf8(part.formatted()).unwrap(),
             concat!(
-                "Content-Type: multipart/signed; \r\n",
+                "Content-Type: multipart/signed;\r\n",
-                " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"; \r\n",
+                " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\";\r\n",
                 " protocol=\"application/pgp-signature\";",
                 " micalg=\"pgp-sha256\"\r\n",
                 "\r\n",
@@ -622,7 +624,7 @@ mod test {
                              .body(String::from("<p>Текст <em>письма</em> в <a href=\"https://ru.wikipedia.org/wiki/Юникод\">уникоде</a><p>")));
 
         assert_eq!(String::from_utf8(part.formatted()).unwrap(),
-                   concat!("Content-Type: multipart/alternative; \r\n",
+                   concat!("Content-Type: multipart/alternative;\r\n",
                            " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"\r\n",
                            "\r\n",
                            "--0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\r\n",
@@ -660,11 +662,11 @@ mod test {
                              .body(String::from("int main() { return 0; }")));
 
         assert_eq!(String::from_utf8(part.formatted()).unwrap(),
-                   concat!("Content-Type: multipart/mixed; \r\n",
+                   concat!("Content-Type: multipart/mixed;\r\n",
                            " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"\r\n",
                            "\r\n",
                            "--0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\r\n",
-                           "Content-Type: multipart/related; \r\n",
+                           "Content-Type: multipart/related;\r\n",
                            " boundary=\"0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\"\r\n",
                            "\r\n",
                            "--0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1\r\n",

src/message/mod.rs

@@ -14,7 +14,7 @@
 //! The easiest way of creating a message, which uses a plain text body.
 //!
 //! ```rust
-//! use lettre::message::Message;
+//! use lettre::message::{header::ContentType, Message};
 //!
 //! # use std::error::Error;
 //! # fn main() -> Result<(), Box<dyn Error>> {
@@ -23,6 +23,7 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //! # Ok(())
 //! # }
@@ -38,6 +39,7 @@
 //! To: Hei <hei@domain.tld>
 //! Subject: Happy new year
 //! Date: Sat, 12 Dec 2020 16:33:19 GMT
+//! Content-Type: text/plain; charset=utf-8
 //! Content-Transfer-Encoding: 7bit
 //!
 //! Be happy!
@@ -106,9 +108,10 @@
 //!
 //! ```rust
 //! # use std::error::Error;
-//! use lettre::message::{header, Attachment, Body, Message, MultiPart, SinglePart};
 //! use std::fs;
 //!
+//! use lettre::message::{header, Attachment, Body, Message, MultiPart, SinglePart};
+//!
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! let image = fs::read("docs/lettre.png")?;
 //! // this image_body can be cloned and reused between emails.
@@ -195,15 +198,19 @@
 //! ```
 //! </details>
 
-use std::{convert::TryFrom, io::Write, iter, time::SystemTime};
+use std::{io::Write, iter, time::SystemTime};
 
 pub use attachment::Attachment;
 pub use body::{Body, IntoBody, MaybeString};
+#[cfg(feature = "dkim")]
+pub use dkim::*;
 pub use mailbox::*;
 pub use mimebody::*;
 
 mod attachment;
 mod body;
+#[cfg(feature = "dkim")]
+pub mod dkim;
 pub mod header;
 mod mailbox;
 mod mimebody;
@@ -227,6 +234,7 @@ trait EmailFormat {
 pub struct MessageBuilder {
     headers: Headers,
     envelope: Option<Envelope>,
+    drop_bcc: bool,
 }
 
 impl MessageBuilder {
@@ -235,24 +243,26 @@ impl MessageBuilder {
         Self {
             headers: Headers::new(),
             envelope: None,
+            drop_bcc: true,
         }
     }
 
-    /// Set custom header to message
+    /// Set or add mailbox to `From` header
-    pub fn header<H: Header>(mut self, header: H) -> Self {
+    ///
-        self.headers.set(header);
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
-        self
+    ///
+    /// Shortcut for `self.mailbox(header::From(mbox))`.
+    pub fn from(self, mbox: Mailbox) -> Self {
+        self.mailbox(header::From::from(Mailboxes::from(mbox)))
     }
 
-    /// Add mailbox to header
+    /// Set `Sender` header. Should be used when providing several `From` mailboxes.
-    pub fn mailbox<H: Header + MailboxesHeader>(self, header: H) -> Self {
+    ///
-        match self.headers.get::<H>() {
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
-            Some(mut header_) => {
+    ///
-                header_.join_mailboxes(header);
+    /// Shortcut for `self.header(header::Sender(mbox))`.
-                self.header(header_)
+    pub fn sender(self, mbox: Mailbox) -> Self {
-            }
+        self.header(header::Sender::from(mbox))
-            None => self.header(header),
-        }
     }
 
     /// Add `Date` header to message
@@ -270,41 +280,6 @@ impl MessageBuilder {
         self.date(SystemTime::now())
     }
 
-    /// Set `Subject` header to message
-    ///
-    /// Shortcut for `self.header(header::Subject(subject.into()))`.
-    pub fn subject<S: Into<String>>(self, subject: S) -> Self {
-        let s: String = subject.into();
-        self.header(header::Subject::from(s))
-    }
-
-    /// Set `MIME-Version` header to 1.0
-    ///
-    /// Shortcut for `self.header(header::MIME_VERSION_1_0)`.
-    ///
-    /// Not exposed as it is set by body methods
-    fn mime_1_0(self) -> Self {
-        self.header(header::MIME_VERSION_1_0)
-    }
-
-    /// Set `Sender` header. Should be used when providing several `From` mailboxes.
-    ///
-    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
-    ///
-    /// Shortcut for `self.header(header::Sender(mbox))`.
-    pub fn sender(self, mbox: Mailbox) -> Self {
-        self.header(header::Sender::from(mbox))
-    }
-
-    /// Set or add mailbox to `From` header
-    ///
-    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
-    ///
-    /// Shortcut for `self.mailbox(header::From(mbox))`.
-    pub fn from(self, mbox: Mailbox) -> Self {
-        self.mailbox(header::From::from(Mailboxes::from(mbox)))
-    }
-
     /// Set or add mailbox to `ReplyTo` header
     ///
     /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
@@ -347,6 +322,14 @@ impl MessageBuilder {
         self.header(header::References::from(id))
     }
 
+    /// Set `Subject` header to message
+    ///
+    /// Shortcut for `self.header(header::Subject(subject.into()))`.
+    pub fn subject<S: Into<String>>(self, subject: S) -> Self {
+        let s: String = subject.into();
+        self.header(header::Subject::from(s))
+    }
+
     /// Set [Message-ID
     /// header](https://tools.ietf.org/html/rfc5322#section-3.6.4)
     ///
@@ -362,9 +345,9 @@ impl MessageBuilder {
                 let hostname = hostname::get()
                     .map_err(|_| ())
                     .and_then(|s| s.into_string().map_err(|_| ()))
-                    .unwrap_or_else(|_| DEFAULT_MESSAGE_ID_DOMAIN.to_string());
+                    .unwrap_or_else(|_| DEFAULT_MESSAGE_ID_DOMAIN.to_owned());
                 #[cfg(not(feature = "hostname"))]
-                let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_string();
+                let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_owned();
 
                 self.header(header::MessageId::from(
                     // https://tools.ietf.org/html/rfc5322#section-3.6.4
@@ -375,17 +358,48 @@ impl MessageBuilder {
     }
 
     /// Set [User-Agent
-    /// header](https://tools.ietf.org/html/draft-melnikov-email-user-agent-004)
+    /// header](https://tools.ietf.org/html/draft-melnikov-email-user-agent-00)
     pub fn user_agent(self, id: String) -> Self {
         self.header(header::UserAgent::from(id))
     }
 
+    /// Set custom header to message
+    pub fn header<H: Header>(mut self, header: H) -> Self {
+        self.headers.set(header);
+        self
+    }
+
+    /// Add mailbox to header
+    pub fn mailbox<H: Header + MailboxesHeader>(self, header: H) -> Self {
+        match self.headers.get::<H>() {
+            Some(mut header_) => {
+                header_.join_mailboxes(header);
+                self.header(header_)
+            }
+            None => self.header(header),
+        }
+    }
+
     /// Force specific envelope (by default it is derived from headers)
     pub fn envelope(mut self, envelope: Envelope) -> Self {
         self.envelope = Some(envelope);
         self
     }
 
+    /// Keep the `Bcc` header
+    ///
+    /// By default the `Bcc` header is removed from the email after
+    /// using it to generate the message envelope. In some cases though,
+    /// like when saving the email as an `.eml`, or sending through
+    /// some transports (like the Gmail API) that don't take a separate
+    /// envelope value, it becomes necessary to keep the `Bcc` header.
+    ///
+    /// Calling this method overrides the default behaviour.
+    pub fn keep_bcc(mut self) -> Self {
+        self.drop_bcc = false;
+        self
+    }
+
     // TODO: High-level methods for attachments and embedded files
 
     /// Create message from body
@@ -418,8 +432,10 @@ impl MessageBuilder {
             None => Envelope::try_from(&res.headers)?,
         };
 
-        // Remove `Bcc` headers now the envelope is set
+        if res.drop_bcc {
-        res.headers.remove::<header::Bcc>();
+            // Remove `Bcc` headers now the envelope is set
+            res.headers.remove::<header::Bcc>();
+        }
 
         Ok(Message {
             headers: res.headers,
@@ -450,6 +466,15 @@ impl MessageBuilder {
     pub fn singlepart(self, part: SinglePart) -> Result<Message, EmailError> {
         self.mime_1_0().build(MessageBody::Mime(Part::Single(part)))
     }
+
+    /// Set `MIME-Version` header to 1.0
+    ///
+    /// Shortcut for `self.header(header::MIME_VERSION_1_0)`.
+    ///
+    /// Not exposed as it is set by body methods
+    fn mime_1_0(self) -> Self {
+        self.header(header::MIME_VERSION_1_0)
+    }
 }
 
 /// Email message which can be formatted
@@ -489,6 +514,77 @@ impl Message {
         self.format(&mut out);
         out
     }
+
+    #[cfg(feature = "dkim")]
+    /// Format body for signing
+    pub(crate) fn body_raw(&self) -> Vec<u8> {
+        let mut out = Vec::new();
+        match &self.body {
+            MessageBody::Mime(p) => p.format(&mut out),
+            MessageBody::Raw(r) => out.extend_from_slice(r),
+        };
+        out.extend_from_slice(b"\r\n");
+        out
+    }
+
+    /// Sign the message using Dkim
+    ///
+    /// Example:
+    /// ```rust
+    /// use lettre::{
+    ///     message::dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
+    ///     Message,
+    /// };
+    ///
+    /// let mut message = Message::builder()
+    ///     .from("Alice <alice@example.org>".parse().unwrap())
+    ///     .reply_to("Bob <bob@example.org>".parse().unwrap())
+    ///     .to("Carla <carla@example.net>".parse().unwrap())
+    ///     .subject("Hello")
+    ///     .body("Hi there, it's a test email, with utf-8 chars ë!\n\n\n".to_owned())
+    ///     .unwrap();
+    /// let key = "-----BEGIN RSA PRIVATE KEY-----
+    /// MIIEowIBAAKCAQEAt2gawjoybf0mAz0mSX0cq1ah5F9cPazZdCwLnFBhRufxaZB8
+    /// NLTdc9xfPIOK8l/xGrN7Nd63J4cTATqZukumczkA46O8YKHwa53pNT6NYwCNtDUL
+    /// eBu+7xUW18GmDzkIFkxGO2R5kkTeWPlKvKpEiicIMfl0OmyW/fI3AbtM7e/gmqQ4
+    /// kEYIO0mTjPT+jTgWE4JIi5KUTHudUBtfMKcSFyM2HkUOExl1c9+A4epjRFQwEXMA
+    /// hM5GrqZoOdUm4fIpvGpLIGIxFgHPpZYbyq6yJZzH3+5aKyCHrsHawPuPiCD45zsU
+    /// re31zCE6b6k1sDiiBR4CaRHnbL7hxFp0aNLOVQIDAQABAoIBAGMK3gBrKxaIcUGo
+    /// gQeIf7XrJ6vK72YC9L8uleqI4a9Hy++E7f4MedZ6eBeWta8jrnEL4Yp6xg+beuDc
+    /// A24+Mhng+6Dyp+TLLqj+8pQlPnbrMprRVms7GIXFrrs+wO1RkBNyhy7FmH0roaMM
+    /// pJZzoGW2pE9QdbqjL3rdlWTi/60xRX9eZ42nNxYnbc+RK03SBd46c3UBha6Y9iQX
+    /// 562yWilDnB5WCX2tBoSN39bEhJvuZDzMwOuGw68Q96Hdz82Iz1xVBnRhH+uNStjR
+    /// VnAssSHVxPSpwWrm3sHlhjBHWPnNIaOKIKl1lbL+qWfVQCj/6a5DquC+vYAeYR6L
+    /// 3mA0z0ECgYEA5YkNYcILSXyE0hZ8eA/t58h8eWvYI5iqt3nT4fznCoYJJ74Vukeg
+    /// 6BTlq/CsanwT1lDtvDKrOaJbA7DPTES/bqT0HoeIdOvAw9w/AZI5DAqYp61i6RMK
+    /// xfAQL/Ik5MDFN8gEMLLXRVMe/aR27f6JFZpShJOK/KCzHqikKfYVJ+UCgYEAzI2F
+    /// ZlTyittWSyUSl5UKyfSnFOx2+6vNy+lu5DeMJu8Wh9rqBk388Bxq98CfkCseWESN
+    /// pTCGdYltz9DvVNBdBLwSMdLuYJAI6U+Zd70MWyuNdHFPyWVHUNqMUBvbUtj2w74q
+    /// Hzu0GI0OrRjdX6C63S17PggmT/N2R9X7P4STxbECgYA+AZAD4I98Ao8+0aQ+Ks9x
+    /// 1c8KXf+9XfiAKAD9A3zGcv72JXtpHwBwsXR5xkJNYcdaFfKi7G0k3J8JmDHnwIqW
+    /// MSlhNeu+6hDg2BaNLhsLDbG/Wi9mFybJ4df9m8Qrp4efUgEPxsAwkgvFKTCXijMu
+    /// CspP1iutoxvAJH50d22voQKBgDIsSFtIXNGYaTs3Va8enK3at5zXP3wNsQXiNRP/
+    /// V/44yNL77EktmewfXFF2yuym1uOZtRCerWxpEClYO0wXa6l8pA3aiiPfUIBByQfo
+    /// s/4s2Z6FKKfikrKPWLlRi+NvWl+65kQQ9eTLvJzSq4IIP61+uWsGvrb/pbSLFPyI
+    /// fWKRAoGBALFCStBXvdMptjq4APUzAdJ0vytZzXkOZHxgmc+R0fQn22OiW0huW6iX
+    /// JcaBbL6ZSBIMA3AdaIjtvNRiomueHqh0GspTgOeCE2585TSFnw6vEOJ8RlR4A0Mw
+    /// I45fbR4l+3D/30WMfZlM6bzZbwPXEnr2s1mirmuQpjumY9wLhK25
+    /// -----END RSA PRIVATE KEY-----";
+    /// let signing_key = DkimSigningKey::new(key, DkimSigningAlgorithm::Rsa).unwrap();
+    /// message.sign(&DkimConfig::default_config(
+    ///     "dkimtest".to_owned(),
+    ///     "example.org".to_owned(),
+    ///     signing_key,
+    /// ));
+    /// println!(
+    ///     "message: {}",
+    ///     std::str::from_utf8(&message.formatted()).unwrap()
+    /// );
+    /// ```
+    #[cfg(feature = "dkim")]
+    pub fn sign(&mut self, dkim_config: &DkimConfig) {
+        dkim_sign(self, dkim_config);
+    }
 }
 
 impl EmailFormat for Message {
@@ -522,6 +618,8 @@ fn make_message_id() -> String {
 mod test {
     use std::time::{Duration, SystemTime};
 
+    use pretty_assertions::assert_eq;
+
     use super::{header, mailbox::Mailbox, make_message_id, Message, MultiPart, SinglePart};
 
     #[test]
@@ -550,7 +648,7 @@ mod test {
     }
 
     #[test]
-    fn email_message() {
+    fn email_message_no_bcc() {
         // Tue, 15 Nov 1994 08:12:31 GMT
         let date = SystemTime::UNIX_EPOCH + Duration::from_secs(784887151);
 
@@ -574,9 +672,47 @@ mod test {
         assert_eq!(
             String::from_utf8(email.formatted()).unwrap(),
             concat!(
-                "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "From: =?utf-8?b?0JrQsNC4?= <kayo@example.com>\r\n",
-                "To: Pony O.P. <pony@domain.tld>\r\n",
+                "To: \"Pony O.P.\" <pony@domain.tld>\r\n",
+                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvSE=?=\r\n",
+                "Content-Transfer-Encoding: 7bit\r\n",
+                "\r\n",
+                "Happy new year!"
+            )
+        );
+    }
+
+    #[test]
+    fn email_message_keep_bcc() {
+        // Tue, 15 Nov 1994 08:12:31 GMT
+        let date = SystemTime::UNIX_EPOCH + Duration::from_secs(784887151);
+
+        let email = Message::builder()
+            .date(date)
+            .bcc("hidden@example.com".parse().unwrap())
+            .keep_bcc()
+            .header(header::From(
+                vec![Mailbox::new(
+                    Some("Каи".into()),
+                    "kayo@example.com".parse().unwrap(),
+                )]
+                .into(),
+            ))
+            .header(header::To(
+                vec!["Pony O.P. <pony@domain.tld>".parse().unwrap()].into(),
+            ))
+            .header(header::Subject::from(String::from("яңа ел белән!")))
+            .body(String::from("Happy new year!"))
+            .unwrap();
+
+        assert_eq!(
+            String::from_utf8(email.formatted()).unwrap(),
+            concat!(
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
+                "Bcc: hidden@example.com\r\n",
+                "From: =?utf-8?b?0JrQsNC4?= <kayo@example.com>\r\n",
+                "To: \"Pony O.P.\" <pony@domain.tld>\r\n",
                 "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvSE=?=\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",

src/transport/file/error.rs

@@ -1,8 +1,9 @@
 //! Error and result type for file transport
 
-use crate::BoxError;
 use std::{error::Error as StdError, fmt};
 
+use crate::BoxError;
+
 /// The Errors that may occur when sending an email over SMTP
 pub struct Error {
     inner: Box<Inner>,
@@ -70,7 +71,7 @@ impl fmt::Display for Error {
         };
 
         if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+            write!(f, ": {e}")?;
         }
 
         Ok(())

src/transport/file/mod.rs

@@ -9,9 +9,10 @@
 //! #
 //! # #[cfg(all(feature = "file-transport", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
 //!
+//! use lettre::{FileTransport, Message, Transport};
+//!
 //! // Write to the local temp directory
 //! let sender = FileTransport::new(temp_dir());
 //! let email = Message::builder()
@@ -41,9 +42,10 @@
 //! #
 //! # #[cfg(all(feature = "file-transport-envelope", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
 //!
+//! use lettre::{FileTransport, Message, Transport};
+//!
 //! // Write to the local temp directory
 //! let sender = FileTransport::with_envelope(temp_dir());
 //! let email = Message::builder()
@@ -70,7 +72,8 @@
 //! # #[cfg(all(feature = "tokio1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{AsyncTransport, Tokio1Executor, Message, AsyncFileTransport};
+//!
+//! use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -95,7 +98,8 @@
 //! # #[cfg(all(feature = "async-std1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{AsyncTransport, AsyncStd1Executor, Message, AsyncFileTransport};
+//!
+//! use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
@@ -132,20 +136,22 @@
 //! {"forward_path":["hei@domain.tld"],"reverse_path":"nobody@domain.tld"}
 //! ```
 
-pub use self::error::Error;
-use crate::{address::Envelope, Transport};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use crate::{AsyncTransport, Executor};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use async_trait::async_trait;
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
 use std::marker::PhantomData;
 use std::{
     path::{Path, PathBuf},
     str,
 };
+
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use async_trait::async_trait;
 use uuid::Uuid;
 
+pub use self::error::Error;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use crate::{AsyncTransport, Executor};
+
 mod error;
 
 type Id = String;
@@ -202,18 +208,18 @@ impl FileTransport {
     pub fn read(&self, email_id: &str) -> Result<(Envelope, Vec<u8>), Error> {
         use std::fs;
 
-        let eml_file = self.path.join(format!("{}.eml", email_id));
+        let eml_file = self.path.join(format!("{email_id}.eml"));
         let eml = fs::read(eml_file).map_err(error::io)?;
 
-        let json_file = self.path.join(format!("{}.json", email_id));
+        let json_file = self.path.join(format!("{email_id}.json"));
-        let json = fs::read(&json_file).map_err(error::io)?;
+        let json = fs::read(json_file).map_err(error::io)?;
         let envelope = serde_json::from_slice(&json).map_err(error::envelope)?;
 
         Ok((envelope, eml))
     }
 
     fn path(&self, email_id: &Uuid, extension: &str) -> PathBuf {
-        self.path.join(format!("{}.{}", email_id, extension))
+        self.path.join(format!("{email_id}.{extension}"))
     }
 }
 
@@ -249,10 +255,10 @@ where
     /// Reads the envelope and the raw message content.
     #[cfg(feature = "file-transport-envelope")]
     pub async fn read(&self, email_id: &str) -> Result<(Envelope, Vec<u8>), Error> {
-        let eml_file = self.inner.path.join(format!("{}.eml", email_id));
+        let eml_file = self.inner.path.join(format!("{email_id}.eml"));
         let eml = E::fs_read(&eml_file).await.map_err(error::io)?;
 
-        let json_file = self.inner.path.join(format!("{}.json", email_id));
+        let json_file = self.inner.path.join(format!("{email_id}.json"));
         let json = E::fs_read(&json_file).await.map_err(error::io)?;
         let envelope = serde_json::from_slice(&json).map_err(error::envelope)?;
 
@@ -270,6 +276,8 @@ impl Transport for FileTransport {
         let email_id = Uuid::new_v4();
 
         let file = self.path(&email_id, "eml");
+        #[cfg(feature = "tracing")]
+        tracing::debug!(?file, "writing email to");
         fs::write(file, email).map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]
@@ -300,6 +308,8 @@ where
         let email_id = Uuid::new_v4();
 
         let file = self.inner.path(&email_id, "eml");
+        #[cfg(feature = "tracing")]
+        tracing::debug!(?file, "writing email to");
         E::fs_write(&file, email).await.map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]

src/transport/mod.rs

@@ -56,8 +56,7 @@
 //! #
 //! # #[cfg(all(feature = "builder", feature = "smtp-transport"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::transport::smtp::authentication::Credentials;
+//! use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
-//! use lettre::{Message, SmtpTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -66,7 +65,7 @@
 //!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+//! let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 //!
 //! // Open a remote connection to the SMTP relay server
 //! let mailer = SmtpTransport::relay("smtp.gmail.com")?
@@ -128,6 +127,9 @@ pub trait Transport {
     #[cfg(feature = "builder")]
     #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
     fn send(&self, message: &Message) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::trace!("starting to send an email");
+
         let raw = message.formatted();
         self.send_raw(message.envelope(), &raw)
     }
@@ -150,6 +152,9 @@ pub trait AsyncTransport {
     #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
     // TODO take &Message
     async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::trace!("starting to send an email");
+
         let raw = message.formatted();
         let envelope = message.envelope();
         self.send_raw(envelope, &raw).await

src/transport/sendmail/error.rs

@@ -1,8 +1,9 @@
 //! Error and result type for sendmail transport
 
-use crate::BoxError;
 use std::{error::Error as StdError, fmt};
 
+use crate::BoxError;
+
 /// The Errors that may occur when sending an email over sendmail
 pub struct Error {
     inner: Box<Inner>,
@@ -67,7 +68,7 @@ impl fmt::Display for Error {
         };
 
         if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+            write!(f, ": {e}")?;
         }
 
         Ok(())

src/transport/sendmail/mod.rs

@@ -33,7 +33,9 @@
 //! #
 //! # #[cfg(all(feature = "tokio1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, AsyncTransport, Tokio1Executor, AsyncSendmailTransport, SendmailTransport};
+//! use lettre::{
+//!     AsyncSendmailTransport, AsyncTransport, Message, SendmailTransport, Tokio1Executor,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -72,6 +74,17 @@
 //! # }
 //! ```
 
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use std::marker::PhantomData;
+use std::{
+    ffi::OsString,
+    io::Write,
+    process::{Command, Stdio},
+};
+
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use async_trait::async_trait;
+
 pub use self::error::Error;
 #[cfg(feature = "async-std1")]
 use crate::AsyncStd1Executor;
@@ -80,15 +93,6 @@ use crate::Tokio1Executor;
 use crate::{address::Envelope, Transport};
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
 use crate::{AsyncTransport, Executor};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use async_trait::async_trait;
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use std::marker::PhantomData;
-use std::{
-    ffi::OsString,
-    io::Write,
-    process::{Command, Stdio},
-};
 
 mod error;
 
@@ -228,6 +232,9 @@ impl Transport for SendmailTransport {
     type Error = Error;
 
     fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.command, "sending email with");
+
         // Spawn the sendmail command
         let mut process = self.command(envelope).spawn().map_err(error::client)?;
 
@@ -257,6 +264,9 @@ impl AsyncTransport for AsyncSendmailTransport<AsyncStd1Executor> {
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         use async_std::io::prelude::WriteExt;
 
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.inner.command, "sending email with");
+
         let mut command = self.async_std_command(envelope);
 
         // Spawn the sendmail command
@@ -289,6 +299,9 @@ impl AsyncTransport for AsyncSendmailTransport<Tokio1Executor> {
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         use tokio1_crate::io::AsyncWriteExt;
 
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.inner.command, "sending email with");
+
         let mut command = self.tokio1_command(envelope);
 
         // Spawn the sendmail command

src/transport/smtp/async_transport.rs

@@ -1,7 +1,8 @@
+#[cfg(feature = "pool")]
+use std::sync::Arc;
 use std::{
     fmt::{self, Debug},
     marker::PhantomData,
-    sync::Arc,
     time::Duration,
 };
 
@@ -198,6 +199,9 @@ where
 {
     fn clone(&self) -> Self {
         Self {
+            #[cfg(feature = "pool")]
+            inner: Arc::clone(&self.inner),
+            #[cfg(not(feature = "pool"))]
             inner: self.inner.clone(),
         }
     }

src/transport/smtp/authentication.rs

@@ -1,8 +1,9 @@
 //! Provides limited SASL authentication mechanisms
 
-use crate::transport::smtp::error::{self, Error};
 use std::fmt::{self, Debug, Display, Formatter};
 
+use crate::transport::smtp::error::{self, Error};
+
 /// Accepted authentication mechanisms
 ///
 /// Trying LOGIN last as it is deprecated.
@@ -97,12 +98,12 @@ impl Mechanism {
                 let decoded_challenge = challenge
                     .ok_or_else(|| error::client("This mechanism does expect a challenge"))?;
 
-                if vec!["User Name", "Username:", "Username"].contains(&decoded_challenge) {
+                if ["User Name", "Username:", "Username"].contains(&decoded_challenge) {
-                    return Ok(credentials.authentication_identity.to_string());
+                    return Ok(credentials.authentication_identity.clone());
                 }
 
-                if vec!["Password", "Password:"].contains(&decoded_challenge) {
+                if ["Password", "Password:"].contains(&decoded_challenge) {
-                    return Ok(credentials.secret.to_string());
+                    return Ok(credentials.secret.clone());
                 }
 
                 Err(error::client("Unrecognized challenge"))
@@ -126,7 +127,7 @@ mod test {
     fn test_plain() {
         let mechanism = Mechanism::Plain;
 
-        let credentials = Credentials::new("username".to_string(), "password".to_string());
+        let credentials = Credentials::new("username".to_owned(), "password".to_owned());
 
         assert_eq!(
             mechanism.response(&credentials, None).unwrap(),
@@ -139,7 +140,7 @@ mod test {
     fn test_login() {
         let mechanism = Mechanism::Login;
 
-        let credentials = Credentials::new("alice".to_string(), "wonderland".to_string());
+        let credentials = Credentials::new("alice".to_owned(), "wonderland".to_owned());
 
         assert_eq!(
             mechanism.response(&credentials, Some("Username")).unwrap(),
@@ -157,8 +158,8 @@ mod test {
         let mechanism = Mechanism::Xoauth2;
 
         let credentials = Credentials::new(
-            "username".to_string(),
+            "username".to_owned(),
-            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_string(),
+            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_owned(),
         );
 
         assert_eq!(
@@ -171,7 +172,7 @@ mod test {
     #[test]
     fn test_from_user_pass_for_credentials() {
         assert_eq!(
-            Credentials::new("alice".to_string(), "wonderland".to_string()),
+            Credentials::new("alice".to_owned(), "wonderland".to_owned()),
             Credentials::from(("alice", "wonderland"))
         );
     }

src/transport/smtp/client/async_connection.rs

@@ -1,8 +1,16 @@
+use std::{fmt::Display, net::IpAddr, time::Duration};
+
+use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+
+#[cfg(feature = "tokio1")]
+use super::async_net::AsyncTokioStream;
+#[cfg(feature = "tracing")]
+use super::escape_crlf;
 use super::{AsyncNetworkStream, ClientCodec, TlsParameters};
 use crate::{
     transport::smtp::{
         authentication::{Credentials, Mechanism},
-        commands::*,
+        commands::{Auth, Data, Ehlo, Mail, Noop, Quit, Rcpt, Starttls},
         error,
         error::Error,
         extension::{ClientId, Extension, MailBodyParameter, MailParameter, ServerInfo},
@@ -10,11 +18,6 @@ use crate::{
     },
     Envelope,
 };
-use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
-use std::{fmt::Display, time::Duration};
-
-#[cfg(feature = "tracing")]
-use super::escape_crlf;
 
 macro_rules! try_smtp (
     ($err: expr, $client: ident) => ({
@@ -40,10 +43,23 @@ pub struct AsyncSmtpConnection {
 }
 
 impl AsyncSmtpConnection {
+    /// Get information about the server
     pub fn server_info(&self) -> &ServerInfo {
         &self.server_info
     }
 
+    /// Connects with existing async stream
+    ///
+    /// Sends EHLO and parses server information
+    #[cfg(feature = "tokio1")]
+    pub async fn connect_with_transport(
+        stream: Box<dyn AsyncTokioStream>,
+        hello_name: &ClientId,
+    ) -> Result<AsyncSmtpConnection, Error> {
+        let stream = AsyncNetworkStream::use_existing_tokio1(stream);
+        Self::connect_impl(stream, hello_name).await
+    }
+
     /// Connects to the configured server
     ///
     /// Sends EHLO and parses server information
@@ -53,8 +69,11 @@ impl AsyncSmtpConnection {
         timeout: Option<Duration>,
         hello_name: &ClientId,
         tls_parameters: Option<TlsParameters>,
+        local_address: Option<IpAddr>,
     ) -> Result<AsyncSmtpConnection, Error> {
-        let stream = AsyncNetworkStream::connect_tokio1(server, timeout, tls_parameters).await?;
+        let stream =
+            AsyncNetworkStream::connect_tokio1(server, timeout, tls_parameters, local_address)
+                .await?;
         Self::connect_impl(stream, hello_name).await
     }
 
@@ -189,6 +208,7 @@ impl AsyncSmtpConnection {
             self.panic = true;
             let _ = self.command(Quit).await;
         }
+        let _ = self.stream.close().await;
     }
 
     /// Sets the underlying stream
@@ -295,7 +315,10 @@ impl AsyncSmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code()))
+                        Err(error::code(
+                            response.code(),
+                            Some(response.message().collect()),
+                        ))
                     }
                 }
                 Err(nom::Err::Failure(e)) => {
@@ -310,4 +333,10 @@ impl AsyncSmtpConnection {
 
         Err(error::response("incomplete response"))
     }
+
+    /// The X509 certificate of the server (DER encoded)
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        self.stream.get_ref().peer_certificate()
+    }
 }

src/transport/smtp/client/async_net.rs

@@ -1,63 +1,85 @@
 use std::{
-    io, mem,
+    fmt, io, mem,
-    net::SocketAddr,
+    net::{IpAddr, SocketAddr},
     pin::Pin,
     task::{Context, Poll},
     time::Duration,
 };
 
+#[cfg(feature = "async-std1-native-tls")]
+use async_native_tls::TlsStream as AsyncStd1TlsStream;
+#[cfg(feature = "async-std1")]
+use async_std::net::{TcpStream as AsyncStd1TcpStream, ToSocketAddrs as AsyncStd1ToSocketAddrs};
 use futures_io::{
     AsyncRead as FuturesAsyncRead, AsyncWrite as FuturesAsyncWrite, Error as IoError, ErrorKind,
     Result as IoResult,
 };
-#[cfg(feature = "tokio1")]
-use tokio1_crate::io::{AsyncRead as _, AsyncWrite as _, ReadBuf as Tokio1ReadBuf};
-
-#[cfg(feature = "async-std1")]
-use async_std::net::{TcpStream as AsyncStd1TcpStream, ToSocketAddrs as AsyncStd1ToSocketAddrs};
-#[cfg(feature = "tokio1")]
-use tokio1_crate::net::{TcpStream as Tokio1TcpStream, ToSocketAddrs as Tokio1ToSocketAddrs};
-
-#[cfg(feature = "async-std1-native-tls")]
-use async_native_tls::TlsStream as AsyncStd1TlsStream;
-#[cfg(feature = "tokio1-native-tls")]
-use tokio1_native_tls_crate::TlsStream as Tokio1TlsStream;
-
 #[cfg(feature = "async-std1-rustls-tls")]
 use futures_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
+#[cfg(feature = "tokio1-boring-tls")]
+use tokio1_boring::SslStream as Tokio1SslStream;
+#[cfg(feature = "tokio1")]
+use tokio1_crate::io::{AsyncRead, AsyncWrite, ReadBuf as Tokio1ReadBuf};
+#[cfg(feature = "tokio1")]
+use tokio1_crate::net::{
+    TcpSocket as Tokio1TcpSocket, TcpStream as Tokio1TcpStream,
+    ToSocketAddrs as Tokio1ToSocketAddrs,
+};
+#[cfg(feature = "tokio1-native-tls")]
+use tokio1_native_tls_crate::TlsStream as Tokio1TlsStream;
 #[cfg(feature = "tokio1-rustls-tls")]
 use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;
 
 #[cfg(any(
     feature = "tokio1-native-tls",
     feature = "tokio1-rustls-tls",
+    feature = "tokio1-boring-tls",
     feature = "async-std1-native-tls",
     feature = "async-std1-rustls-tls"
 ))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
+#[cfg(feature = "tokio1")]
+use crate::transport::smtp::client::net::resolved_address_filter;
 use crate::transport::smtp::{error, Error};
 
 /// A network stream
+#[derive(Debug)]
 pub struct AsyncNetworkStream {
     inner: InnerAsyncNetworkStream,
 }
 
+#[cfg(feature = "tokio1")]
+pub trait AsyncTokioStream: AsyncRead + AsyncWrite + Send + Sync + Unpin + fmt::Debug {
+    fn peer_addr(&self) -> io::Result<SocketAddr>;
+}
+
+#[cfg(feature = "tokio1")]
+impl AsyncTokioStream for Tokio1TcpStream {
+    fn peer_addr(&self) -> io::Result<SocketAddr> {
+        self.peer_addr()
+    }
+}
+
 /// Represents the different types of underlying network streams
 // usually only one TLS backend at a time is going to be enabled,
 // so clippy::large_enum_variant doesn't make sense here
 #[allow(clippy::large_enum_variant)]
 #[allow(dead_code)]
+#[derive(Debug)]
 enum InnerAsyncNetworkStream {
     /// Plain Tokio 1.x TCP stream
     #[cfg(feature = "tokio1")]
-    Tokio1Tcp(Tokio1TcpStream),
+    Tokio1Tcp(Box<dyn AsyncTokioStream>),
     /// Encrypted Tokio 1.x TCP stream
     #[cfg(feature = "tokio1-native-tls")]
-    Tokio1NativeTls(Tokio1TlsStream<Tokio1TcpStream>),
+    Tokio1NativeTls(Tokio1TlsStream<Box<dyn AsyncTokioStream>>),
     /// Encrypted Tokio 1.x TCP stream
     #[cfg(feature = "tokio1-rustls-tls")]
-    Tokio1RustlsTls(Tokio1RustlsTlsStream<Tokio1TcpStream>),
+    Tokio1RustlsTls(Tokio1RustlsTlsStream<Box<dyn AsyncTokioStream>>),
+    /// Encrypted Tokio 1.x TCP stream
+    #[cfg(feature = "tokio1-boring-tls")]
+    Tokio1BoringTls(Tokio1SslStream<Box<dyn AsyncTokioStream>>),
     /// Plain Tokio 1.x TCP stream
     #[cfg(feature = "async-std1")]
     AsyncStd1Tcp(AsyncStd1TcpStream),
@@ -91,6 +113,8 @@ impl AsyncNetworkStream {
             }
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(ref s) => s.get_ref().0.peer_addr(),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(ref s) => s.get_ref().peer_addr(),
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(ref s) => s.peer_addr(),
             #[cfg(feature = "async-std1-native-tls")]
@@ -107,50 +131,71 @@ impl AsyncNetworkStream {
         }
     }
 
+    #[cfg(feature = "tokio1")]
+    pub fn use_existing_tokio1(stream: Box<dyn AsyncTokioStream>) -> AsyncNetworkStream {
+        AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(stream))
+    }
+
     #[cfg(feature = "tokio1")]
     pub async fn connect_tokio1<T: Tokio1ToSocketAddrs>(
         server: T,
         timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
+        local_addr: Option<IpAddr>,
     ) -> Result<AsyncNetworkStream, Error> {
-        async fn try_connect_timeout<T: Tokio1ToSocketAddrs>(
+        async fn try_connect<T: Tokio1ToSocketAddrs>(
             server: T,
-            timeout: Duration,
+            timeout: Option<Duration>,
+            local_addr: Option<IpAddr>,
         ) -> Result<Tokio1TcpStream, Error> {
             let addrs = tokio1_crate::net::lookup_host(server)
                 .await
-                .map_err(error::connection)?;
+                .map_err(error::connection)?
+                .filter(|resolved_addr| resolved_address_filter(resolved_addr, local_addr));
 
             let mut last_err = None;
 
             for addr in addrs {
-                let connect_future = Tokio1TcpStream::connect(&addr);
+                let socket = match addr.ip() {
-                match tokio1_crate::time::timeout(timeout, connect_future).await {
+                    IpAddr::V4(_) => Tokio1TcpSocket::new_v4(),
-                    Ok(Ok(stream)) => return Ok(stream),
+                    IpAddr::V6(_) => Tokio1TcpSocket::new_v6(),
-                    Ok(Err(err)) => last_err = Some(err),
+                }
-                    Err(_) => {
+                .map_err(error::connection)?;
-                        last_err = Some(io::Error::new(
+                if let Some(local_addr) = local_addr {
-                            io::ErrorKind::TimedOut,
+                    socket
-                            "connection timed out",
+                        .bind(SocketAddr::new(local_addr, 0))
-                        ))
+                        .map_err(error::connection)?;
+                }
+
+                let connect_future = socket.connect(addr);
+                if let Some(timeout) = timeout {
+                    match tokio1_crate::time::timeout(timeout, connect_future).await {
+                        Ok(Ok(stream)) => return Ok(stream),
+                        Ok(Err(err)) => last_err = Some(err),
+                        Err(_) => {
+                            last_err = Some(io::Error::new(
+                                io::ErrorKind::TimedOut,
+                                "connection timed out",
+                            ))
+                        }
+                    }
+                } else {
+                    match connect_future.await {
+                        Ok(stream) => return Ok(stream),
+                        Err(err) => last_err = Some(err),
                     }
                 }
             }
 
             Err(match last_err {
                 Some(last_err) => error::connection(last_err),
-                None => error::connection("could not resolve to any address"),
+                None => error::connection("could not resolve to any supported address"),
             })
         }
 
-        let tcp_stream = match timeout {
+        let tcp_stream = try_connect(server, timeout, local_addr).await?;
-            Some(t) => try_connect_timeout(server, t).await?,
+        let mut stream =
-            None => Tokio1TcpStream::connect(server)
+            AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(Box::new(tcp_stream)));
-                .await
-                .map_err(error::connection)?,
-        };
-
-        let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
             stream.upgrade_tls(tls_parameters).await?;
         }
@@ -163,6 +208,9 @@ impl AsyncNetworkStream {
         timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
+        // Unfortunately there doesn't currently seem to be a way to set the local address
+        // Whilst we can create a AsyncStd1TcpStream from an existing socket, it needs to first have
+        // connected which is a blocking operation.
         async fn try_connect_timeout<T: AsyncStd1ToSocketAddrs>(
             server: T,
             timeout: Duration,
@@ -209,14 +257,22 @@ impl AsyncNetworkStream {
         match &self.inner {
             #[cfg(all(
                 feature = "tokio1",
-                not(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))
+                not(any(
+                    feature = "tokio1-native-tls",
+                    feature = "tokio1-rustls-tls",
+                    feature = "tokio1-boring-tls"
+                ))
             ))]
             InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                 let _ = tls_parameters;
                 panic!("Trying to upgrade an AsyncNetworkStream without having enabled either the tokio1-native-tls or the tokio1-rustls-tls feature");
             }
 
-            #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
+            #[cfg(any(
+                feature = "tokio1-native-tls",
+                feature = "tokio1-rustls-tls",
+                feature = "tokio1-boring-tls"
+            ))]
             InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
@@ -258,12 +314,16 @@ impl AsyncNetworkStream {
     }
 
     #[allow(unused_variables)]
-    #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
+    #[cfg(any(
+        feature = "tokio1-native-tls",
+        feature = "tokio1-rustls-tls",
+        feature = "tokio1-boring-tls"
+    ))]
     async fn upgrade_tokio1_tls(
-        tcp_stream: Tokio1TcpStream,
+        tcp_stream: Box<dyn AsyncTokioStream>,
         tls_parameters: TlsParameters,
     ) -> Result<InnerAsyncNetworkStream, Error> {
-        let domain = tls_parameters.domain().to_string();
+        let domain = tls_parameters.domain().to_owned();
 
         match tls_parameters.connector {
             #[cfg(feature = "native-tls")]
@@ -290,8 +350,6 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "tokio1-rustls-tls")]
                 return {
-                    use std::convert::TryFrom;
-
                     use rustls::ServerName;
                     use tokio1_rustls::TlsConnector;
 
@@ -306,11 +364,31 @@ impl AsyncNetworkStream {
                     Ok(InnerAsyncNetworkStream::Tokio1RustlsTls(stream))
                 };
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                #[cfg(not(feature = "tokio1-boring-tls"))]
+                panic!("built without the tokio1-boring-tls feature");
+
+                #[cfg(feature = "tokio1-boring-tls")]
+                return {
+                    let mut config = connector.configure().map_err(error::connection)?;
+                    config.set_verify_hostname(tls_parameters.accept_invalid_hostnames);
+
+                    let stream = tokio1_boring::connect(config, &domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
+                    Ok(InnerAsyncNetworkStream::Tokio1BoringTls(stream))
+                };
+            }
         }
     }
 
     #[allow(unused_variables)]
-    #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
+    #[cfg(any(
+        feature = "async-std1-native-tls",
+        feature = "async-std1-rustls-tls",
+        feature = "async-std1-boring-tls"
+    ))]
     async fn upgrade_asyncstd1_tls(
         tcp_stream: AsyncStd1TcpStream,
         mut tls_parameters: TlsParameters,
@@ -345,8 +423,6 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "async-std1-rustls-tls")]
                 return {
-                    use std::convert::TryFrom;
-
                     use futures_rustls::TlsConnector;
                     use rustls::ServerName;
 
@@ -361,6 +437,10 @@ impl AsyncNetworkStream {
                     Ok(InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream))
                 };
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                panic!("boring-tls isn't supported with async-std yet.");
+            }
         }
     }
 
@@ -372,6 +452,8 @@ impl AsyncNetworkStream {
             InnerAsyncNetworkStream::Tokio1NativeTls(_) => true,
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(_) => true,
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(_) => true,
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(_) => false,
             #[cfg(feature = "async-std1-native-tls")]
@@ -381,6 +463,57 @@ impl AsyncNetworkStream {
             InnerAsyncNetworkStream::None => false,
         }
     }
+
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        match &self.inner {
+            #[cfg(feature = "tokio1")]
+            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "tokio1-native-tls")]
+            InnerAsyncNetworkStream::Tokio1NativeTls(stream) => Ok(stream
+                .get_ref()
+                .peer_certificate()
+                .map_err(error::tls)?
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            #[cfg(feature = "tokio1-rustls-tls")]
+            InnerAsyncNetworkStream::Tokio1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_certificate()
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            #[cfg(feature = "async-std1")]
+            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "async-std1-native-tls")]
+            InnerAsyncNetworkStream::AsyncStd1NativeTls(t) => panic!("Unsupported"),
+            #[cfg(feature = "async-std1-rustls-tls")]
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            InnerAsyncNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
 }
 
 impl FuturesAsyncRead for AsyncNetworkStream {
@@ -417,6 +550,15 @@ impl FuturesAsyncRead for AsyncNetworkStream {
                     Poll::Pending => Poll::Pending,
                 }
             }
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(ref mut s) => {
+                let mut b = Tokio1ReadBuf::new(buf);
+                match Pin::new(s).poll_read(cx, &mut b) {
+                    Poll::Ready(Ok(())) => Poll::Ready(Ok(b.filled().len())),
+                    Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
+                    Poll::Pending => Poll::Pending,
+                }
+            }
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(ref mut s) => Pin::new(s).poll_read(cx, buf),
             #[cfg(feature = "async-std1-native-tls")]
@@ -448,6 +590,8 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
             InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_write(cx, buf),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(ref mut s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(ref mut s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "async-std1-native-tls")]
@@ -473,6 +617,8 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
             InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_flush(cx),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(ref mut s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(ref mut s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "async-std1-native-tls")]
@@ -494,6 +640,8 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
             InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_shutdown(cx),
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_shutdown(cx),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(ref mut s) => Pin::new(s).poll_shutdown(cx),
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(ref mut s) => Pin::new(s).poll_close(cx),
             #[cfg(feature = "async-std1-native-tls")]

src/transport/smtp/client/connection.rs

@@ -1,16 +1,18 @@
 use std::{
     fmt::Display,
     io::{self, BufRead, BufReader, Write},
-    net::ToSocketAddrs,
+    net::{IpAddr, ToSocketAddrs},
     time::Duration,
 };
 
+#[cfg(feature = "tracing")]
+use super::escape_crlf;
 use super::{ClientCodec, NetworkStream, TlsParameters};
 use crate::{
     address::Envelope,
     transport::smtp::{
         authentication::{Credentials, Mechanism},
-        commands::*,
+        commands::{Auth, Data, Ehlo, Mail, Noop, Quit, Rcpt, Starttls},
         error,
         error::Error,
         extension::{ClientId, Extension, MailBodyParameter, MailParameter, ServerInfo},
@@ -18,9 +20,6 @@ use crate::{
     },
 };
 
-#[cfg(feature = "tracing")]
-use super::escape_crlf;
-
 macro_rules! try_smtp (
     ($err: expr, $client: ident) => ({
         match $err {
@@ -45,6 +44,7 @@ pub struct SmtpConnection {
 }
 
 impl SmtpConnection {
+    /// Get information about the server
     pub fn server_info(&self) -> &ServerInfo {
         &self.server_info
     }
@@ -59,8 +59,9 @@ impl SmtpConnection {
         timeout: Option<Duration>,
         hello_name: &ClientId,
         tls_parameters: Option<&TlsParameters>,
+        local_address: Option<IpAddr>,
     ) -> Result<SmtpConnection, Error> {
-        let stream = NetworkStream::connect(server, timeout, tls_parameters)?;
+        let stream = NetworkStream::connect(server, timeout, tls_parameters, local_address)?;
         let stream = BufReader::new(stream);
         let mut conn = SmtpConnection {
             stream,
@@ -142,7 +143,7 @@ impl SmtpConnection {
         hello_name: &ClientId,
     ) -> Result<(), Error> {
         if self.server_info.supports_feature(Extension::StartTls) {
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             {
                 try_smtp!(self.command(Starttls), self);
                 self.stream.get_mut().upgrade_tls(tls_parameters)?;
@@ -152,7 +153,11 @@ impl SmtpConnection {
                 try_smtp!(self.ehlo(hello_name), self);
                 Ok(())
             }
-            #[cfg(not(any(feature = "native-tls", feature = "rustls-tls")))]
+            #[cfg(not(any(
+                feature = "native-tls",
+                feature = "rustls-tls",
+                feature = "boring-tls"
+            )))]
             // This should never happen as `Tls` can only be created
             // when a TLS library is enabled
             unreachable!("TLS support required but not supported");
@@ -178,6 +183,7 @@ impl SmtpConnection {
             self.panic = true;
             let _ = self.command(Quit);
         }
+        let _ = self.stream.get_mut().shutdown(std::net::Shutdown::Both);
     }
 
     /// Sets the underlying stream
@@ -237,11 +243,12 @@ impl SmtpConnection {
 
     /// Sends the message content
     pub fn message(&mut self, message: &[u8]) -> Result<Response, Error> {
-        let mut out_buf: Vec<u8> = vec![];
         let mut codec = ClientCodec::new();
+        let mut out_buf = Vec::with_capacity(message.len());
         codec.encode(message, &mut out_buf);
         self.write(out_buf.as_slice())?;
         self.write(b"\r\n.\r\n")?;
+
         self.read_response()
     }
 
@@ -276,7 +283,10 @@ impl SmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code()))
+                        Err(error::code(
+                            response.code(),
+                            Some(response.message().collect()),
+                        ))
                     };
                 }
                 Err(nom::Err::Failure(e)) => {
@@ -291,4 +301,10 @@ impl SmtpConnection {
 
         Err(error::response("incomplete response"))
     }
+
+    /// The X509 certificate of the server (DER encoded)
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        self.stream.get_ref().peer_certificate()
+    }
 }

src/transport/smtp/client/mod.rs

@@ -7,16 +7,14 @@
 //!
 //! # #[cfg(feature = "smtp-transport")]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::transport::smtp::{SMTP_PORT, extension::ClientId, commands::*, client::SmtpConnection};
+//! use lettre::transport::smtp::{
+//!     client::SmtpConnection, commands::*, extension::ClientId, SMTP_PORT,
+//! };
 //!
-//! let hello = ClientId::Domain("my_hostname".to_string());
+//! let hello = ClientId::Domain("my_hostname".to_owned());
-//! let mut client = SmtpConnection::connect(&("localhost", SMTP_PORT), None, &hello, None)?;
+//! let mut client = SmtpConnection::connect(&("localhost", SMTP_PORT), None, &hello, None, None)?;
-//! client.command(
+//! client.command(Mail::new(Some("user@example.com".parse()?), vec![]))?;
-//!         Mail::new(Some("user@example.com".parse()?), vec![])
+//! client.command(Rcpt::new("user@example.org".parse()?, vec![]))?;
-//!     )?;
-//! client.command(
-//!         Rcpt::new("user@example.org".parse()?, vec![])
-//!       )?;
 //! client.command(Data)?;
 //! client.message("Test email".as_bytes())?;
 //! client.command(Quit)?;
@@ -28,15 +26,19 @@
 use std::fmt::Debug;
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-pub(crate) use self::async_connection::AsyncSmtpConnection;
+pub use self::async_connection::AsyncSmtpConnection;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-pub(crate) use self::async_net::AsyncNetworkStream;
+pub use self::async_net::AsyncNetworkStream;
+#[cfg(feature = "tokio1")]
+pub use self::async_net::AsyncTokioStream;
 use self::net::NetworkStream;
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(super) use self::tls::InnerTlsParameters;
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+pub use self::tls::TlsVersion;
 pub use self::{
     connection::SmtpConnection,
-    tls::{Certificate, Tls, TlsParameters, TlsParametersBuilder},
+    tls::{Certificate, CertificateStore, Tls, TlsParameters, TlsParametersBuilder},
 };
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
@@ -48,60 +50,57 @@ mod net;
 mod tls;
 
 /// The codec used for transparency
-#[derive(Default, Clone, Copy, Debug)]
+#[derive(Debug)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 struct ClientCodec {
-    escape_count: u8,
+    status: CodecStatus,
 }
 
 impl ClientCodec {
     /// Creates a new client codec
     pub fn new() -> Self {
-        ClientCodec::default()
+        Self {
+            status: CodecStatus::StartOfNewLine,
+        }
     }
 
     /// Adds transparency
     fn encode(&mut self, frame: &[u8], buf: &mut Vec<u8>) {
-        match frame.len() {
+        for &b in frame {
-            0 => {
+            buf.push(b);
-                match self.escape_count {
+            match (b, self.status) {
-                    0 => buf.extend_from_slice(b"\r\n.\r\n"),
+                (b'\r', _) => {
-                    1 => buf.extend_from_slice(b"\n.\r\n"),
+                    self.status = CodecStatus::StartingNewLine;
-                    2 => buf.extend_from_slice(b".\r\n"),
-                    _ => unreachable!(),
                 }
-                self.escape_count = 0;
+                (b'\n', CodecStatus::StartingNewLine) => {
-            }
+                    self.status = CodecStatus::StartOfNewLine;
-            _ => {
-                let mut start = 0;
-                for (idx, byte) in frame.iter().enumerate() {
-                    match self.escape_count {
-                        0 => self.escape_count = if *byte == b'\r' { 1 } else { 0 },
-                        1 => self.escape_count = if *byte == b'\n' { 2 } else { 0 },
-                        2 => {
-                            self.escape_count = if *byte == b'.' {
-                                3
-                            } else if *byte == b'\r' {
-                                1
-                            } else {
-                                0
-                            }
-                        }
-                        _ => unreachable!(),
-                    }
-                    if self.escape_count == 3 {
-                        self.escape_count = 0;
-                        buf.extend_from_slice(&frame[start..idx]);
-                        buf.extend_from_slice(b".");
-                        start = idx;
-                    }
                 }
-                buf.extend_from_slice(&frame[start..]);
+                (_, CodecStatus::StartingNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                }
+                (b'.', CodecStatus::StartOfNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                    buf.push(b'.');
+                }
+                (_, CodecStatus::StartOfNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                }
+                _ => {}
             }
         }
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+#[allow(clippy::enum_variant_names)]
+enum CodecStatus {
+    /// We are past the first character of the current line
+    MiddleOfLine,
+    /// We just read a `\r` character
+    StartingNewLine,
+    /// We are at the start of a new line
+    StartOfNewLine,
+}
+
 /// Returns the string replacing all the CRLF with "\<CRLF\>"
 /// Used for debug displays
 #[cfg(feature = "tracing")]
@@ -115,9 +114,10 @@ mod test {
 
     #[test]
     fn test_codec() {
+        let mut buf = Vec::new();
         let mut codec = ClientCodec::new();
-        let mut buf: Vec<u8> = vec![];
 
+        codec.encode(b".\r\n", &mut buf);
         codec.encode(b"test\r\n", &mut buf);
         codec.encode(b"test\r\n\r\n", &mut buf);
         codec.encode(b".\r\n", &mut buf);
@@ -128,9 +128,13 @@ mod test {
         codec.encode(b"test\n", &mut buf);
         codec.encode(b".test\n", &mut buf);
         codec.encode(b"test", &mut buf);
+        codec.encode(b"test", &mut buf);
+        codec.encode(b"test\r\n", &mut buf);
+        codec.encode(b".test\r\n", &mut buf);
+        codec.encode(b"test.\r\n", &mut buf);
         assert_eq!(
             String::from_utf8(buf).unwrap(),
-            "test\r\ntest\r\n\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntest"
+            "..\r\ntest\r\ntest\r\n\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntesttesttest\r\n..test\r\ntest.\r\n"
         );
     }
 

src/transport/smtp/client/net.rs

@@ -1,19 +1,21 @@
 #[cfg(feature = "rustls-tls")]
-use std::convert::TryFrom;
+use std::sync::Arc;
 use std::{
     io::{self, Read, Write},
     mem,
-    net::{Ipv4Addr, Shutdown, SocketAddr, SocketAddrV4, TcpStream, ToSocketAddrs},
+    net::{IpAddr, Ipv4Addr, Shutdown, SocketAddr, SocketAddrV4, TcpStream, ToSocketAddrs},
     time::Duration,
 };
 
+#[cfg(feature = "boring-tls")]
+use boring::ssl::SslStream;
 #[cfg(feature = "native-tls")]
 use native_tls::TlsStream;
-
 #[cfg(feature = "rustls-tls")]
 use rustls::{ClientConnection, ServerName, StreamOwned};
+use socket2::{Domain, Protocol, Type};
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
 use crate::transport::smtp::{error, Error};
@@ -36,6 +38,8 @@ enum InnerNetworkStream {
     /// Encrypted TCP stream
     #[cfg(feature = "rustls-tls")]
     RustlsTls(StreamOwned<ClientConnection, TcpStream>),
+    #[cfg(feature = "boring-tls")]
+    BoringTls(SslStream<TcpStream>),
     /// Can't be built
     None,
 }
@@ -57,6 +61,8 @@ impl NetworkStream {
             InnerNetworkStream::NativeTls(ref s) => s.get_ref().peer_addr(),
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(ref s) => s.get_ref().peer_addr(),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref s) => s.get_ref().peer_addr(),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(SocketAddr::V4(SocketAddrV4::new(
@@ -75,6 +81,8 @@ impl NetworkStream {
             InnerNetworkStream::NativeTls(ref s) => s.get_ref().shutdown(how),
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(ref s) => s.get_ref().shutdown(how),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref s) => s.get_ref().shutdown(how),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -86,19 +94,39 @@ impl NetworkStream {
         server: T,
         timeout: Option<Duration>,
         tls_parameters: Option<&TlsParameters>,
+        local_addr: Option<IpAddr>,
     ) -> Result<NetworkStream, Error> {
-        fn try_connect_timeout<T: ToSocketAddrs>(
+        fn try_connect<T: ToSocketAddrs>(
             server: T,
-            timeout: Duration,
+            timeout: Option<Duration>,
+            local_addr: Option<IpAddr>,
         ) -> Result<TcpStream, Error> {
-            let addrs = server.to_socket_addrs().map_err(error::connection)?;
+            let addrs = server
+                .to_socket_addrs()
+                .map_err(error::connection)?
+                .filter(|resolved_addr| resolved_address_filter(resolved_addr, local_addr));
 
             let mut last_err = None;
 
             for addr in addrs {
-                match TcpStream::connect_timeout(&addr, timeout) {
+                let socket = socket2::Socket::new(
-                    Ok(stream) => return Ok(stream),
+                    Domain::for_address(addr),
-                    Err(err) => last_err = Some(err),
+                    Type::STREAM,
+                    Some(Protocol::TCP),
+                )
+                .map_err(error::connection)?;
+                bind_local_address(&socket, &addr, local_addr)?;
+
+                if let Some(timeout) = timeout {
+                    match socket.connect_timeout(&addr.into(), timeout) {
+                        Ok(_) => return Ok(socket.into()),
+                        Err(err) => last_err = Some(err),
+                    }
+                } else {
+                    match socket.connect(&addr.into()) {
+                        Ok(_) => return Ok(socket.into()),
+                        Err(err) => last_err = Some(err),
+                    }
                 }
             }
 
@@ -108,11 +136,7 @@ impl NetworkStream {
             })
         }
 
-        let tcp_stream = match timeout {
+        let tcp_stream = try_connect(server, timeout, local_addr)?;
-            Some(t) => try_connect_timeout(server, t)?,
-            None => TcpStream::connect(server).map_err(error::connection)?,
-        };
-
         let mut stream = NetworkStream::new(InnerNetworkStream::Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
             stream.upgrade_tls(tls_parameters)?;
@@ -122,13 +146,17 @@ impl NetworkStream {
 
     pub fn upgrade_tls(&mut self, tls_parameters: &TlsParameters) -> Result<(), Error> {
         match &self.inner {
-            #[cfg(not(any(feature = "native-tls", feature = "rustls-tls")))]
+            #[cfg(not(any(
+                feature = "native-tls",
+                feature = "rustls-tls",
+                feature = "boring-tls"
+            )))]
             InnerNetworkStream::Tcp(_) => {
                 let _ = tls_parameters;
                 panic!("Trying to upgrade an NetworkStream without having enabled either the native-tls or the rustls-tls feature");
             }
 
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             InnerNetworkStream::Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerNetworkStream::None);
@@ -144,7 +172,7 @@ impl NetworkStream {
         }
     }
 
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     fn upgrade_tls_impl(
         tcp_stream: TcpStream,
         tls_parameters: &TlsParameters,
@@ -161,11 +189,21 @@ impl NetworkStream {
             InnerTlsParameters::RustlsTls(connector) => {
                 let domain = ServerName::try_from(tls_parameters.domain())
                     .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
-                let connection =
+                let connection = ClientConnection::new(Arc::clone(connector), domain)
-                    ClientConnection::new(connector.clone(), domain).map_err(error::connection)?;
+                    .map_err(error::connection)?;
                 let stream = StreamOwned::new(connection, tcp_stream);
                 InnerNetworkStream::RustlsTls(stream)
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                let stream = connector
+                    .configure()
+                    .map_err(error::connection)?
+                    .verify_hostname(tls_parameters.accept_invalid_hostnames)
+                    .connect(tls_parameters.domain(), tcp_stream)
+                    .map_err(error::connection)?;
+                InnerNetworkStream::BoringTls(stream)
+            }
         })
     }
 
@@ -176,6 +214,8 @@ impl NetworkStream {
             InnerNetworkStream::NativeTls(_) => true,
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(_) => true,
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(_) => true,
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 false
@@ -183,6 +223,37 @@ impl NetworkStream {
         }
     }
 
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        match &self.inner {
+            InnerNetworkStream::Tcp(_) => Err(error::client("Connection is not encrypted")),
+            #[cfg(feature = "native-tls")]
+            InnerNetworkStream::NativeTls(stream) => Ok(stream
+                .peer_certificate()
+                .map_err(error::tls)?
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            #[cfg(feature = "rustls-tls")]
+            InnerNetworkStream::RustlsTls(stream) => Ok(stream
+                .conn
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_certificate()
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            InnerNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
+
     pub fn set_read_timeout(&mut self, duration: Option<Duration>) -> io::Result<()> {
         match self.inner {
             InnerNetworkStream::Tcp(ref mut stream) => stream.set_read_timeout(duration),
@@ -194,6 +265,10 @@ impl NetworkStream {
             InnerNetworkStream::RustlsTls(ref mut stream) => {
                 stream.get_ref().set_read_timeout(duration)
             }
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref mut stream) => {
+                stream.get_ref().set_read_timeout(duration)
+            }
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -214,7 +289,10 @@ impl NetworkStream {
             InnerNetworkStream::RustlsTls(ref mut stream) => {
                 stream.get_ref().set_write_timeout(duration)
             }
-
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref mut stream) => {
+                stream.get_ref().set_write_timeout(duration)
+            }
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -231,6 +309,8 @@ impl Read for NetworkStream {
             InnerNetworkStream::NativeTls(ref mut s) => s.read(buf),
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(ref mut s) => s.read(buf),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref mut s) => s.read(buf),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(0)
@@ -247,6 +327,8 @@ impl Write for NetworkStream {
             InnerNetworkStream::NativeTls(ref mut s) => s.write(buf),
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(ref mut s) => s.write(buf),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref mut s) => s.write(buf),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(0)
@@ -261,6 +343,8 @@ impl Write for NetworkStream {
             InnerNetworkStream::NativeTls(ref mut s) => s.flush(),
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(ref mut s) => s.flush(),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(ref mut s) => s.flush(),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -268,3 +352,47 @@ impl Write for NetworkStream {
         }
     }
 }
+
+/// If the local address is set, binds the socket to this address.
+/// If local address is not set, then destination address is required to determine to the default
+/// local address on some platforms.
+/// See: https://github.com/hyperium/hyper/blob/faf24c6ad8eee1c3d5ccc9a4d4835717b8e2903f/src/client/connect/http.rs#L560
+fn bind_local_address(
+    socket: &socket2::Socket,
+    dst_addr: &SocketAddr,
+    local_addr: Option<IpAddr>,
+) -> Result<(), Error> {
+    match local_addr {
+        Some(local_addr) => {
+            socket
+                .bind(&SocketAddr::new(local_addr, 0).into())
+                .map_err(error::connection)?;
+        }
+        _ => {
+            if cfg!(windows) {
+                // Windows requires a socket be bound before calling connect
+                let any: SocketAddr = match dst_addr {
+                    SocketAddr::V4(_) => ([0, 0, 0, 0], 0).into(),
+                    SocketAddr::V6(_) => ([0, 0, 0, 0, 0, 0, 0, 0], 0).into(),
+                };
+                socket.bind(&any.into()).map_err(error::connection)?;
+            }
+        }
+    }
+    Ok(())
+}
+
+/// When we have an iterator of resolved remote addresses, we must filter them to be the same
+/// protocol as the local address binding. If no local address is set, then all will be matched.
+pub(crate) fn resolved_address_filter(
+    resolved_addr: &SocketAddr,
+    local_addr: Option<IpAddr>,
+) -> bool {
+    match local_addr {
+        Some(local_addr) => match resolved_addr.ip() {
+            IpAddr::V4(_) => local_addr.is_ipv4(),
+            IpAddr::V6(_) => local_addr.is_ipv6(),
+        },
+        None => true,
+    }
+}

src/transport/smtp/client/tls.rs

@@ -1,21 +1,58 @@
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+use std::fmt::{self, Debug};
-use crate::transport::smtp::{error, Error};
+#[cfg(feature = "rustls-tls")]
+use std::{sync::Arc, time::SystemTime};
+
+#[cfg(feature = "boring-tls")]
+use boring::{
+    ssl::{SslConnector, SslVersion},
+    x509::store::X509StoreBuilder,
+};
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls-tls")]
 use rustls::{
     client::{ServerCertVerified, ServerCertVerifier, WebPkiVerifier},
-    ClientConfig, Error as TlsError, OwnedTrustAnchor, RootCertStore, ServerName,
+    ClientConfig, Error as TlsError, RootCertStore, ServerName,
 };
-use std::fmt::{self, Debug};
-#[cfg(feature = "rustls-tls")]
-use std::{sync::Arc, time::SystemTime};
 
-/// Accepted protocols by default.
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-/// This removes TLS 1.0 and 1.1 compared to tls-native defaults.
+use crate::transport::smtp::{error, Error};
-// This is also rustls' default behavior
+
-#[cfg(feature = "native-tls")]
+/// TLS protocol versions.
-const DEFAULT_TLS_MIN_PROTOCOL: Protocol = Protocol::Tlsv12;
+#[derive(Debug, Copy, Clone)]
+#[non_exhaustive]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+pub enum TlsVersion {
+    /// TLS 1.0
+    ///
+    /// Should only be used when trying to support legacy
+    /// SMTP servers that haven't updated to
+    /// at least TLS 1.2 yet.
+    ///
+    /// Supported by `native-tls` and `boring-tls`.
+    Tlsv10,
+    /// TLS 1.1
+    ///
+    /// Should only be used when trying to support legacy
+    /// SMTP servers that haven't updated to
+    /// at least TLS 1.2 yet.
+    ///
+    /// Supported by `native-tls` and `boring-tls`.
+    Tlsv11,
+    /// TLS 1.2
+    ///
+    /// A good option for most SMTP servers.
+    ///
+    /// Supported by all TLS backends.
+    Tlsv12,
+    /// TLS 1.3
+    ///
+    /// The most secure option, altough not supported by all SMTP servers.
+    ///
+    /// Altough it is technically supported by all TLS backends,
+    /// trying to set it for `native-tls` will give a runtime error.
+    Tlsv13,
+}
 
 /// How to apply TLS to a client connection
 #[derive(Clone)]
@@ -24,16 +61,25 @@ pub enum Tls {
     /// Insecure connection only (for testing purposes)
     None,
     /// Start with insecure connection and use `STARTTLS` when available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Opportunistic(TlsParameters),
     /// Start with insecure connection and require `STARTTLS`
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Required(TlsParameters),
     /// Use TLS wrapped connection
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Wrapper(TlsParameters),
 }
 
@@ -41,31 +87,65 @@ impl Debug for Tls {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match &self {
             Self::None => f.pad("None"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
             Self::Opportunistic(_) => f.pad("Opportunistic"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
             Self::Required(_) => f.pad("Required"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
             Self::Wrapper(_) => f.pad("Wrapper"),
         }
     }
 }
 
+/// Source for the base set of root certificates to trust.
+#[allow(missing_copy_implementations)]
+#[derive(Clone, Debug)]
+pub enum CertificateStore {
+    /// Use the default for the TLS backend.
+    ///
+    /// For native-tls, this will use the system certificate store on Windows, the keychain on
+    /// macOS, and OpenSSL directories on Linux (usually `/etc/ssl`).
+    ///
+    /// For rustls, this will also use the the system store if the `rustls-native-certs` feature is
+    /// enabled, or will fall back to `webpki-roots`.
+    ///
+    /// The boring-tls backend uses the same logic as OpenSSL on all platforms.
+    Default,
+    /// Use a hardcoded set of Mozilla roots via the `webpki-roots` crate.
+    ///
+    /// This option is only available in the rustls backend.
+    #[cfg(feature = "webpki-roots")]
+    WebpkiRoots,
+    /// Don't use any system certificates.
+    None,
+}
+
+impl Default for CertificateStore {
+    fn default() -> Self {
+        CertificateStore::Default
+    }
+}
+
 /// Parameters to use for secure clients
 #[derive(Clone)]
 pub struct TlsParameters {
     pub(crate) connector: InnerTlsParameters,
     /// The domain name which is expected in the TLS certificate from the server
     pub(super) domain: String,
+    #[cfg(feature = "boring-tls")]
+    pub(super) accept_invalid_hostnames: bool,
 }
 
 /// Builder for `TlsParameters`
 #[derive(Debug, Clone)]
 pub struct TlsParametersBuilder {
     domain: String,
+    cert_store: CertificateStore,
     root_certs: Vec<Certificate>,
     accept_invalid_hostnames: bool,
     accept_invalid_certs: bool,
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    min_tls_version: TlsVersion,
 }
 
 impl TlsParametersBuilder {
@@ -73,12 +153,21 @@ impl TlsParametersBuilder {
     pub fn new(domain: String) -> Self {
         Self {
             domain,
+            cert_store: CertificateStore::Default,
             root_certs: Vec::new(),
             accept_invalid_hostnames: false,
             accept_invalid_certs: false,
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+            min_tls_version: TlsVersion::Tlsv12,
         }
     }
 
+    /// Set the source for the base set of root certificates to trust.
+    pub fn certificate_store(mut self, cert_store: CertificateStore) -> Self {
+        self.cert_store = cert_store;
+        self
+    }
+
     /// Add a custom root certificate
     ///
     /// Can be used to safely connect to a server using a self signed certificate, for example.
@@ -100,13 +189,22 @@ impl TlsParametersBuilder {
     /// This method introduces significant vulnerabilities to man-in-the-middle attacks.
     ///
     /// Hostname verification can only be disabled with the `native-tls` TLS backend.
-    #[cfg(feature = "native-tls")]
+    #[cfg(any(feature = "native-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(feature = "native-tls")))]
+    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "boring-tls"))))]
     pub fn dangerous_accept_invalid_hostnames(mut self, accept_invalid_hostnames: bool) -> Self {
         self.accept_invalid_hostnames = accept_invalid_hostnames;
         self
     }
 
+    /// Controls which minimum TLS version is allowed
+    ///
+    /// Defaults to [`Tlsv12`][TlsVersion::Tlsv12].
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn set_min_tls_version(mut self, min_tls_version: TlsVersion) -> Self {
+        self.min_tls_version = min_tls_version;
+        self
+    }
+
     /// Controls whether invalid certificates are accepted
     ///
     /// Defaults to `false`.
@@ -128,16 +226,20 @@ impl TlsParametersBuilder {
         self
     }
 
-    /// Creates a new `TlsParameters` using native-tls or rustls
+    /// Creates a new `TlsParameters` using native-tls, boring-tls or rustls
     /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn build(self) -> Result<TlsParameters, Error> {
         #[cfg(feature = "rustls-tls")]
         return self.build_rustls();
-
+        #[cfg(all(not(feature = "rustls-tls"), feature = "native-tls"))]
-        #[cfg(not(feature = "rustls-tls"))]
         return self.build_native();
+        #[cfg(all(not(feature = "rustls-tls"), feature = "boring-tls"))]
+        return self.build_boring();
     }
 
     /// Creates a new `TlsParameters` using native-tls with the provided configuration
@@ -146,17 +248,93 @@ impl TlsParametersBuilder {
     pub fn build_native(self) -> Result<TlsParameters, Error> {
         let mut tls_builder = TlsConnector::builder();
 
+        match self.cert_store {
+            CertificateStore::Default => {}
+            CertificateStore::None => {
+                tls_builder.disable_built_in_roots(true);
+            }
+            #[allow(unreachable_patterns)]
+            other => {
+                return Err(error::tls(format!(
+                    "{other:?} is not supported in native tls"
+                )))
+            }
+        }
         for cert in self.root_certs {
             tls_builder.add_root_certificate(cert.native_tls);
         }
         tls_builder.danger_accept_invalid_hostnames(self.accept_invalid_hostnames);
         tls_builder.danger_accept_invalid_certs(self.accept_invalid_certs);
 
-        tls_builder.min_protocol_version(Some(DEFAULT_TLS_MIN_PROTOCOL));
+        let min_tls_version = match self.min_tls_version {
+            TlsVersion::Tlsv10 => Protocol::Tlsv10,
+            TlsVersion::Tlsv11 => Protocol::Tlsv11,
+            TlsVersion::Tlsv12 => Protocol::Tlsv12,
+            TlsVersion::Tlsv13 => {
+                return Err(error::tls(
+                    "min tls version Tlsv13 not supported in native tls",
+                ))
+            }
+        };
+
+        tls_builder.min_protocol_version(Some(min_tls_version));
         let connector = tls_builder.build().map_err(error::tls)?;
         Ok(TlsParameters {
             connector: InnerTlsParameters::NativeTls(connector),
             domain: self.domain,
+            #[cfg(feature = "boring-tls")]
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
+        })
+    }
+
+    /// Creates a new `TlsParameters` using boring-tls with the provided configuration
+    #[cfg(feature = "boring-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "boring-tls")))]
+    pub fn build_boring(self) -> Result<TlsParameters, Error> {
+        use boring::ssl::{SslMethod, SslVerifyMode};
+
+        let mut tls_builder = SslConnector::builder(SslMethod::tls_client()).map_err(error::tls)?;
+
+        if self.accept_invalid_certs {
+            tls_builder.set_verify(SslVerifyMode::NONE);
+        } else {
+            match self.cert_store {
+                CertificateStore::Default => {}
+                CertificateStore::None => {
+                    // Replace the default store with an empty store.
+                    tls_builder
+                        .set_cert_store(X509StoreBuilder::new().map_err(error::tls)?.build());
+                }
+                #[allow(unreachable_patterns)]
+                other => {
+                    return Err(error::tls(format!(
+                        "{other:?} is not supported in boring tls"
+                    )))
+                }
+            }
+
+            let cert_store = tls_builder.cert_store_mut();
+
+            for cert in self.root_certs {
+                cert_store.add_cert(cert.boring_tls).map_err(error::tls)?;
+            }
+        }
+
+        let min_tls_version = match self.min_tls_version {
+            TlsVersion::Tlsv10 => SslVersion::TLS1,
+            TlsVersion::Tlsv11 => SslVersion::TLS1_1,
+            TlsVersion::Tlsv12 => SslVersion::TLS1_2,
+            TlsVersion::Tlsv13 => SslVersion::TLS1_3,
+        };
+
+        tls_builder
+            .set_min_proto_version(Some(min_tls_version))
+            .map_err(error::tls)?;
+        let connector = tls_builder.build();
+        Ok(TlsParameters {
+            connector: InnerTlsParameters::BoringTls(connector),
+            domain: self.domain,
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
         })
     }
 
@@ -165,26 +343,81 @@ impl TlsParametersBuilder {
     #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
     pub fn build_rustls(self) -> Result<TlsParameters, Error> {
         let tls = ClientConfig::builder();
-        let tls = tls.with_safe_defaults();
+
+        let just_version3 = &[&rustls::version::TLS13];
+        let supported_versions = match self.min_tls_version {
+            TlsVersion::Tlsv10 => {
+                return Err(error::tls("min tls version Tlsv10 not supported in rustls"))
+            }
+            TlsVersion::Tlsv11 => {
+                return Err(error::tls("min tls version Tlsv11 not supported in rustls"))
+            }
+            TlsVersion::Tlsv12 => rustls::ALL_VERSIONS,
+            TlsVersion::Tlsv13 => just_version3,
+        };
+
+        let tls = tls
+            .with_safe_default_cipher_suites()
+            .with_safe_default_kx_groups()
+            .with_protocol_versions(supported_versions)
+            .map_err(error::tls)?;
 
         let tls = if self.accept_invalid_certs {
             tls.with_custom_certificate_verifier(Arc::new(InvalidCertsVerifier {}))
         } else {
             let mut root_cert_store = RootCertStore::empty();
+
+            #[cfg(feature = "rustls-native-certs")]
+            fn load_native_roots(store: &mut RootCertStore) -> Result<(), Error> {
+                let native_certs = rustls_native_certs::load_native_certs().map_err(error::tls)?;
+                let mut valid_count = 0;
+                let mut invalid_count = 0;
+                for cert in native_certs {
+                    match store.add(&rustls::Certificate(cert.0)) {
+                        Ok(_) => valid_count += 1,
+                        Err(err) => {
+                            #[cfg(feature = "tracing")]
+                            tracing::debug!("certificate parsing failed: {:?}", err);
+                            invalid_count += 1;
+                        }
+                    }
+                }
+                #[cfg(feature = "tracing")]
+                tracing::debug!(
+                    "loaded platform certs with {valid_count} valid and {invalid_count} invalid certs"
+                );
+                Ok(())
+            }
+
+            #[cfg(feature = "webpki-roots")]
+            fn load_webpki_roots(store: &mut RootCertStore) {
+                store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(|ta| {
+                    rustls::OwnedTrustAnchor::from_subject_spki_name_constraints(
+                        ta.subject,
+                        ta.spki,
+                        ta.name_constraints,
+                    )
+                }));
+            }
+
+            match self.cert_store {
+                CertificateStore::Default => {
+                    #[cfg(feature = "rustls-native-certs")]
+                    load_native_roots(&mut root_cert_store)?;
+                    #[cfg(all(not(feature = "rustls-native-certs"), feature = "webpki-roots"))]
+                    load_webpki_roots(&mut root_cert_store);
+                }
+                #[cfg(feature = "webpki-roots")]
+                CertificateStore::WebpkiRoots => {
+                    load_webpki_roots(&mut root_cert_store);
+                }
+                CertificateStore::None => {}
+            }
             for cert in self.root_certs {
                 for rustls_cert in cert.rustls {
                     root_cert_store.add(&rustls_cert).map_err(error::tls)?;
                 }
             }
-            root_cert_store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(
-                |ta| {
-                    OwnedTrustAnchor::from_subject_spki_name_constraints(
-                        ta.subject,
-                        ta.spki,
-                        ta.name_constraints,
-                    )
-                },
-            ));
 
             tls.with_custom_certificate_verifier(Arc::new(WebPkiVerifier::new(
                 root_cert_store,
@@ -196,27 +429,36 @@ impl TlsParametersBuilder {
         Ok(TlsParameters {
             connector: InnerTlsParameters::RustlsTls(Arc::new(tls)),
             domain: self.domain,
+            #[cfg(feature = "boring-tls")]
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
         })
     }
 }
 
 #[derive(Clone)]
+#[allow(clippy::enum_variant_names)]
 pub enum InnerTlsParameters {
     #[cfg(feature = "native-tls")]
     NativeTls(TlsConnector),
     #[cfg(feature = "rustls-tls")]
     RustlsTls(Arc<ClientConfig>),
+    #[cfg(feature = "boring-tls")]
+    BoringTls(SslConnector),
 }
 
 impl TlsParameters {
     /// Creates a new `TlsParameters` using native-tls or rustls
     /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn new(domain: String) -> Result<Self, Error> {
         TlsParametersBuilder::new(domain).build()
     }
 
+    /// Creates a new `TlsParameters` builder
     pub fn builder(domain: String) -> TlsParametersBuilder {
         TlsParametersBuilder::new(domain)
     }
@@ -235,6 +477,13 @@ impl TlsParameters {
         TlsParametersBuilder::new(domain).build_rustls()
     }
 
+    /// Creates a new `TlsParameters` using boring
+    #[cfg(feature = "boring-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "boring-tls")))]
+    pub fn new_boring(domain: String) -> Result<Self, Error> {
+        TlsParametersBuilder::new(domain).build_boring()
+    }
+
     pub fn domain(&self) -> &str {
         &self.domain
     }
@@ -248,20 +497,27 @@ pub struct Certificate {
     native_tls: native_tls::Certificate,
     #[cfg(feature = "rustls-tls")]
     rustls: Vec<rustls::Certificate>,
+    #[cfg(feature = "boring-tls")]
+    boring_tls: boring::x509::X509,
 }
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 impl Certificate {
     /// Create a `Certificate` from a DER encoded certificate
     pub fn from_der(der: Vec<u8>) -> Result<Self, Error> {
         #[cfg(feature = "native-tls")]
         let native_tls_cert = native_tls::Certificate::from_der(&der).map_err(error::tls)?;
 
+        #[cfg(feature = "boring-tls")]
+        let boring_tls_cert = boring::x509::X509::from_der(&der).map_err(error::tls)?;
+
         Ok(Self {
             #[cfg(feature = "native-tls")]
             native_tls: native_tls_cert,
             #[cfg(feature = "rustls-tls")]
             rustls: vec![rustls::Certificate(der)],
+            #[cfg(feature = "boring-tls")]
+            boring_tls: boring_tls_cert,
         })
     }
 
@@ -270,6 +526,9 @@ impl Certificate {
         #[cfg(feature = "native-tls")]
         let native_tls_cert = native_tls::Certificate::from_pem(pem).map_err(error::tls)?;
 
+        #[cfg(feature = "boring-tls")]
+        let boring_tls_cert = boring::x509::X509::from_pem(pem).map_err(error::tls)?;
+
         #[cfg(feature = "rustls-tls")]
         let rustls_cert = {
             use std::io::Cursor;
@@ -287,6 +546,8 @@ impl Certificate {
             native_tls: native_tls_cert,
             #[cfg(feature = "rustls-tls")]
             rustls: rustls_cert,
+            #[cfg(feature = "boring-tls")]
+            boring_tls: boring_tls_cert,
         })
     }
 }

src/transport/smtp/commands.rs

@@ -1,5 +1,7 @@
 //! SMTP commands
 
+use std::fmt::{self, Display, Formatter};
+
 use crate::{
     address::Address,
     transport::smtp::{
@@ -9,10 +11,9 @@ use crate::{
         response::Response,
     },
 };
-use std::fmt::{self, Display, Formatter};
 
 /// EHLO command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Ehlo {
     client_id: ClientId,
@@ -32,7 +33,7 @@ impl Ehlo {
 }
 
 /// STARTTLS command
-#[derive(PartialEq, Clone, Debug, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Copy)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Starttls;
 
@@ -43,7 +44,7 @@ impl Display for Starttls {
 }
 
 /// MAIL command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Mail {
     sender: Option<Address>,
@@ -58,7 +59,7 @@ impl Display for Mail {
             self.sender.as_ref().map_or("", |s| s.as_ref())
         )?;
         for parameter in &self.parameters {
-            write!(f, " {}", parameter)?;
+            write!(f, " {parameter}")?;
         }
         f.write_str("\r\n")
     }
@@ -72,7 +73,7 @@ impl Mail {
 }
 
 /// RCPT command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Rcpt {
     recipient: Address,
@@ -83,7 +84,7 @@ impl Display for Rcpt {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         write!(f, "RCPT TO:<{}>", self.recipient)?;
         for parameter in &self.parameters {
-            write!(f, " {}", parameter)?;
+            write!(f, " {parameter}")?;
         }
         f.write_str("\r\n")
     }
@@ -100,7 +101,7 @@ impl Rcpt {
 }
 
 /// DATA command
-#[derive(PartialEq, Clone, Debug, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Copy)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Data;
 
@@ -111,7 +112,7 @@ impl Display for Data {
 }
 
 /// QUIT command
-#[derive(PartialEq, Clone, Debug, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Copy)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Quit;
 
@@ -122,7 +123,7 @@ impl Display for Quit {
 }
 
 /// NOOP command
-#[derive(PartialEq, Clone, Debug, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Copy)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Noop;
 
@@ -133,7 +134,7 @@ impl Display for Noop {
 }
 
 /// HELP command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Help {
     argument: Option<String>,
@@ -143,7 +144,7 @@ impl Display for Help {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         f.write_str("HELP")?;
         if let Some(argument) = &self.argument {
-            write!(f, " {}", argument)?;
+            write!(f, " {argument}")?;
         }
         f.write_str("\r\n")
     }
@@ -157,7 +158,7 @@ impl Help {
 }
 
 /// VRFY command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Vrfy {
     argument: String,
@@ -177,7 +178,7 @@ impl Vrfy {
 }
 
 /// EXPN command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Expn {
     argument: String,
@@ -197,7 +198,7 @@ impl Expn {
 }
 
 /// RSET command
-#[derive(PartialEq, Clone, Debug, Copy)]
+#[derive(PartialEq, Eq, Clone, Debug, Copy)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Rset;
 
@@ -208,7 +209,7 @@ impl Display for Rset {
 }
 
 /// AUTH command
-#[derive(PartialEq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Auth {
     mechanism: Mechanism,
@@ -219,7 +220,7 @@ pub struct Auth {
 
 impl Display for Auth {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        let encoded_response = self.response.as_ref().map(base64::encode);
+        let encoded_response = self.response.as_ref().map(crate::base64::encode);
 
         if self.mechanism.supports_initial_response() {
             write!(f, "AUTH {} {}", self.mechanism, encoded_response.unwrap())?;
@@ -270,7 +271,7 @@ impl Auth {
         #[cfg(feature = "tracing")]
         tracing::debug!("auth encoded challenge: {}", encoded_challenge);
 
-        let decoded_base64 = base64::decode(&encoded_challenge).map_err(error::response)?;
+        let decoded_base64 = crate::base64::decode(encoded_challenge).map_err(error::response)?;
         let decoded_challenge = String::from_utf8(decoded_base64).map_err(error::response)?;
         #[cfg(feature = "tracing")]
         tracing::debug!("auth decoded challenge: {}", decoded_challenge);
@@ -288,21 +289,22 @@ impl Auth {
 
 #[cfg(test)]
 mod test {
+    use std::str::FromStr;
+
     use super::*;
     use crate::transport::smtp::extension::MailBodyParameter;
-    use std::str::FromStr;
 
     #[test]
     fn test_display() {
-        let id = ClientId::Domain("localhost".to_string());
+        let id = ClientId::Domain("localhost".to_owned());
         let email = Address::from_str("test@example.com").unwrap();
         let mail_parameter = MailParameter::Other {
-            keyword: "TEST".to_string(),
+            keyword: "TEST".to_owned(),
-            value: Some("value".to_string()),
+            value: Some("value".to_owned()),
         };
         let rcpt_parameter = RcptParameter::Other {
-            keyword: "TEST".to_string(),
+            keyword: "TEST".to_owned(),
-            value: Some("value".to_string()),
+            value: Some("value".to_owned()),
         };
         assert_eq!(format!("{}", Ehlo::new(id)), "EHLO localhost\r\n");
         assert_eq!(
@@ -339,24 +341,18 @@ mod test {
             format!("{}", Rcpt::new(email, vec![rcpt_parameter])),
             "RCPT TO:<test@example.com> TEST=value\r\n"
         );
-        assert_eq!(format!("{}", Quit), "QUIT\r\n");
+        assert_eq!(format!("{Quit}"), "QUIT\r\n");
-        assert_eq!(format!("{}", Data), "DATA\r\n");
+        assert_eq!(format!("{Data}"), "DATA\r\n");
-        assert_eq!(format!("{}", Noop), "NOOP\r\n");
+        assert_eq!(format!("{Noop}"), "NOOP\r\n");
         assert_eq!(format!("{}", Help::new(None)), "HELP\r\n");
         assert_eq!(
-            format!("{}", Help::new(Some("test".to_string()))),
+            format!("{}", Help::new(Some("test".to_owned()))),
             "HELP test\r\n"
         );
-        assert_eq!(
+        assert_eq!(format!("{}", Vrfy::new("test".to_owned())), "VRFY test\r\n");
-            format!("{}", Vrfy::new("test".to_string())),
+        assert_eq!(format!("{}", Expn::new("test".to_owned())), "EXPN test\r\n");
-            "VRFY test\r\n"
+        assert_eq!(format!("{Rset}"), "RSET\r\n");
-        );
+        let credentials = Credentials::new("user".to_owned(), "password".to_owned());
-        assert_eq!(
-            format!("{}", Expn::new("test".to_string())),
-            "EXPN test\r\n"
-        );
-        assert_eq!(format!("{}", Rset), "RSET\r\n");
-        let credentials = Credentials::new("user".to_string(), "password".to_string());
         assert_eq!(
             format!(
                 "{}",

src/transport/smtp/error.rs

@@ -1,10 +1,11 @@
 //! Error and result type for SMTP clients
 
+use std::{error::Error as StdError, fmt};
+
 use crate::{
     transport::smtp::response::{Code, Severity},
     BoxError,
 };
-use std::{error::Error as StdError, fmt};
 
 // Inspired by https://github.com/seanmonstar/reqwest/blob/a8566383168c0ef06c21f38cbc9213af6ff6db31/src/error.rs
 
@@ -67,8 +68,11 @@ impl Error {
     }
 
     /// Returns true if the error is from TLS
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn is_tls(&self) -> bool {
         matches!(self.inner.kind, Kind::Tls)
     }
@@ -101,8 +105,11 @@ pub(crate) enum Kind {
     /// Underlying network i/o error
     Network,
     /// TLS error
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     Tls,
 }
 
@@ -127,18 +134,18 @@ impl fmt::Display for Error {
             Kind::Client => f.write_str("internal client error")?,
             Kind::Network => f.write_str("network error")?,
             Kind::Connection => f.write_str("Connection error")?,
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Kind::Tls => f.write_str("tls error")?,
             Kind::Transient(ref code) => {
-                write!(f, "transient error ({})", code)?;
+                write!(f, "transient error ({code})")?;
             }
             Kind::Permanent(ref code) => {
-                write!(f, "permanent error ({})", code)?;
+                write!(f, "permanent error ({code})")?;
             }
         };
 
         if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+            write!(f, ": {e}")?;
         }
 
         Ok(())
@@ -154,10 +161,10 @@ impl StdError for Error {
     }
 }
 
-pub(crate) fn code(c: Code) -> Error {
+pub(crate) fn code(c: Code, s: Option<String>) -> Error {
     match c.severity {
-        Severity::TransientNegativeCompletion => Error::new::<Error>(Kind::Transient(c), None),
+        Severity::TransientNegativeCompletion => Error::new(Kind::Transient(c), s),
-        Severity::PermanentNegativeCompletion => Error::new::<Error>(Kind::Permanent(c), None),
+        Severity::PermanentNegativeCompletion => Error::new(Kind::Permanent(c), s),
         _ => client("Unknown error code"),
     }
 }
@@ -178,7 +185,7 @@ pub(crate) fn connection<E: Into<BoxError>>(e: E) -> Error {
     Error::new(Kind::Connection, Some(e))
 }
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(crate) fn tls<E: Into<BoxError>>(e: E) -> Error {
     Error::new(Kind::Tls, Some(e))
 }

src/transport/smtp/extension.rs

@@ -1,17 +1,18 @@
 //! ESMTP features
 
+use std::{
+    collections::HashSet,
+    fmt::{self, Display, Formatter},
+    net::{Ipv4Addr, Ipv6Addr},
+    result::Result,
+};
+
 use crate::transport::smtp::{
     authentication::Mechanism,
     error::{self, Error},
     response::Response,
     util::XText,
 };
-use std::{
-    collections::HashSet,
-    fmt::{self, Display, Formatter},
-    net::{Ipv4Addr, Ipv6Addr},
-    result::Result,
-};
 
 /// Client identifier, the parameter to `EHLO`
 #[derive(PartialEq, Eq, Clone, Debug)]
@@ -54,8 +55,8 @@ impl Display for ClientId {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match *self {
             Self::Domain(ref value) => f.write_str(value),
-            Self::Ipv4(ref value) => write!(f, "[{}]", value),
+            Self::Ipv4(ref value) => write!(f, "[{value}]"),
-            Self::Ipv6(ref value) => write!(f, "[IPv6:{}]", value),
+            Self::Ipv6(ref value) => write!(f, "[IPv6:{value}]"),
         }
     }
 }
@@ -96,7 +97,7 @@ impl Display for Extension {
             Extension::EightBitMime => f.write_str("8BITMIME"),
             Extension::SmtpUtfEight => f.write_str("SMTPUTF8"),
             Extension::StartTls => f.write_str("STARTTLS"),
-            Extension::Authentication(ref mechanism) => write!(f, "AUTH {}", mechanism),
+            Extension::Authentication(ref mechanism) => write!(f, "AUTH {mechanism}"),
         }
     }
 }
@@ -118,7 +119,7 @@ pub struct ServerInfo {
 impl Display for ServerInfo {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         let features = if self.features.is_empty() {
-            "no supported features".to_string()
+            "no supported features".to_owned()
         } else {
             format!("{:?}", self.features)
         };
@@ -173,7 +174,7 @@ impl ServerInfo {
         }
 
         Ok(ServerInfo {
-            name: name.to_string(),
+            name: name.to_owned(),
             features,
         })
     }
@@ -227,8 +228,8 @@ pub enum MailParameter {
 impl Display for MailParameter {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match *self {
-            MailParameter::Body(ref value) => write!(f, "BODY={}", value),
+            MailParameter::Body(ref value) => write!(f, "BODY={value}"),
-            MailParameter::Size(size) => write!(f, "SIZE={}", size),
+            MailParameter::Size(size) => write!(f, "SIZE={size}"),
             MailParameter::SmtpUtfEight => f.write_str("SMTPUTF8"),
             MailParameter::Other {
                 ref keyword,
@@ -292,31 +293,32 @@ impl Display for RcptParameter {
 #[cfg(test)]
 mod test {
 
+    use std::collections::HashSet;
+
     use super::*;
     use crate::transport::smtp::{
         authentication::Mechanism,
         response::{Category, Code, Detail, Response, Severity},
     };
-    use std::collections::HashSet;
 
     #[test]
     fn test_clientid_fmt() {
         assert_eq!(
-            format!("{}", ClientId::Domain("test".to_string())),
+            format!("{}", ClientId::Domain("test".to_owned())),
-            "test".to_string()
+            "test".to_owned()
         );
-        assert_eq!(format!("{}", LOCALHOST_CLIENT), "[127.0.0.1]".to_string());
+        assert_eq!(format!("{LOCALHOST_CLIENT}"), "[127.0.0.1]".to_owned());
     }
 
     #[test]
     fn test_extension_fmt() {
         assert_eq!(
             format!("{}", Extension::EightBitMime),
-            "8BITMIME".to_string()
+            "8BITMIME".to_owned()
         );
         assert_eq!(
             format!("{}", Extension::Authentication(Mechanism::Plain)),
-            "AUTH PLAIN".to_string()
+            "AUTH PLAIN".to_owned()
         );
     }
 
@@ -329,11 +331,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: eightbitmime,
                 }
             ),
-            "name with {EightBitMime}".to_string()
+            "name with {EightBitMime}".to_owned()
         );
 
         let empty = HashSet::new();
@@ -342,11 +344,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: empty,
                 }
             ),
-            "name with no supported features".to_string()
+            "name with no supported features".to_owned()
         );
 
         let mut plain = HashSet::new();
@@ -356,11 +358,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: plain,
                 }
             ),
-            "name with {Authentication(Plain)}".to_string()
+            "name with {Authentication(Plain)}".to_owned()
         );
     }
 
@@ -372,18 +374,14 @@ mod test {
                 Category::Unspecified4,
                 Detail::One,
             ),
-            vec![
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned()],
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
         );
 
         let mut features = HashSet::new();
         assert!(features.insert(Extension::EightBitMime));
 
         let server_info = ServerInfo {
-            name: "me".to_string(),
+            name: "me".to_owned(),
             features,
         };
 
@@ -399,10 +397,10 @@ mod test {
                 Detail::One,
             ),
             vec![
-                "me".to_string(),
+                "me".to_owned(),
-                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_string(),
+                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_owned(),
-                "8BITMIME".to_string(),
+                "8BITMIME".to_owned(),
-                "SIZE 42".to_string(),
+                "SIZE 42".to_owned(),
             ],
         );
 
@@ -412,7 +410,7 @@ mod test {
         assert!(features2.insert(Extension::Authentication(Mechanism::Xoauth2),));
 
         let server_info2 = ServerInfo {
-            name: "me".to_string(),
+            name: "me".to_owned(),
             features: features2,
         };
 

src/transport/smtp/mod.rs

@@ -33,7 +33,7 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport};
+//! use lettre::{Message, SmtpTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -43,8 +43,7 @@
 //!     .body(String::from("Be happy!"))?;
 //!
 //! // Create TLS transport on port 465
-//! let sender = SmtpTransport::relay("smtp.example.com")?
+//! let sender = SmtpTransport::relay("smtp.example.com")?.build();
-//!     .build();
 //! // Send the email via remote relay
 //! let result = sender.send(&email);
 //! assert!(result.is_ok());
@@ -59,7 +58,13 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::{PoolConfig, authentication::{Credentials, Mechanism}}};
+//! use lettre::{
+//!     transport::smtp::{
+//!         authentication::{Credentials, Mechanism},
+//!         PoolConfig,
+//!     },
+//!     Message, SmtpTransport, Transport,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -71,11 +76,14 @@
 //! // Create TLS transport on port 587 with STARTTLS
 //! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
 //!     // Add credentials for authentication
-//!     .credentials(Credentials::new("username".to_string(), "password".to_string()))
+//!     .credentials(Credentials::new(
+//!         "username".to_owned(),
+//!         "password".to_owned(),
+//!     ))
 //!     // Configure expected authentication mechanism
 //!     .authentication(vec![Mechanism::Plain])
 //!     // Connection pool settings
-//!     .pool_config( PoolConfig::new().max_size(20))
+//!     .pool_config(PoolConfig::new().max_size(20))
 //!     .build();
 //!
 //! // Send the email via remote relay
@@ -90,7 +98,10 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::client::{TlsParameters, Tls}};
+//! use lettre::{
+//!     transport::smtp::client::{Tls, TlsParameters},
+//!     Message, SmtpTransport, Transport,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -100,8 +111,9 @@
 //!     .body(String::from("Be happy!"))?;
 //!
 //! // Custom TLS configuration
-//! let tls = TlsParameters::builder("smtp.example.com".to_string())
+//! let tls = TlsParameters::builder("smtp.example.com".to_owned())
-//!           .dangerous_accept_invalid_certs(true).build()?;
+//!     .dangerous_accept_invalid_certs(true)
+//!     .build()?;
 //!
 //! // Create TLS transport on port 465
 //! let sender = SmtpTransport::relay("smtp.example.com")?
@@ -116,6 +128,10 @@
 //! # }
 //! ```
 
+use std::time::Duration;
+
+use client::Tls;
+
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 pub use self::async_transport::{AsyncSmtpTransport, AsyncSmtpTransportBuilder};
 #[cfg(feature = "pool")]
@@ -124,7 +140,7 @@ pub use self::{
     error::Error,
     transport::{SmtpTransport, SmtpTransportBuilder},
 };
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::client::TlsParameters;
 use crate::transport::smtp::{
     authentication::{Credentials, Mechanism, DEFAULT_MECHANISMS},
@@ -132,8 +148,6 @@ use crate::transport::smtp::{
     extension::ClientId,
     response::Response,
 };
-use client::Tls;
-use std::time::Duration;
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 mod async_transport;
@@ -186,7 +200,7 @@ struct SmtpInfo {
 impl Default for SmtpInfo {
     fn default() -> Self {
         Self {
-            server: "localhost".to_string(),
+            server: "localhost".to_owned(),
             port: SMTP_PORT,
             hello_name: ClientId::default(),
             credentials: None,

src/transport/smtp/pool/async_impl.rs

@@ -1,20 +1,22 @@
-use std::fmt::{self, Debug};
+use std::{
-use std::mem;
+    fmt::{self, Debug},
-use std::ops::{Deref, DerefMut};
+    mem,
-use std::sync::Arc;
+    ops::{Deref, DerefMut},
-use std::time::{Duration, Instant};
+    sync::Arc,
+    time::{Duration, Instant},
+};
 
-use futures_util::lock::Mutex;
+use futures_util::{
-use futures_util::stream::{self, StreamExt};
+    lock::Mutex,
+    stream::{self, StreamExt},
+};
 use once_cell::sync::OnceCell;
 
-use crate::executor::SpawnHandle;
+use super::{
-use crate::transport::smtp::async_transport::AsyncSmtpClient;
+    super::{client::AsyncSmtpConnection, Error},
-use crate::Executor;
+    PoolConfig,
-
+};
-use super::super::client::AsyncSmtpConnection;
+use crate::{executor::SpawnHandle, transport::smtp::async_transport::AsyncSmtpClient, Executor};
-use super::super::Error;
-use super::PoolConfig;
 
 pub struct Pool<E: Executor> {
     config: PoolConfig,
@@ -156,14 +158,14 @@ impl<E: Executor> Pool<E> {
                     #[cfg(feature = "tracing")]
                     tracing::debug!("reusing a pooled connection");
 
-                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                    return Ok(PooledConnection::wrap(conn, Arc::clone(self)));
                 }
                 None => {
                     #[cfg(feature = "tracing")]
                     tracing::debug!("creating a new connection");
 
                     let conn = self.client.connection().await?;
-                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                    return Ok(PooledConnection::wrap(conn, Arc::clone(self)));
                 }
             }
         }
@@ -201,7 +203,7 @@ impl<E: Executor> Debug for Pool<E> {
                 &match self.connections.try_lock() {
                     Some(connections) => format!("{} connections", connections.len()),
 
-                    None => "LOCKED".to_string(),
+                    None => "LOCKED".to_owned(),
                 },
             )
             .field("client", &self.client)

src/transport/smtp/pool/sync_impl.rs

@@ -1,15 +1,18 @@
-use std::fmt::{self, Debug};
+use std::{
-use std::ops::{Deref, DerefMut};
+    fmt::{self, Debug},
-use std::sync::{Arc, Mutex, TryLockError};
+    mem,
-use std::time::{Duration, Instant};
+    ops::{Deref, DerefMut},
-use std::{mem, thread};
+    sync::{Arc, Mutex, TryLockError},
+    thread,
+    time::{Duration, Instant},
+};
 
+use super::{
+    super::{client::SmtpConnection, Error},
+    PoolConfig,
+};
 use crate::transport::smtp::transport::SmtpClient;
 
-use super::super::client::SmtpConnection;
-use super::super::Error;
-use super::PoolConfig;
-
 pub struct Pool {
     config: PoolConfig,
     connections: Mutex<Vec<ParkedConnection>>,
@@ -138,14 +141,14 @@ impl Pool {
                     #[cfg(feature = "tracing")]
                     tracing::debug!("reusing a pooled connection");
 
-                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                    return Ok(PooledConnection::wrap(conn, Arc::clone(self)));
                 }
                 None => {
                     #[cfg(feature = "tracing")]
                     tracing::debug!("creating a new connection");
 
                     let conn = self.client.connection()?;
-                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                    return Ok(PooledConnection::wrap(conn, Arc::clone(self)));
                 }
             }
         }
@@ -183,8 +186,8 @@ impl Debug for Pool {
                 &match self.connections.try_lock() {
                     Ok(connections) => format!("{} connections", connections.len()),
 
-                    Err(TryLockError::WouldBlock) => "LOCKED".to_string(),
+                    Err(TryLockError::WouldBlock) => "LOCKED".to_owned(),
-                    Err(TryLockError::Poisoned(_)) => "POISONED".to_string(),
+                    Err(TryLockError::Poisoned(_)) => "POISONED".to_owned(),
                 },
             )
             .field("client", &self.client)

src/transport/smtp/response.rs

@@ -1,7 +1,13 @@
 //! SMTP response, containing a mandatory return code and an optional text
 //! message
 
-use crate::transport::smtp::{error, Error};
+use std::{
+    fmt::{Display, Formatter, Result},
+    result,
+    str::FromStr,
+    string::ToString,
+};
+
 use nom::{
     branch::alt,
     bytes::streaming::{tag, take_until},
@@ -10,12 +16,8 @@ use nom::{
     sequence::{preceded, tuple},
     IResult,
 };
-use std::{
+
-    fmt::{Display, Formatter, Result},
+use crate::transport::smtp::{error, Error};
-    result,
-    str::FromStr,
-    string::ToString,
-};
 
 /// First digit indicates severity
 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
@@ -149,7 +151,7 @@ impl FromStr for Response {
     fn from_str(s: &str) -> result::Result<Response, Error> {
         parse_response(s)
             .map(|(_, r)| r)
-            .map_err(|e| error::response(e.to_string()))
+            .map_err(|e| error::response(e.to_owned()))
     }
 }
 
@@ -327,10 +329,10 @@ mod test {
                     detail: Detail::Zero,
                 },
                 message: vec![
-                    "me".to_string(),
+                    "me".to_owned(),
-                    "8BITMIME".to_string(),
+                    "8BITMIME".to_owned(),
-                    "SIZE 42".to_string(),
+                    "SIZE 42".to_owned(),
-                    "AUTH PLAIN CRAM-MD5".to_string(),
+                    "AUTH PLAIN CRAM-MD5".to_owned(),
                 ],
             }
         );
@@ -350,11 +352,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
         )
         .is_positive());
         assert!(!Response::new(
@@ -363,11 +361,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
         )
         .is_positive());
     }
@@ -380,11 +374,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
         )
         .has_code(451));
         assert!(!Response::new(
@@ -393,11 +383,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
         )
         .has_code(251));
     }
@@ -411,11 +397,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
             )
             .first_word(),
             Some("me")
@@ -428,9 +410,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
+                    "me mo".to_owned(),
-                    "8BITMIME".to_string(),
+                    "8BITMIME".to_owned(),
-                    "SIZE 42".to_string(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_word(),
@@ -455,7 +437,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_word(),
             None
@@ -467,7 +449,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_word(),
             None
@@ -479,7 +461,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["".to_string()],
+                vec!["".to_owned()],
             )
             .first_word(),
             None
@@ -492,7 +474,7 @@ mod test {
         let res = parse_response(raw_response);
         match res {
             Err(nom::Err::Incomplete(_)) => {}
-            _ => panic!("Expected incomplete response, got {:?}", res),
+            _ => panic!("Expected incomplete response, got {res:?}"),
         }
     }
 
@@ -505,11 +487,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
             )
             .first_line(),
             Some("me")
@@ -522,9 +500,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
+                    "me mo".to_owned(),
-                    "8BITMIME".to_string(),
+                    "8BITMIME".to_owned(),
-                    "SIZE 42".to_string(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_line(),
@@ -549,7 +527,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_line(),
             Some(" ")
@@ -561,7 +539,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_line(),
             Some("  ")
@@ -573,7 +551,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["".to_string()],
+                vec!["".to_owned()],
             )
             .first_line(),
             Some("")

src/transport/smtp/transport.rs

@@ -7,7 +7,7 @@ use super::pool::sync_impl::Pool;
 #[cfg(feature = "pool")]
 use super::PoolConfig;
 use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{address::Envelope, Transport};
 
@@ -45,8 +45,11 @@ impl SmtpTransport {
     ///
     /// Creates an encrypted transport over submissions port, using the provided domain
     /// to validate TLS certificates.
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -66,8 +69,11 @@ impl SmtpTransport {
     ///
     /// An error is returned if the connection can't be upgraded. No credentials
     /// or emails will be sent to the server, protecting from downgrade attacks.
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn starttls_relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -166,8 +172,11 @@ impl SmtpTransportBuilder {
     }
 
     /// Set the TLS settings to use
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn tls(mut self, tls: Tls) -> Self {
         self.info.tls = tls;
         self
@@ -210,7 +219,7 @@ impl SmtpClient {
     pub fn connection(&self) -> Result<SmtpConnection, Error> {
         #[allow(clippy::match_single_binding)]
         let tls_parameters = match self.info.tls {
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Tls::Wrapper(ref tls_parameters) => Some(tls_parameters),
             _ => None,
         };
@@ -221,9 +230,10 @@ impl SmtpClient {
             self.info.timeout,
             &self.info.hello_name,
             tls_parameters,
+            None,
         )?;
 
-        #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+        #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
         match self.info.tls {
             Tls::Opportunistic(ref tls_parameters) => {
                 if conn.can_starttls() {

src/transport/smtp/util.rs

@@ -41,7 +41,7 @@ mod tests {
         ]
         .iter()
         {
-            assert_eq!(format!("{}", XText(input)), expect.to_string());
+            assert_eq!(format!("{}", XText(input)), (*expect).to_owned());
         }
     }
 }

src/transport/stub/mod.rs

@@ -1,10 +1,12 @@
-//! The stub transport only logs message envelope and drops the content. It can be useful for
+//! The stub transport logs message envelopes as well as contents. It can be useful for testing
-//! testing purposes.
+//! purposes.
 //!
-//! #### Stub Transport
+//! # Stub Transport
 //!
-//! The stub transport returns provided result and drops the content. It can be useful for
+//! The stub transport logs message envelopes as well as contents. It can be useful for testing
-//! testing purposes.
+//! purposes.
+//!
+//! # Examples
 //!
 //! ```rust
 //! # #[cfg(feature = "builder")]
@@ -12,7 +14,7 @@
 //! use lettre::{transport::stub::StubTransport, Message, Transport};
 //!
 //! # use std::error::Error;
-//! # fn main() -> Result<(), Box<dyn Error>> {
+//! # fn try_main() -> Result<(), Box<dyn Error>> {
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
@@ -23,18 +25,36 @@
 //! let mut sender = StubTransport::new_ok();
 //! let result = sender.send(&email);
 //! assert!(result.is_ok());
+//! assert_eq!(
+//!     sender.messages(),
+//!     vec![(
+//!         email.envelope().clone(),
+//!         String::from_utf8(email.formatted()).unwrap()
+//!     )],
+//! );
 //! # Ok(())
 //! # }
+//! # try_main().unwrap();
 //! # }
 //! ```
 
+use std::{
+    error::Error as StdError,
+    fmt,
+    sync::{Arc, Mutex as StdMutex},
+};
+
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+use async_trait::async_trait;
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+use futures_util::lock::Mutex as FuturesMutex;
+
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 use crate::AsyncTransport;
 use crate::{address::Envelope, Transport};
-#[cfg(any(feature = "tokio1", feature = "async-std1"))]
-use async_trait::async_trait;
-use std::{error::Error as StdError, fmt};
 
+/// An error returned by the stub transport
+#[non_exhaustive]
 #[derive(Debug, Copy, Clone)]
 pub struct Error;
 
@@ -46,47 +66,113 @@ impl fmt::Display for Error {
 
 impl StdError for Error {}
 
-/// This transport logs the message envelope and returns the given response
+/// This transport logs messages and always returns the given response
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone)]
 pub struct StubTransport {
     response: Result<(), Error>,
+    message_log: Arc<StdMutex<Vec<(Envelope, String)>>>,
+}
+
+/// This transport logs messages and always returns the given response
+#[derive(Debug, Clone)]
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+#[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
+pub struct AsyncStubTransport {
+    response: Result<(), Error>,
+    message_log: Arc<FuturesMutex<Vec<(Envelope, String)>>>,
 }
 
 impl StubTransport {
     /// Creates a new transport that always returns the given Result
-    pub fn new(response: Result<(), Error>) -> StubTransport {
+    pub fn new(response: Result<(), Error>) -> Self {
-        StubTransport { response }
+        Self {
+            response,
+            message_log: Arc::new(StdMutex::new(vec![])),
+        }
     }
 
     /// Creates a new transport that always returns a success response
-    pub fn new_ok() -> StubTransport {
+    pub fn new_ok() -> Self {
-        StubTransport { response: Ok(()) }
+        Self {
+            response: Ok(()),
+            message_log: Arc::new(StdMutex::new(vec![])),
+        }
     }
 
     /// Creates a new transport that always returns an error
-    pub fn new_error() -> StubTransport {
+    pub fn new_error() -> Self {
-        StubTransport {
+        Self {
             response: Err(Error),
+            message_log: Arc::new(StdMutex::new(vec![])),
         }
     }
+
+    /// Return all logged messages sent using [`Transport::send_raw`]
+    pub fn messages(&self) -> Vec<(Envelope, String)> {
+        self.message_log
+            .lock()
+            .expect("Couldn't acquire lock to write message log")
+            .clone()
+    }
+}
+
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+impl AsyncStubTransport {
+    /// Creates a new transport that always returns the given Result
+    pub fn new(response: Result<(), Error>) -> Self {
+        Self {
+            response,
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Creates a new transport that always returns a success response
+    pub fn new_ok() -> Self {
+        Self {
+            response: Ok(()),
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Creates a new transport that always returns an error
+    pub fn new_error() -> Self {
+        Self {
+            response: Err(Error),
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Return all logged messages sent using [`AsyncTransport::send_raw`]
+    #[cfg(any(feature = "tokio1", feature = "async-std1"))]
+    pub async fn messages(&self) -> Vec<(Envelope, String)> {
+        self.message_log.lock().await.clone()
+    }
 }
 
 impl Transport for StubTransport {
     type Ok = ();
     type Error = Error;
 
-    fn send_raw(&self, _envelope: &Envelope, _email: &[u8]) -> Result<Self::Ok, Self::Error> {
+    fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        self.message_log
+            .lock()
+            .expect("Couldn't acquire lock to write message log")
+            .push((envelope.clone(), String::from_utf8_lossy(email).into()));
         self.response
     }
 }
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 #[async_trait]
-impl AsyncTransport for StubTransport {
+impl AsyncTransport for AsyncStubTransport {
     type Ok = ();
     type Error = Error;
 
-    async fn send_raw(&self, _envelope: &Envelope, _email: &[u8]) -> Result<Self::Ok, Self::Error> {
+    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        self.message_log
+            .lock()
+            .await
+            .push((envelope.clone(), String::from_utf8_lossy(email).into()));
         self.response
     }
 }

testdata/coredns.conf

@@ -0,0 +1,7 @@
+. {
+  bind 127.0.0.54
+  forward . 9.9.9.9 8.8.8.8 1.1.1.1 {
+    except example.org
+  }
+  file testdata/db.example.org example.org
+}

testdata/db.example.org

@@ -0,0 +1,2 @@
+@       600     IN      SOA     ns.example.org hostmaster.example.org 1 10800 3600 604800 3600
+dkimtest._domainkey 600 IN TXT  "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAz+FHbM8BwkBBz/Ux5OYLQ5Bp1HVuCHTP6Rr3HXTnome/2cGl/ze0tsmmFbCjjsS89MXbMGs9xJhjv18LmL1N0UTllblOizzVjorQyN4RwBOfG34j7SS56pwzrA738Ry8FAbL5InPWEgVzbOhXuTCs8yuzcqTnm4sH/csnIl7cMWeQkVn1FR9LKMtUG0fjhDPkdX0jx3qTX1L3Z7a7gX6geY191yNd9i9DvE2/+wMigMYz1LAts4alk2g86MQhtbjc8AOR7EC15hSw37/lmamlunYLa3wC+PzHNMA8sAfnmkgNvipssjh8LnelD9qn+VtsjQB5ppkeQx3TcUPvz5z+QIDAQAB"

testdata/email_with_png.eml

@@ -1,10 +1,10 @@
-Date: Tue, 15 Nov 1994 08:12:31 -0000
+Date: Tue, 15 Nov 1994 08:12:31 +0000
 From: NoBody <nobody@domain.tld>
 Reply-To: Yuin <yuin@domain.tld>
 To: Hei <hei@domain.tld>
 Subject: Happy new year
 MIME-Version: 1.0
-Content-Type: multipart/related; 
+Content-Type: multipart/related;
  boundary="GUEEoEeTXtLcK2sMhmH1RfC1co13g4rtnRUFjQFA"
 
 --0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1

tests/transport_file.rs

@@ -9,13 +9,15 @@ fn default_date() -> std::time::SystemTime {
 #[cfg(test)]
 #[cfg(all(feature = "file-transport", feature = "builder"))]
 mod sync {
-    use crate::default_date;
-    use lettre::{FileTransport, Message, Transport};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{FileTransport, Message, Transport};
+
+    use crate::default_date;
+
     #[test]
     fn file_transport() {
         let sender = FileTransport::new(temp_dir());
@@ -31,7 +33,7 @@ mod sync {
         let result = sender.send(&email);
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(
@@ -41,7 +43,7 @@ mod sync {
                 "Reply-To: Yuin <yuin@domain.tld>\r\n",
                 "To: Hei <hei@domain.tld>\r\n",
                 "Subject: Happy new year\r\n",
-                "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Be happy!"
@@ -66,10 +68,10 @@ mod sync {
         let result = sender.send(&email);
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
-        let json_file = temp_dir().join(format!("{}.json", id));
+        let json_file = temp_dir().join(format!("{id}.json"));
         let json = read_to_string(&json_file).unwrap();
 
         assert_eq!(
@@ -79,7 +81,7 @@ mod sync {
                 "Reply-To: Yuin <yuin@domain.tld>\r\n",
                 "To: Hei <hei@domain.tld>\r\n",
                 "Subject: Happy new year\r\n",
-                "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Be happy!"
@@ -104,15 +106,16 @@ mod sync {
 #[cfg(test)]
 #[cfg(all(feature = "file-transport", feature = "builder", feature = "tokio1"))]
 mod tokio_1 {
-    use crate::default_date;
-    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
     use tokio1_crate as tokio;
 
+    use crate::default_date;
+
     #[tokio::test]
     async fn file_transport_tokio1() {
         let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -128,7 +131,7 @@ mod tokio_1 {
         let result = sender.send(email).await;
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(
@@ -138,7 +141,7 @@ mod tokio_1 {
                 "Reply-To: Yuin <yuin@domain.tld>\r\n",
                 "To: Hei <hei@domain.tld>\r\n",
                 "Subject: Happy new year\r\n",
-                "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Be happy!"
@@ -155,13 +158,15 @@ mod tokio_1 {
     feature = "async-std1"
 ))]
 mod asyncstd_1 {
-    use crate::default_date;
-    use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
+
+    use crate::default_date;
+
     #[async_std::test]
     async fn file_transport_asyncstd1() {
         let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
@@ -177,7 +182,7 @@ mod asyncstd_1 {
         let result = sender.send(email).await;
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(
@@ -187,7 +192,7 @@ mod asyncstd_1 {
                 "Reply-To: Yuin <yuin@domain.tld>\r\n",
                 "To: Hei <hei@domain.tld>\r\n",
                 "Subject: Happy new year\r\n",
-                "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Be happy!"

tests/transport_sendmail.rs

@@ -15,7 +15,7 @@ mod sync {
             .unwrap();
 
         let result = sender.send(&email);
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }
@@ -42,7 +42,7 @@ mod tokio_1 {
             .unwrap();
 
         let result = sender.send(email).await;
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }
@@ -68,7 +68,7 @@ mod asyncstd_1 {
             .unwrap();
 
         let result = sender.send(email).await;
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }

tests/transport_smtp.rs

@@ -24,7 +24,6 @@ mod sync {
 #[cfg(all(feature = "smtp-transport", feature = "builder", feature = "tokio1"))]
 mod tokio_1 {
     use lettre::{AsyncSmtpTransport, AsyncTransport, Message, Tokio1Executor};
-
     use tokio1_crate as tokio;
 
     #[tokio::test]

tests/transport_smtp_pool.rs

@@ -1,8 +1,9 @@
 #[cfg(all(test, feature = "smtp-transport", feature = "pool"))]
 mod sync {
-    use lettre::{address::Envelope, SmtpTransport, Transport};
     use std::{sync::mpsc, thread};
 
+    use lettre::{address::Envelope, SmtpTransport, Transport};
+
     fn envelope() -> Envelope {
         Envelope::new(
             Some("user@localhost".parse().unwrap()),

tests/transport_stub.rs

@@ -17,20 +17,25 @@ mod sync {
 
         sender_ok.send(&email).unwrap();
         sender_ko.send(&email).unwrap_err();
+
+        let expected_messages = [(
+            email.envelope().clone(),
+            String::from_utf8(email.formatted()).unwrap(),
+        )];
+        assert_eq!(sender_ok.messages(), expected_messages);
     }
 }
 
 #[cfg(test)]
 #[cfg(all(feature = "builder", feature = "tokio1"))]
 mod tokio_1 {
-    use lettre::{transport::stub::StubTransport, AsyncTransport, Message};
+    use lettre::{transport::stub::AsyncStubTransport, AsyncTransport, Message};
-
     use tokio1_crate as tokio;
 
     #[tokio::test]
     async fn stub_transport_tokio1() {
-        let sender_ok = StubTransport::new_ok();
+        let sender_ok = AsyncStubTransport::new_ok();
-        let sender_ko = StubTransport::new_error();
+        let sender_ko = AsyncStubTransport::new_error();
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
             .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
@@ -40,19 +45,25 @@ mod tokio_1 {
             .unwrap();
 
         sender_ok.send(email.clone()).await.unwrap();
-        sender_ko.send(email).await.unwrap_err();
+        sender_ko.send(email.clone()).await.unwrap_err();
+
+        let expected_messages = [(
+            email.envelope().clone(),
+            String::from_utf8(email.formatted()).unwrap(),
+        )];
+        assert_eq!(sender_ok.messages().await, expected_messages);
     }
 }
 
 #[cfg(test)]
 #[cfg(all(feature = "builder", feature = "async-std1"))]
 mod asyncstd_1 {
-    use lettre::{transport::stub::StubTransport, AsyncTransport, Message};
+    use lettre::{transport::stub::AsyncStubTransport, AsyncTransport, Message};
 
     #[async_std::test]
     async fn stub_transport_asyncstd1() {
-        let sender_ok = StubTransport::new_ok();
+        let sender_ok = AsyncStubTransport::new_ok();
-        let sender_ko = StubTransport::new_error();
+        let sender_ko = AsyncStubTransport::new_error();
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
             .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
@@ -62,6 +73,12 @@ mod asyncstd_1 {
             .unwrap();
 
         sender_ok.send(email.clone()).await.unwrap();
-        sender_ko.send(email).await.unwrap_err();
+        sender_ko.send(email.clone()).await.unwrap_err();
+
+        let expected_messages = [(
+            email.envelope().clone(),
+            String::from_utf8(email.formatted()).unwrap(),
+        )];
+        assert_eq!(sender_ok.messages().await, expected_messages);
     }
 }