Prepare 0.10.4 (#871)

Write a trace message when sending an email. Further, write a debug
message when using the sendmail transport method, containing which
program is called. And write a debug message containing the target
file name when the file transport method is used.

This should help improve #556 a tiny bit.

Co-authored-by: finga <finga@onders.org>

.github/workflows/test.yml

@@ -13,7 +13,7 @@ env:
 
 jobs:
   rustfmt:
-    name: rustfmt / nightly-2022-02-11
+    name: rustfmt / nightly-2022-11-12
     runs-on: ubuntu-latest
 
     steps:
@@ -22,7 +22,7 @@ jobs:
 
       - name: Install rust
         run: |
-          rustup default nightly-2022-02-11
+          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: 1.56.0
-            rust: 1.56.0
+          - name: 1.60.0
+            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
@@ -133,9 +121,12 @@ jobs:
       - name: Test with default features
         run: cargo test
 
-      - name: Test with all features
-        run: cargo test --all-features
+      - name: Test with all features (-native-tls)
+        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

@@ -1,5 +1,103 @@
+<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
 
@@ -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.7"
+version = "0.10.4"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -11,7 +11,7 @@ authors = ["Alexis Mousset <contact@amousset.me>", "Paolo Barbolini <paolo@paolo
 categories = ["email", "network-programming"]
 keywords = ["email", "smtp", "mailer", "message", "sendmail"]
 edition = "2021"
-rust-version = "1.56"
+rust-version = "1.60"
 
 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -19,7 +19,7 @@ is-it-maintained-open-issues = { repository = "lettre/lettre" }
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-idna = "0.2"
+idna = "0.3"
 once_cell = { version = "1", optional = true }
 tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # feature
 
@@ -27,9 +27,9 @@ tracing = { version = "0.1.16", default-features = false, features = ["std"], op
 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 }
-base64 = { version = "0.13", optional = true }
-email-encoding = { version = "0.1.1", optional = true }
+quoted_printable = { version = "0.4.6", optional = true }
+base64 = { version = "0.21", optional = true }
+email-encoding = { version = "0.2", optional = true }
 
 # file transport
 uuid = { version = "1", features = ["v4"], optional = true }
@@ -43,9 +43,11 @@ 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 = "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 }
@@ -53,34 +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", "rt", "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 }
-rsa = { version = "0.6.0", optional = true }
+sha2 = { version = "0.10", optional = true, features = ["oid"] }
+rsa = { version = "0.8", optional = true }
 ed25519-dalek = { version = "1.0.1", optional = true }
-regex = { version = "1", default-features = false, features = ["std"], optional = true }
 
 # email formats
 email_address = { version = "0.2.1", default-features = false }
 
 [dev-dependencies]
 pretty_assertions = "1"
-criterion = "0.3"
+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 = "0.23"
+maud = "0.24"
 
 [[bench]]
 harness = false
@@ -92,15 +95,17 @@ 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 = []
-smtp-transport = ["base64", "nom", "socket2", "once_cell"]
+sendmail-transport = ["tokio1_crate?/process", "tokio1_crate?/io-util", "async-std?/unstable"]
+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"]
@@ -108,13 +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", "regex", "once_cell"]
+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"]

README.md

@@ -28,27 +28,14 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0-rc.7">
-    <img src="https://deps.rs/crate/lettre/0.10.0-rc.7/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.10.4">
+    <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 1.56.0 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.7"
+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")
@@ -101,6 +95,14 @@ 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 

examples/asyncstd1_smtp_starttls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
-    AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    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,
-    AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    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

@@ -2,8 +2,8 @@
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
-    Tokio1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncTransport, Message, Tokio1Executor,
 };
 use tokio1_crate as tokio;
 
@@ -16,10 +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> =
@@ -31,6 +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

@@ -2,8 +2,8 @@
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
-    Tokio1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncTransport, Message, Tokio1Executor,
 };
 use tokio1_crate as tokio;
 
@@ -16,10 +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> =
@@ -31,6 +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:?}"),
     }
 }

src/address/types.rs

@@ -184,7 +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(),
@@ -226,7 +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

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

@@ -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,
@@ -166,13 +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
@@ -210,7 +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
@@ -220,14 +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,
@@ -267,13 +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
@@ -289,15 +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 1.56.0 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_
@@ -100,7 +109,7 @@
 //! [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.7")]
+#![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)]
@@ -112,12 +121,32 @@
     unused_import_braces,
     rust_2018_idioms,
     clippy::string_add,
-    clippy::string_add_assign
+    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",
@@ -136,6 +165,15 @@ mod compiletime_checks {
     If you'd like to use `native-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(
+        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(
         feature = "async-std1",
@@ -167,6 +205,8 @@ Make sure to apply the same to any of your crate dependencies that use the `lett
 }
 
 pub mod address;
+#[cfg(any(feature = "smtp-transport", feature = "dkim"))]
+mod base64;
 pub mod error;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 mod executor;

src/message/attachment.rs

@@ -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/dkim.rs

@@ -7,9 +7,7 @@ use std::{
 };
 
 use ed25519_dalek::Signer;
-use once_cell::sync::Lazy;
-use regex::bytes::Regex;
-use rsa::{pkcs1::DecodeRsaPrivateKey, Hash, PaddingScheme, RsaPrivateKey};
+use rsa::{pkcs1::DecodeRsaPrivateKey, pkcs1v15::Pkcs1v15Sign, RsaPrivateKey};
 use sha2::{Digest, Sha256};
 
 use crate::message::{
@@ -96,9 +94,9 @@ impl Display for DkimSigningKeyError {
 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,
+            InnerDkimSigningKeyError::Base64(err) => err,
+            InnerDkimSigningKeyError::Rsa(err) => err,
+            InnerDkimSigningKeyError::Ed25519(err) => err,
         })
     }
 }
@@ -123,14 +121,14 @@ impl DkimSigningKey {
                 RsaPrivateKey::from_pkcs1_pem(private_key)
                     .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Rsa(err)))?,
             ),
-            DkimSigningAlgorithm::Ed25519 => {
-                InnerDkimSigningKey::Ed25519(
-                    ed25519_dalek::Keypair::from_bytes(&base64::decode(private_key).map_err(
-                        |err| DkimSigningKeyError(InnerDkimSigningKeyError::Base64(err)),
-                    )?)
-                    .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Ed25519(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 {
@@ -219,24 +217,34 @@ fn dkim_header_format(
 
 /// Canonicalize the body of an email
 fn dkim_canonicalize_body(
-    body: &[u8],
+    mut body: &[u8],
     canonicalization: DkimCanonicalizationType,
 ) -> Cow<'_, [u8]> {
-    static RE: Lazy<Regex> = Lazy::new(|| Regex::new("(\r\n)+$").unwrap());
-    static RE_DOUBLE_SPACE: Lazy<Regex> = Lazy::new(|| Regex::new("[\\t ]+").unwrap());
-    static RE_SPACE_EOL: Lazy<Regex> = Lazy::new(|| Regex::new("[\t ]\r\n").unwrap());
     match canonicalization {
-        DkimCanonicalizationType::Simple => RE.replace(body, &b"\r\n"[..]),
-        DkimCanonicalizationType::Relaxed => {
-            let body = RE_DOUBLE_SPACE.replace_all(body, &b" "[..]);
-            let body = match RE_SPACE_EOL.replace_all(&body, &b"\r\n"[..]) {
-                Cow::Borrowed(_body) => body,
-                Cow::Owned(body) => Cow::Owned(body),
-            };
-            match RE.replace(&body, &b"\r\n"[..]) {
-                Cow::Borrowed(_body) => body,
-                Cow::Owned(body) => Cow::Owned(body),
+        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)
         }
     }
 }
@@ -245,7 +253,7 @@ 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().get(0) {
+        match h.as_bytes().first() {
             Some(b' ' | b'\t') => skip_whitespace(&h[1..]),
             _ => h,
         }
@@ -346,11 +354,11 @@ fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timesta
         .unwrap()
         .as_secs();
     let headers = message.headers();
-    let body_hash = Sha256::digest(&dkim_canonicalize_body(
+    let body_hash = Sha256::digest(dkim_canonicalize_body(
         &message.body_raw(),
         dkim_config.canonicalization.body,
     ));
-    let bh = base64::encode(body_hash);
+    let bh = crate::base64::encode(body_hash);
     let mut signed_headers_list =
         dkim_config
             .headers
@@ -382,16 +390,13 @@ fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timesta
     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) => base64::encode(
+        InnerDkimSigningKey::Rsa(private_key) => crate::base64::encode(
             private_key
-                .sign(
-                    PaddingScheme::new_pkcs1v15_sign(Some(Hash::SHA2_256)),
-                    &hashed_headers,
-                )
+                .sign(Pkcs1v15Sign::new::<Sha256>(), &hashed_headers)
                 .unwrap(),
         ),
         InnerDkimSigningKey::Ed25519(private_key) => {
-            base64::encode(private_key.sign(&hashed_headers).to_bytes())
+            crate::base64::encode(private_key.sign(&hashed_headers).to_bytes())
         }
     };
     let dkim_header = dkim_header_format(
@@ -416,8 +421,9 @@ mod test {
             header::{HeaderName, HeaderValue},
             Header, Message,
         },
-        dkim_canonicalize_headers, dkim_sign_fixed_time, DkimCanonicalization,
-        DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm, DkimSigningKey,
+        dkim_canonicalize_body, dkim_canonicalize_headers, dkim_sign_fixed_time,
+        DkimCanonicalization, DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm,
+        DkimSigningKey,
     };
     use crate::StdError;
 
@@ -471,9 +477,9 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
             .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_string()))
+            .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_string()).unwrap()
+            .body("test\r\n\r\ntest   \ttest\r\n\r\n\r\n".to_owned()).unwrap()
     }
 
     #[test]
@@ -490,6 +496,24 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         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();
@@ -497,8 +521,8 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         dkim_sign_fixed_time(
             &mut message,
             &DkimConfig::new(
-                "dkimtest".to_string(),
-                "example.org".to_string(),
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
                 signing_key,
                 vec![
                     HeaderName::new_from_ascii_str("Date"),
@@ -546,8 +570,8 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         dkim_sign_fixed_time(
             &mut message,
             &DkimConfig::new(
-                "dkimtest".to_string(),
-                "example.org".to_string(),
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
                 signing_key,
                 vec![
                     HeaderName::new_from_ascii_str("Date"),
@@ -564,7 +588,7 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         );
         let signed = message.formatted();
         let signed = std::str::from_utf8(&signed).unwrap();
-        println!("{}", signed);
+        println!("{signed}");
         assert_eq!(
             signed,
             std::concat!(

src/message/header/content.rs

@@ -99,7 +99,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "7bit".to_string(),
+            "7bit".to_owned(),
         ));
 
         assert_eq!(
@@ -109,7 +109,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "base64".to_string(),
+            "base64".to_owned(),
         ));
 
         assert_eq!(

src/message/header/content_disposition.rs

@@ -16,8 +16,8 @@ impl ContentDisposition {
     pub fn inline() -> Self {
         Self(HeaderValue::dangerous_new_pre_encoded(
             Self::name(),
-            "inline".to_string(),
-            "inline".to_string(),
+            "inline".to_owned(),
+            "inline".to_owned(),
         ))
     }
 
@@ -33,17 +33,19 @@ impl ContentDisposition {
     }
 
     fn with_name(kind: &str, file_name: &str) -> Self {
-        let raw_value = format!("{}; filename=\"{}\"", kind, file_name);
+        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, false);
-        w.write_str(kind).expect("writing `kind` returned an error");
-        w.write_char(';').expect("writing `;` returned an error");
-        w.space();
+        {
+            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");
+            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(),
@@ -88,12 +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"
         );
     }
@@ -104,7 +106,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
-            "inline".to_string(),
+            "inline".to_owned(),
         ));
 
         assert_eq!(
@@ -114,7 +116,7 @@ mod test {
 
         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!(

src/message/header/content_type.rs

@@ -135,8 +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: {}",
-                            mime
+                            "Couldn't parse the following MIME-Type: {mime}"
                         ))),
                     }
                 }
@@ -179,14 +178,14 @@ mod test {
 
         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(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

@@ -8,7 +8,7 @@ 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 {
@@ -90,7 +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
@@ -110,7 +110,7 @@ mod test {
 
         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!(
@@ -122,7 +122,7 @@ mod test {
 
         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!(

src/message/header/mailbox.rs

@@ -14,7 +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 {
@@ -30,8 +30,10 @@ macro_rules! mailbox_header {
             fn display(&self) -> HeaderValue {
                 let mut encoded_value = String::new();
                 let line_len = $header_name.len() + ": ".len();
-                let mut w = EmailWriter::new(&mut encoded_value, line_len, false);
-                self.0.encode(&mut w).expect("writing `Mailbox` returned an error");
+                {
+                    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)
             }
@@ -56,7 +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 {
@@ -78,8 +80,10 @@ macro_rules! mailboxes_header {
             fn display(&self) -> HeaderValue {
                 let mut encoded_value = String::new();
                 let line_len = $header_name.len() + ": ".len();
-                let mut w = EmailWriter::new(&mut encoded_value, line_len, false);
-                self.0.encode(&mut w).expect("writing `Mailboxes` returned an error");
+                {
+                    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)
             }
@@ -248,7 +252,7 @@ mod test {
         let mut headers = Headers::new();
         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)));
@@ -261,7 +265,7 @@ mod test {
         let mut headers = Headers::new();
         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)));
@@ -277,7 +281,7 @@ mod test {
         let mut headers = Headers::new();
         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())));
@@ -293,7 +297,7 @@ mod test {
         let mut headers = Headers::new();
         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())));
@@ -309,7 +313,7 @@ mod test {
         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_string(),
+            "Test, test <1@example.com>, Test2, test2 <2@example.com>".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
@@ -320,7 +324,7 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Test, test <1@example.com>, Test2, test2".to_string(),
+            "Test, test <1@example.com>, Test2, test2".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), None);

src/message/header/mod.rs

@@ -277,6 +277,7 @@ impl PartialEq<HeaderName> for &str {
     }
 }
 
+/// A safe for use header value
 #[derive(Debug, Clone, PartialEq)]
 pub struct HeaderValue {
     name: HeaderName,
@@ -285,6 +286,12 @@ pub struct HeaderValue {
 }
 
 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();
@@ -296,6 +303,14 @@ impl HeaderValue {
         }
     }
 
+    /// 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,
@@ -308,10 +323,12 @@ impl HeaderValue {
         }
     }
 
+    #[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
     }
@@ -325,28 +342,21 @@ struct HeaderValueEncoder<'a> {
 
 impl<'a> HeaderValueEncoder<'a> {
     fn encode(name: &str, value: &'a str, f: &'a mut impl fmt::Write) -> fmt::Result {
-        let (words_iter, encoder) = Self::new(name, value, f);
-        encoder.format(words_iter)
+        let encoder = Self::new(name, f);
+        encoder.format(value.split_inclusive(' '))
     }
 
-    fn new(
-        name: &str,
-        value: &'a str,
-        writer: &'a mut dyn Write,
-    ) -> (WordsPlusFillIterator<'a>, Self) {
+    fn new(name: &str, writer: &'a mut dyn Write) -> Self {
         let line_len = name.len() + ": ".len();
-        let writer = EmailWriter::new(writer, line_len, false);
+        let writer = EmailWriter::new(writer, line_len, 0, false, false);
 
-        (
-            WordsPlusFillIterator { s: value },
-            Self {
-                writer,
-                encode_buf: String::new(),
-            },
-        )
+        Self {
+            writer,
+            encode_buf: String::new(),
+        }
     }
 
-    fn format(mut self, words_iter: WordsPlusFillIterator<'_>) -> fmt::Result {
+    fn format(mut self, words_iter: impl Iterator<Item = &'a str>) -> fmt::Result {
         for next_word in words_iter {
             let allowed = allowed_str(next_word);
 
@@ -374,71 +384,20 @@ impl<'a> HeaderValueEncoder<'a> {
             return Ok(());
         }
 
-        // It is important that we don't encode leading whitespace otherwise it breaks wrapping.
-        let first_not_allowed = self
-            .encode_buf
-            .bytes()
-            .enumerate()
-            .find(|(_i, c)| !allowed_char(*c))
-            .map(|(i, _)| i);
-        // May as well also write the tail in plain text.
-        let last_not_allowed = self
-            .encode_buf
-            .bytes()
-            .enumerate()
-            .rev()
-            .find(|(_i, c)| !allowed_char(*c))
-            .map(|(i, _)| i + 1);
+        let prefix = self.encode_buf.trim_end_matches(' ');
+        email_encoding::headers::rfc2047::encode(prefix, &mut self.writer)?;
 
-        let (prefix, to_encode, suffix) = match first_not_allowed {
-            Some(first_not_allowed) => {
-                let last_not_allowed = last_not_allowed.unwrap();
-
-                let (remaining, suffix) = self.encode_buf.split_at(last_not_allowed);
-                let (prefix, to_encode) = remaining.split_at(first_not_allowed);
-
-                (prefix, to_encode, suffix)
-            }
-            None => ("", self.encode_buf.as_str(), ""),
-        };
-
-        self.writer.folding().write_str(prefix)?;
-        email_encoding::headers::rfc2047::encode(to_encode, &mut self.writer)?;
-        self.writer.folding().write_str(suffix)?;
+        // TODO: add a better API for doing this in email-encoding
+        let spaces = self.encode_buf.len() - prefix.len();
+        for _ in 0..spaces {
+            self.writer.space();
+        }
 
         self.encode_buf.clear();
         Ok(())
     }
 }
 
-/// Iterator yielding a string split by space, but spaces are included before 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
-            .bytes()
-            .enumerate()
-            .skip(1)
-            .find(|&(_i, c)| c == b' ')
-            .map(|(i, _)| i)
-            .unwrap_or(self.s.len());
-
-        let word = &self.s[..next_word];
-        self.s = &self.s[word.len()..];
-        Some(word)
-    }
-}
-
 fn allowed_str(s: &str) -> bool {
     s.bytes().all(allowed_char)
 }
@@ -451,7 +410,8 @@ const fn allowed_char(c: u8) -> bool {
 mod tests {
     use pretty_assertions::assert_eq;
 
-    use super::{HeaderName, HeaderValue, Headers};
+    use super::{HeaderName, HeaderValue, Headers, To};
+    use crate::message::Mailboxes;
 
     #[test]
     fn valid_headername() {
@@ -514,7 +474,7 @@ mod tests {
         let mut headers = Headers::new();
         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!(
@@ -528,7 +488,7 @@ mod tests {
         let mut headers = Headers::new();
         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!(
@@ -546,7 +506,7 @@ mod tests {
         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()
         ));
 
         assert_eq!(
@@ -565,7 +525,7 @@ mod tests {
         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!(
@@ -583,7 +543,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_string()
+            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_owned()
         ));
 
         assert_eq!(
@@ -597,12 +557,12 @@ mod tests {
         let mut headers = Headers::new();
         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(),
-            "To: Se=?utf-8?b?w6E=?=n <sean@example.com>\r\n"
+            "To: =?utf-8?b?U2XDoW4=?= <sean@example.com>\r\n"
         );
     }
 
@@ -611,7 +571,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "🌎 <world@example.com>".to_string(),
+            "🌎 <world@example.com>".to_owned(),
         ));
 
         assert_eq!(
@@ -623,19 +583,46 @@ mod tests {
     #[test]
     fn format_special_with_folding() {
         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_string(),
-         ) );
+        let to = To::from(Mailboxes::from_iter([
+            "🌍 <world@example.com>".parse().unwrap(),
+            "🦆 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==?= Everywhere\r\n",
-                " <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9INCY0LLQsNC9?=\r\n",
-                " =?utf-8?b?0L7QstC40Yc=?= <ivanov@example.com>, J=?utf-8?b?xIFuaXMgQsST?=\r\n",
-                " =?utf-8?b?cnppxYbFoQ==?= <janis@example.com>, Se=?utf-8?b?w6FuIMOTIFJ1?=\r\n",
-                " =?utf-8?b?ZGHDrQ==?= <sean@example.com>\r\n",
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhiBFdmVyeXdo?=\r\n",
+                " =?utf-8?b?ZXJl?= <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LI=?=\r\n",
+                " =?utf-8?b?0LDQvSDQmNCy0LDQvdC+0LLQuNGH?= <ivanov@example.com>,\r\n",
+                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, =?utf-8?b?U2U=?=\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",
             )
         );
     }
@@ -646,7 +633,7 @@ mod tests {
         headers.insert_raw(
             HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_string(),)
+            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_owned(),)
         );
 
         assert_eq!(
@@ -667,7 +654,7 @@ mod tests {
         let mut headers = Headers::new();
         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!(
@@ -682,35 +669,38 @@ mod tests {
         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(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Someone <somewhere@example.com>".to_string(),
+            "Someone <somewhere@example.com>".to_owned(),
         ));
         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",
                 " 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==?= Everywhere\r\n",
-                " <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9INCY0LLQsNC9?=\r\n",
-                " =?utf-8?b?0L7QstC40Yc=?= <ivanov@example.com>, J=?utf-8?b?xIFuaXMgQsST?=\r\n",
-                " =?utf-8?b?cnppxYbFoQ==?= <janis@example.com>, Se=?utf-8?b?w6FuIMOTIFJ1?=\r\n",
-                " =?utf-8?b?ZGHDrQ==?= <sean@example.com>\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?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",
                 "From: Someone <somewhere@example.com>\r\n",
                 "Content-Transfer-Encoding: quoted-printable\r\n",
             )
@@ -722,14 +712,14 @@ mod tests {
         let mut headers = Headers::new();
         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",
-                " ;;;;;s;;;;;;;;;;;;;;;;fffeinmjggggggggg=?utf-8?b?772G44Gj?=\r\n"
+                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go; =?utf-8?b?Ozs7OztzOzs7Ozs7Ozs7?=\r\n",
+                " =?utf-8?b?Ozs7Ozs7O2ZmZmVpbm1qZ2dnZ2dnZ2dn772G44Gj?=\r\n",
             )
         );
     }

src/message/header/special.rs

@@ -4,7 +4,7 @@ use crate::{
 };
 
 /// 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
@@ -88,14 +91,14 @@ mod test {
 
         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(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

@@ -4,7 +4,7 @@ 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 {
@@ -109,12 +109,23 @@ 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(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Sample subject".to_string(),
+            "Sample subject".to_owned(),
         ));
 
         assert_eq!(

src/message/mailbox/serde.rs

@@ -13,7 +13,7 @@ impl Serialize for Mailbox {
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 
@@ -111,7 +111,7 @@ impl Serialize for Mailboxes {
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 

src/message/mailbox/types.rs

@@ -70,7 +70,7 @@ impl Mailbox {
     pub(crate) fn encode(&self, w: &mut EmailWriter<'_>) -> FmtResult {
         if let Some(name) = &self.name {
             email_encoding::headers::quoted_string::encode(name, w)?;
-            w.space();
+            w.optional_breakpoint();
             w.write_char('<')?;
         }
 
@@ -275,7 +275,7 @@ impl Mailboxes {
         for mailbox in self.iter() {
             if !mem::take(&mut first) {
                 w.write_char(',')?;
-                w.space();
+                w.optional_breakpoint();
             }
 
             mailbox.encode(w)?;
@@ -315,6 +315,18 @@ 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>;
@@ -324,14 +336,6 @@ 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();
@@ -393,7 +397,7 @@ fn write_word(f: &mut Formatter<'_>, s: &str) -> FmtResult {
     } else {
         // Quoted string: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
         f.write_char('"')?;
-        for &c in s.as_bytes() {
+        for c in s.chars() {
             write_quoted_string_char(f, c)?;
         }
         f.write_char('"')?;
@@ -437,34 +441,37 @@ fn is_valid_atom_char(c: u8) -> bool {
 }
 
 // https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
-fn write_quoted_string_char(f: &mut Formatter<'_>, c: u8) -> FmtResult {
+fn write_quoted_string_char(f: &mut Formatter<'_>, c: char) -> FmtResult {
     match c {
-		// NO-WS-CTL: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
-		1..=8 | 11 | 12 | 14..=31 | 127 |
+        // Can not be encoded.
+        '\n' | '\r' => Err(std::fmt::Error),
 
-		// Note, not qcontent but can be put before or after any qcontent.
-		b'\t' |
-		b' ' |
+        // Note, not qcontent but can be put before or after any qcontent.
+        '\t' | ' ' => f.write_char(c),
 
-		// The rest of the US-ASCII except \ and "
-		33 |
-		35..=91 |
-		93..=126 |
+        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 |
 
-		// Non-ascii characters will be escaped separately later.
-		128..=255
+            // The rest of the US-ASCII except \ and "
+            33 |
+            35..=91 |
+            93..=126 |
 
-		=> f.write_char(c.into()),
+            // Non-ascii characters will be escaped separately later.
+            128.. => true,
+            _ => false,
+        } =>
+        {
+            f.write_char(c)
+        }
 
-		// Can not be encoded.
-		b'\n' | b'\r' => Err(std::fmt::Error),
-
-		c => {
-			// quoted-pair https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
-			f.write_char('\\')?;
-			f.write_char(c.into())
-		}
-	}
+        _ => {
+            // quoted-pair https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
+            f.write_char('\\')?;
+            f.write_char(c)
+        }
+    }
 }
 
 #[cfg(test)]
@@ -511,6 +518,34 @@ mod test {
         );
     }
 
+    #[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!(
@@ -585,7 +620,7 @@ 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

@@ -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(),
             }
         )

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!
@@ -232,6 +234,7 @@ trait EmailFormat {
 pub struct MessageBuilder {
     headers: Headers,
     envelope: Option<Envelope>,
+    drop_bcc: bool,
 }
 
 impl MessageBuilder {
@@ -240,24 +243,26 @@ impl MessageBuilder {
         Self {
             headers: Headers::new(),
             envelope: None,
+            drop_bcc: true,
         }
     }
 
-    /// Set custom header to message
-    pub fn header<H: Header>(mut self, header: H) -> Self {
-        self.headers.set(header);
-        self
+    /// 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)))
     }
 
-    /// 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),
-        }
+    /// 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))
     }
 
     /// Add `Date` header to message
@@ -275,41 +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).
@@ -352,6 +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)
     ///
@@ -367,9 +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
@@ -380,17 +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
@@ -423,8 +432,10 @@ impl MessageBuilder {
             None => Envelope::try_from(&res.headers)?,
         };
 
-        // Remove `Bcc` headers now the envelope is set
-        res.headers.remove::<header::Bcc>();
+        if res.drop_bcc {
+            // Remove `Bcc` headers now the envelope is set
+            res.headers.remove::<header::Bcc>();
+        }
 
         Ok(Message {
             headers: res.headers,
@@ -455,6 +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
@@ -521,7 +541,7 @@ impl Message {
     ///     .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_string())
+    ///     .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
@@ -552,8 +572,8 @@ impl Message {
     /// -----END RSA PRIVATE KEY-----";
     /// let signing_key = DkimSigningKey::new(key, DkimSigningAlgorithm::Rsa).unwrap();
     /// message.sign(&DkimConfig::default_config(
-    ///     "dkimtest".to_string(),
-    ///     "example.org".to_string(),
+    ///     "dkimtest".to_owned(),
+    ///     "example.org".to_owned(),
     ///     signing_key,
     /// ));
     /// println!(
@@ -628,7 +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);
 
@@ -655,7 +675,45 @@ mod test {
                 "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",
-                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvQ==?=!\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",
                 "Happy new year!"

src/transport/file/error.rs

@@ -71,7 +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

@@ -208,18 +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 = fs::read(&json_file).map_err(error::io)?;
+        let json_file = self.path.join(format!("{email_id}.json"));
+        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}"))
     }
 }
 
@@ -255,10 +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)?;
 
@@ -276,6 +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")]
@@ -306,6 +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

@@ -65,7 +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")?
@@ -127,6 +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)
     }
@@ -149,6 +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

@@ -68,7 +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

@@ -232,6 +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)?;
 
@@ -261,6 +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
@@ -293,6 +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

@@ -98,12 +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) {
-                    return Ok(credentials.authentication_identity.to_string());
+                if ["User Name", "Username:", "Username"].contains(&decoded_challenge) {
+                    return Ok(credentials.authentication_identity.clone());
                 }
 
-                if vec!["Password", "Password:"].contains(&decoded_challenge) {
-                    return Ok(credentials.secret.to_string());
+                if ["Password", "Password:"].contains(&decoded_challenge) {
+                    return Ok(credentials.secret.clone());
                 }
 
                 Err(error::client("Unrecognized challenge"))
@@ -127,7 +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(),
@@ -140,7 +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(),
@@ -158,8 +158,8 @@ mod test {
         let mechanism = Mechanism::Xoauth2;
 
         let credentials = Credentials::new(
-            "username".to_string(),
-            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_string(),
+            "username".to_owned(),
+            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_owned(),
         );
 
         assert_eq!(
@@ -172,7 +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

@@ -2,13 +2,15 @@ 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},
@@ -41,10 +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
@@ -193,6 +208,7 @@ impl AsyncSmtpConnection {
             self.panic = true;
             let _ = self.command(Quit).await;
         }
+        let _ = self.stream.close().await;
     }
 
     /// Sets the underlying stream
@@ -301,7 +317,7 @@ impl AsyncSmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     }
                 }
@@ -319,7 +335,7 @@ impl AsyncSmtpConnection {
     }
 
     /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[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

@@ -1,5 +1,5 @@
 use std::{
-    io, mem,
+    fmt, io, mem,
     net::{IpAddr, SocketAddr},
     pin::Pin,
     task::{Context, Poll},
@@ -16,8 +16,10 @@ use futures_io::{
 };
 #[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 as _, AsyncWrite as _, ReadBuf as Tokio1ReadBuf};
+use tokio1_crate::io::{AsyncRead, AsyncWrite, ReadBuf as Tokio1ReadBuf};
 #[cfg(feature = "tokio1")]
 use tokio1_crate::net::{
     TcpSocket as Tokio1TcpSocket, TcpStream as Tokio1TcpStream,
@@ -31,6 +33,7 @@ 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"
 ))]
@@ -41,25 +44,42 @@ 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),
@@ -93,6 +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")]
@@ -109,6 +131,11 @@ 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,
@@ -167,7 +194,8 @@ impl AsyncNetworkStream {
         }
 
         let tcp_stream = try_connect(server, timeout, local_addr).await?;
-        let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream));
+        let mut stream =
+            AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(Box::new(tcp_stream)));
         if let Some(tls_parameters) = tls_parameters {
             stream.upgrade_tls(tls_parameters).await?;
         }
@@ -229,14 +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);
@@ -278,12 +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")]
@@ -324,11 +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,
@@ -377,6 +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.");
+            }
         }
     }
 
@@ -388,6 +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")]
@@ -422,6 +488,13 @@ impl AsyncNetworkStream {
                 .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"))
@@ -477,6 +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")]
@@ -508,6 +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")]
@@ -533,6 +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")]
@@ -554,6 +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

@@ -12,7 +12,7 @@ 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},
@@ -44,6 +44,7 @@ pub struct SmtpConnection {
 }
 
 impl SmtpConnection {
+    /// Get information about the server
     pub fn server_info(&self) -> &ServerInfo {
         &self.server_info
     }
@@ -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()
     }
 
@@ -278,7 +285,7 @@ impl SmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     };
                 }
@@ -296,7 +303,7 @@ impl SmtpConnection {
     }
 
     /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[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

@@ -11,7 +11,7 @@
 //!     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, None)?;
 //! client.command(Mail::new(Some("user@example.com".parse()?), vec![]))?;
 //! client.command(Rcpt::new("user@example.org".parse()?, vec![]))?;
@@ -29,12 +29,16 @@ use std::fmt::Debug;
 pub use self::async_connection::AsyncSmtpConnection;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 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"))]
@@ -46,60 +50,57 @@ mod net;
 mod tls;
 
 /// The codec used for transparency
-#[derive(Default, Clone, Copy, Debug)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug)]
 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() {
-            0 => {
-                match self.escape_count {
-                    0 => buf.extend_from_slice(b"\r\n.\r\n"),
-                    1 => buf.extend_from_slice(b"\n.\r\n"),
-                    2 => buf.extend_from_slice(b".\r\n"),
-                    _ => unreachable!(),
+        for &b in frame {
+            buf.push(b);
+            match (b, self.status) {
+                (b'\r', _) => {
+                    self.status = CodecStatus::StartingNewLine;
                 }
-                self.escape_count = 0;
-            }
-            _ => {
-                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;
-                    }
+                (b'\n', CodecStatus::StartingNewLine) => {
+                    self.status = CodecStatus::StartOfNewLine;
                 }
-                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")]
@@ -113,9 +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);
@@ -126,9 +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

@@ -1,3 +1,5 @@
+#[cfg(feature = "rustls-tls")]
+use std::sync::Arc;
 use std::{
     io::{self, Read, Write},
     mem,
@@ -5,13 +7,15 @@ use std::{
     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};
@@ -34,6 +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,
 }
@@ -55,6 +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(
@@ -73,6 +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(())
@@ -136,13 +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);
@@ -158,7 +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,
@@ -175,11 +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 =
-                    ClientConnection::new(connector.clone(), domain).map_err(error::connection)?;
+                let connection = ClientConnection::new(Arc::clone(connector), domain)
+                    .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)
+            }
         })
     }
 
@@ -190,6 +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
@@ -197,7 +223,7 @@ impl NetworkStream {
         }
     }
 
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[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")),
@@ -217,6 +243,13 @@ impl NetworkStream {
                 .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"),
         }
     }
@@ -232,6 +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(())
@@ -252,7 +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(())
@@ -269,6 +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)
@@ -285,6 +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)
@@ -299,6 +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(())

src/transport/smtp/client/tls.rs

@@ -2,22 +2,57 @@ use std::fmt::{self, Debug};
 #[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,
 };
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::{error, Error};
 
-/// Accepted protocols by default.
-/// This removes TLS 1.0 and 1.1 compared to tls-native defaults.
-// This is also rustls' default behavior
-#[cfg(feature = "native-tls")]
-const DEFAULT_TLS_MIN_PROTOCOL: Protocol = Protocol::Tlsv12;
+/// TLS protocol versions.
+#[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)]
@@ -26,16 +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_attr(docsrs, doc(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", feature = "boring-tls")))
+    )]
     Opportunistic(TlsParameters),
     /// Start with insecure connection and require `STARTTLS`
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(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", feature = "boring-tls")))
+    )]
     Required(TlsParameters),
     /// Use TLS wrapped connection
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(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", feature = "boring-tls")))
+    )]
     Wrapper(TlsParameters),
 }
 
@@ -43,31 +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 {
@@ -75,12 +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.
@@ -102,13 +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_attr(docsrs, doc(cfg(feature = "native-tls")))]
+    #[cfg(any(feature = "native-tls", feature = "boring-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`.
@@ -130,16 +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_attr(docsrs, doc(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", feature = "boring-tls")))
+    )]
     pub fn build(self) -> Result<TlsParameters, Error> {
         #[cfg(feature = "rustls-tls")]
         return self.build_rustls();
-
-        #[cfg(not(feature = "rustls-tls"))]
+        #[cfg(all(not(feature = "rustls-tls"), feature = "native-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
@@ -148,17 +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,
         })
     }
 
@@ -167,26 +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,
@@ -198,27 +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_attr(docsrs, doc(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", 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)
     }
@@ -237,6 +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
     }
@@ -250,20 +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,
         })
     }
 
@@ -272,6 +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;
@@ -289,6 +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

@@ -13,7 +13,7 @@ use crate::{
 };
 
 /// 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,
@@ -33,7 +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;
 
@@ -44,7 +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>,
@@ -59,7 +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")
     }
@@ -73,7 +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,
@@ -84,7 +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")
     }
@@ -101,7 +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;
 
@@ -112,7 +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;
 
@@ -123,7 +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;
 
@@ -134,7 +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>,
@@ -144,7 +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")
     }
@@ -158,7 +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,
@@ -178,7 +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,
@@ -198,7 +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;
 
@@ -209,7 +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,
@@ -220,7 +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())?;
@@ -271,7 +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);
@@ -296,15 +296,15 @@ mod test {
 
     #[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(),
-            value: Some("value".to_string()),
+            keyword: "TEST".to_owned(),
+            value: Some("value".to_owned()),
         };
         let rcpt_parameter = RcptParameter::Other {
-            keyword: "TEST".to_string(),
-            value: Some("value".to_string()),
+            keyword: "TEST".to_owned(),
+            value: Some("value".to_owned()),
         };
         assert_eq!(format!("{}", Ehlo::new(id)), "EHLO localhost\r\n");
         assert_eq!(
@@ -341,24 +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!("{}", Data), "DATA\r\n");
-        assert_eq!(format!("{}", Noop), "NOOP\r\n");
+        assert_eq!(format!("{Quit}"), "QUIT\r\n");
+        assert_eq!(format!("{Data}"), "DATA\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!(
-            format!("{}", Vrfy::new("test".to_string())),
-            "VRFY test\r\n"
-        );
-        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!("{}", Vrfy::new("test".to_owned())), "VRFY test\r\n");
+        assert_eq!(format!("{}", Expn::new("test".to_owned())), "EXPN test\r\n");
+        assert_eq!(format!("{Rset}"), "RSET\r\n");
+        let credentials = Credentials::new("user".to_owned(), "password".to_owned());
         assert_eq!(
             format!(
                 "{}",

src/transport/smtp/error.rs

@@ -68,8 +68,11 @@ impl Error {
     }
 
     /// Returns true if the error is from TLS
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(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", feature = "boring-tls")))
+    )]
     pub fn is_tls(&self) -> bool {
         matches!(self.inner.kind, Kind::Tls)
     }
@@ -102,8 +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(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg_attr(
+        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,
 }
 
@@ -128,18 +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(())
@@ -179,7 +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

@@ -55,8 +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::Ipv6(ref value) => write!(f, "[IPv6:{}]", value),
+            Self::Ipv4(ref value) => write!(f, "[{value}]"),
+            Self::Ipv6(ref value) => write!(f, "[IPv6:{value}]"),
         }
     }
 }
@@ -97,7 +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}"),
         }
     }
 }
@@ -119,7 +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)
         };
@@ -174,7 +174,7 @@ impl ServerInfo {
         }
 
         Ok(ServerInfo {
-            name: name.to_string(),
+            name: name.to_owned(),
             features,
         })
     }
@@ -228,8 +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::Size(size) => write!(f, "SIZE={}", size),
+            MailParameter::Body(ref value) => write!(f, "BODY={value}"),
+            MailParameter::Size(size) => write!(f, "SIZE={size}"),
             MailParameter::SmtpUtfEight => f.write_str("SMTPUTF8"),
             MailParameter::Other {
                 ref keyword,
@@ -304,21 +304,21 @@ mod test {
     #[test]
     fn test_clientid_fmt() {
         assert_eq!(
-            format!("{}", ClientId::Domain("test".to_string())),
-            "test".to_string()
+            format!("{}", ClientId::Domain("test".to_owned())),
+            "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()
         );
     }
 
@@ -331,11 +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();
@@ -344,11 +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();
@@ -358,11 +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()
         );
     }
 
@@ -374,18 +374,14 @@ mod test {
                 Category::Unspecified4,
                 Detail::One,
             ),
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned()],
         );
 
         let mut features = HashSet::new();
         assert!(features.insert(Extension::EightBitMime));
 
         let server_info = ServerInfo {
-            name: "me".to_string(),
+            name: "me".to_owned(),
             features,
         };
 
@@ -401,10 +397,10 @@ mod test {
                 Detail::One,
             ),
             vec![
-                "me".to_string(),
-                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
+                "me".to_owned(),
+                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_owned(),
+                "8BITMIME".to_owned(),
+                "SIZE 42".to_owned(),
             ],
         );
 
@@ -414,7 +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

@@ -77,8 +77,8 @@
 //! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
 //!     // Add credentials for authentication
 //!     .credentials(Credentials::new(
-//!         "username".to_string(),
-//!         "password".to_string(),
+//!         "username".to_owned(),
+//!         "password".to_owned(),
 //!     ))
 //!     // Configure expected authentication mechanism
 //!     .authentication(vec![Mechanism::Plain])
@@ -111,7 +111,7 @@
 //!     .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()?;
 //!
@@ -140,7 +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},
@@ -200,7 +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

@@ -158,14 +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)));
                 }
             }
         }
@@ -203,7 +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

@@ -141,14 +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)));
                 }
             }
         }
@@ -186,8 +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::Poisoned(_)) => "POISONED".to_string(),
+                    Err(TryLockError::WouldBlock) => "LOCKED".to_owned(),
+                    Err(TryLockError::Poisoned(_)) => "POISONED".to_owned(),
                 },
             )
             .field("client", &self.client)

src/transport/smtp/response.rs

@@ -151,7 +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()))
     }
 }
 
@@ -329,10 +329,10 @@ mod test {
                     detail: Detail::Zero,
                 },
                 message: vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                    "AUTH PLAIN CRAM-MD5".to_string(),
+                    "me".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
+                    "AUTH PLAIN CRAM-MD5".to_owned(),
                 ],
             }
         );
@@ -352,11 +352,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .is_positive());
         assert!(!Response::new(
@@ -365,11 +361,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .is_positive());
     }
@@ -382,11 +374,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .has_code(451));
         assert!(!Response::new(
@@ -395,11 +383,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .has_code(251));
     }
@@ -413,11 +397,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
             )
             .first_word(),
             Some("me")
@@ -430,9 +410,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
+                    "me mo".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_word(),
@@ -457,7 +437,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_word(),
             None
@@ -469,7 +449,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_word(),
             None
@@ -481,7 +461,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["".to_string()],
+                vec!["".to_owned()],
             )
             .first_word(),
             None
@@ -494,7 +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:?}"),
         }
     }
 
@@ -507,11 +487,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
             )
             .first_line(),
             Some("me")
@@ -524,9 +500,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
+                    "me mo".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_line(),
@@ -551,7 +527,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_line(),
             Some(" ")
@@ -563,7 +539,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_line(),
             Some("  ")
@@ -575,7 +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_attr(docsrs, doc(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", 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_attr(docsrs, doc(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", 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_attr(docsrs, doc(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", 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,
         };
@@ -224,7 +233,7 @@ impl SmtpClient {
             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

@@ -53,6 +53,8 @@ use futures_util::lock::Mutex as FuturesMutex;
 use crate::AsyncTransport;
 use crate::{address::Envelope, Transport};
 
+/// An error returned by the stub transport
+#[non_exhaustive]
 #[derive(Debug, Copy, Clone)]
 pub struct Error;
 

tests/transport_file.rs

@@ -33,7 +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!(
@@ -68,10 +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!(
@@ -131,7 +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!(
@@ -182,7 +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!(

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_stub.rs

@@ -18,7 +18,7 @@ mod sync {
         sender_ok.send(&email).unwrap();
         sender_ko.send(&email).unwrap_err();
 
-        let expected_messages = vec![(
+        let expected_messages = [(
             email.envelope().clone(),
             String::from_utf8(email.formatted()).unwrap(),
         )];
@@ -47,7 +47,7 @@ mod tokio_1 {
         sender_ok.send(email.clone()).await.unwrap();
         sender_ko.send(email.clone()).await.unwrap_err();
 
-        let expected_messages = vec![(
+        let expected_messages = [(
             email.envelope().clone(),
             String::from_utf8(email.formatted()).unwrap(),
         )];
@@ -75,7 +75,7 @@ mod asyncstd_1 {
         sender_ok.send(email.clone()).await.unwrap();
         sender_ko.send(email.clone()).await.unwrap_err();
 
-        let expected_messages = vec![(
+        let expected_messages = [(
             email.envelope().clone(),
             String::from_utf8(email.formatted()).unwrap(),
         )];