Prepare 0.10.0-rc.5 (#750)

This makes it more useful as a testing tool as it now allows you to retrieve
all messages sent via this transport.

.github/workflows/test.yml

@@ -7,11 +7,13 @@ on:
       - master
 
 env:
+  RUSTFLAGS: "--cfg lettre_ignore_tls_mismatch"
+  RUSTDOCFLAGS: "--cfg lettre_ignore_tls_mismatch"
   RUST_BACKTRACE: full
 
 jobs:
   rustfmt:
-    name: rustfmt / stable
+    name: rustfmt / nightly-2022-02-11
     runs-on: ubuntu-latest
 
     steps:
@@ -20,7 +22,7 @@ jobs:
 
       - name: Install rust
         run: |
-          rustup update --no-self-update stable
+          rustup default nightly-2022-02-11
           rustup component add rustfmt
 
       - name: cargo fmt
@@ -79,8 +81,8 @@ jobs:
             rust: stable
           - name: beta
             rust: beta
-          - name: 1.46.0
-            rust: 1.46.0
+          - name: 1.56.0
+            rust: 1.56.0
 
     steps:
       - name: Checkout
@@ -108,6 +110,20 @@ jobs:
       - name: Run SMTP server
         run: smtp-sink 2525 1000&
 
+      - name: Install coredns
+        run: |
+          wget -q https://github.com/coredns/coredns/releases/download/v1.8.6/coredns_1.8.6_linux_amd64.tgz
+          tar xzf coredns_1.8.6_linux_amd64.tgz
+
+      - name: Start coredns
+        run: |
+          sudo ./coredns -conf testdata/coredns.conf &
+          sudo systemctl stop systemd-resolved
+          echo "nameserver 127.0.0.54" | sudo tee /etc/resolv.conf
+
+      - name: Install dkimverify
+        run: sudo apt -y install python3-dkim
+
       - name: Test with no default features
         run: cargo test --no-default-features
 

CHANGELOG.md

@@ -5,7 +5,7 @@
 
 Several breaking changes were made between 0.9 and 0.10, but changes should be straightforward:
 
-* MSRV is now 1.46
+* MSRV is now 1.56.0
 * The `lettre_email` crate has been merged into `lettre`. To migrate, replace `lettre_email` with `lettre::message`
   and make sure to enable the `builder` feature (it's enabled by default).
 * `SendableEmail` has been renamed to `Email` and `EmailBuilder::build()` produces it directly. To migrate,
@@ -31,7 +31,8 @@ Several breaking changes were made between 0.9 and 0.10, but changes should be s
 * 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 `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
+  `/usr/bin/sendmail`.
 
 #### Bug Fixes
 

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.1"
+version = "0.10.0-rc.5"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -10,7 +10,8 @@ license = "MIT"
 authors = ["Alexis Mousset <contact@amousset.me>", "Paolo Barbolini <paolo@paolo565.org>"]
 categories = ["email", "network-programming"]
 keywords = ["email", "smtp", "mailer", "message", "sendmail"]
-edition = "2018"
+edition = "2021"
+rust-version = "1.56"
 
 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -19,6 +20,7 @@ maintenance = { status = "actively-developed" }
 
 [dependencies]
 idna = "0.2"
+once_cell = "1"
 tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # feature
 
 # builder
@@ -27,8 +29,8 @@ 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 }
-once_cell = "1"
 regex = { version = "1", default-features = false, features = ["std", "unicode-case"] }
+email-encoding = { version = "0.1", optional = true }
 
 # file transport
 uuid = { version = "0.8", features = ["v4"], optional = true }
@@ -36,15 +38,14 @@ serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
 
 # smtp
-nom = { version = "6", default-features = false, features = ["alloc", "std"], optional = true }
-r2d2 = { version = "0.8", optional = true } # feature
+nom = { version = "7", optional = true }
 hostname = { version = "0.3", optional = true } # feature
 
 ## tls
 native-tls = { version = "0.2", optional = true } # feature
-rustls = { version = "0.19", features = ["dangerous_configuration"], optional = true }
-webpki = { version = "0.21", optional = true }
-webpki-roots = { version = "0.21", optional = true }
+rustls = { version = "0.20", features = ["dangerous_configuration"], optional = true }
+rustls-pemfile = { version = "0.3", optional = true }
+webpki-roots = { version = "0.22", optional = true }
 
 # async
 futures-io = { version = "0.3.7", optional = true }
@@ -54,16 +55,21 @@ async-trait = { version = "0.1", optional = true }
 ## async-std
 async-std = { version = "1.8", optional = true, features = ["unstable"] }
 #async-native-tls = { version = "0.3.3", optional = true }
-async-rustls = { version = "0.2", optional = true }
+futures-rustls = { version = "0.22", optional = true }
 
 ## tokio
-tokio1_crate = { package = "tokio", version = "1", features = ["fs", "process", "net", "io-util"], optional = true }
+tokio1_crate = { package = "tokio", version = "1", features = ["fs", "rt", "process", "time", "net", "io-util"], optional = true }
 tokio1_native_tls_crate = { package = "tokio-native-tls", version = "0.3", optional = true }
-tokio1_rustls = { package = "tokio-rustls", version = "0.22", optional = true }
+tokio1_rustls = { package = "tokio-rustls", version = "0.23", optional = true }
+
+## dkim
+sha2 = { version = "0.10", optional = true }
+rsa = { version = "0.6.0-pre", optional = true }
+ed25519-dalek = { version = "1.0.1", optional = true }
 
 [dev-dependencies]
 criterion = "0.3"
-tracing-subscriber = "0.2.10"
+tracing-subscriber = "0.3"
 glob = "0.3"
 walkdir = "2"
 tokio1_crate = { package = "tokio", version = "1", features = ["macros", "rt-multi-thread"] }
@@ -76,8 +82,9 @@ harness = false
 name = "transport_smtp"
 
 [features]
-default = ["smtp-transport", "native-tls", "hostname", "r2d2", "builder"]
-builder = ["httpdate", "mime", "base64", "fastrand", "quoted_printable"]
+default = ["smtp-transport", "pool", "native-tls", "hostname", "builder"]
+builder = ["httpdate", "mime", "base64", "fastrand", "quoted_printable", "email-encoding"]
+mime03 = ["mime"]
 
 # transports
 file-transport = ["uuid"]
@@ -85,19 +92,23 @@ file-transport-envelope = ["serde", "serde_json", "file-transport"]
 sendmail-transport = []
 smtp-transport = ["base64", "nom"]
 
-rustls-tls = ["webpki", "webpki-roots", "rustls"]
+pool = ["futures-util"]
+
+rustls-tls = ["webpki-roots", "rustls", "rustls-pemfile"]
 
 # async
 async-std1 = ["async-std", "async-trait", "futures-io", "futures-util"]
 #async-std1-native-tls = ["async-std1", "native-tls", "async-native-tls"]
-async-std1-rustls-tls = ["async-std1", "rustls-tls", "async-rustls"]
+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"]
 
+dkim = ["sha2", "rsa", "ed25519-dalek"]
+
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "docsrs"]
+rustdoc-args = ["--cfg", "docsrs", "--cfg", "lettre_ignore_tls_mismatch"]
 
 [[example]]
 name = "basic_html"

LICENSE

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

README.md

@@ -28,8 +28,8 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0-rc.1">
-    <img src="https://deps.rs/crate/lettre/0.10.0-rc.1/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.10.0-rc.5">
+    <img src="https://deps.rs/crate/lettre/0.10.0-rc.5/status.svg"
       alt="dependency status" />
   </a>
 </div>
@@ -65,13 +65,13 @@ Lettre does not provide (for now):
 
 ## Example
 
-This library requires Rust 1.46 or newer.
+This library requires Rust 1.56.0 or newer.
 To use this library, add the following to your `Cargo.toml`:
 
 
 ```toml
 [dependencies]
-lettre = "0.10.0-rc.1"
+lettre = "0.10.0-rc.5"
 ```
 
 ```rust,no_run
@@ -103,10 +103,22 @@ match mailer.send(&email) {
 
 ## Testing
 
-The `lettre` tests require an open mail server listening locally on port 2525 and the `sendmail` command.
+The `lettre` tests require an open mail server listening locally on port 2525 and the `sendmail` command. If you have python installed 
+such a server can be launched with `python -m smtpd -n -c DebuggingServer localhost:2525`
 
 Alternatively only unit tests can be run by doing `cargo test --lib`.
 
+## Troubleshooting
+
+These are general steps to be followed when troubleshooting SMTP related issues.
+
+- Ensure basic connectivity, ensure requisite ports are open and daemons are listening.
+- Confirm that your service provider allows traffic on the ports being used for mail transfer.
+- Check SMTP relay authentication and configuration.
+- Validate your DNS records. (DMARC, SPF, DKIM, MX)
+- Verify your SSL/TLS certificates are setup properly.
+- Investigate if filtering, formatting, or filesize limits are causing messages to be lost, delayed, or blocked by relays or remote hosts.
+
 ## Code of conduct
 
 Anyone who interacts with Lettre in any space, including but not limited to

examples/tokio1_smtp_starttls.rs

@@ -1,12 +1,11 @@
 // This line is only to make it compile from lettre's examples folder,
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
-use tokio1_crate as tokio;
-
 use lettre::{
     transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
     Tokio1Executor,
 };
+use tokio1_crate as tokio;
 
 #[tokio::main]
 async fn main() {

examples/tokio1_smtp_tls.rs

@@ -1,12 +1,11 @@
 // This line is only to make it compile from lettre's examples folder,
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
-use tokio1_crate as tokio;
-
 use lettre::{
     transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
     Tokio1Executor,
 };
+use tokio1_crate as tokio;
 
 #[tokio::main]
 async fn main() {

rustfmt.toml

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

src/address/envelope.rs

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

src/address/types.rs

@@ -1,10 +1,6 @@
 //! Representation of an email address
 
-use idna::domain_to_ascii;
-use once_cell::sync::Lazy;
-use regex::Regex;
 use std::{
-    convert::{TryFrom, TryInto},
     error::Error,
     ffi::OsStr,
     fmt::{Display, Formatter, Result as FmtResult},
@@ -12,6 +8,10 @@ use std::{
     str::FromStr,
 };
 
+use idna::domain_to_ascii;
+use once_cell::sync::Lazy;
+use regex::Regex;
+
 /// Represents an email address with a user and a domain name.
 ///
 /// This type contains email in canonical form (_user@domain.tld_).
@@ -174,15 +174,10 @@ impl FromStr for Address {
     type Err = AddressError;
 
     fn from_str(val: &str) -> Result<Self, AddressError> {
-        let mut parts = val.rsplitn(2, '@');
-        let domain = parts.next().ok_or(AddressError::MissingParts)?;
-        let user = parts.next().ok_or(AddressError::MissingParts)?;
-
-        Address::check_user(user)?;
-        Address::check_domain(domain)?;
+        let at_start = check_address(val)?;
         Ok(Address {
             serialized: val.into(),
-            at_start: user.len(),
+            at_start,
         })
     }
 }
@@ -209,6 +204,18 @@ where
     }
 }
 
+impl TryFrom<String> for Address {
+    type Error = AddressError;
+
+    fn try_from(serialized: String) -> Result<Self, AddressError> {
+        let at_start = check_address(&serialized)?;
+        Ok(Address {
+            serialized,
+            at_start,
+        })
+    }
+}
+
 impl AsRef<str> for Address {
     fn as_ref(&self) -> &str {
         &self.serialized
@@ -221,6 +228,16 @@ impl AsRef<OsStr> for Address {
     }
 }
 
+fn check_address(val: &str) -> Result<usize, AddressError> {
+    let mut parts = val.rsplitn(2, '@');
+    let domain = parts.next().ok_or(AddressError::MissingParts)?;
+    let user = parts.next().ok_or(AddressError::MissingParts)?;
+
+    Address::check_user(user)?;
+    Address::check_domain(domain)?;
+    Ok(user.len())
+}
+
 #[derive(Debug, PartialEq, Clone, Copy)]
 /// Errors in email addresses parsing
 pub enum AddressError {

src/executor.rs

@@ -1,10 +1,16 @@
-use async_trait::async_trait;
-
 use std::fmt::Debug;
+#[cfg(feature = "smtp-transport")]
+use std::future::Future;
 #[cfg(feature = "file-transport")]
 use std::io::Result as IoResult;
 #[cfg(feature = "file-transport")]
 use std::path::Path;
+#[cfg(feature = "smtp-transport")]
+use std::time::Duration;
+
+use async_trait::async_trait;
+#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
+use futures_util::future::BoxFuture;
 
 #[cfg(all(
     feature = "smtp-transport",
@@ -37,12 +43,29 @@ use crate::transport::smtp::Error;
 /// [`AsyncFileTransport`]: crate::AsyncFileTransport
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
 #[async_trait]
-pub trait Executor: Debug + Send + Sync + private::Sealed {
+pub trait Executor: Debug + Send + Sync + 'static + private::Sealed {
+    #[cfg(feature = "smtp-transport")]
+    type Handle: SpawnHandle;
+    #[cfg(feature = "smtp-transport")]
+    type Sleep: Future<Output = ()> + Send + 'static;
+
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    fn spawn<F>(fut: F) -> Self::Handle
+    where
+        F: Future<Output = ()> + Send + 'static,
+        F::Output: Send + 'static;
+
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    fn sleep(duration: Duration) -> Self::Sleep;
+
     #[doc(hidden)]
     #[cfg(feature = "smtp-transport")]
     async fn connect(
         hostname: &str,
         port: u16,
+        timeout: Option<Duration>,
         hello_name: &ClientId,
         tls: &Tls,
     ) -> Result<AsyncSmtpConnection, Error>;
@@ -56,6 +79,13 @@ pub trait Executor: Debug + Send + Sync + private::Sealed {
     async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()>;
 }
 
+#[doc(hidden)]
+#[cfg(feature = "smtp-transport")]
+#[async_trait]
+pub trait SpawnHandle: Debug + Send + Sync + 'static + private::Sealed {
+    async fn shutdown(self);
+}
+
 /// Async [`Executor`] using `tokio` `1.x`
 ///
 /// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
@@ -74,11 +104,33 @@ pub struct Tokio1Executor;
 #[async_trait]
 #[cfg(feature = "tokio1")]
 impl Executor for Tokio1Executor {
+    #[cfg(feature = "smtp-transport")]
+    type Handle = tokio1_crate::task::JoinHandle<()>;
+    #[cfg(feature = "smtp-transport")]
+    type Sleep = tokio1_crate::time::Sleep;
+
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    fn spawn<F>(fut: F) -> Self::Handle
+    where
+        F: Future<Output = ()> + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        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,
         port: u16,
+        timeout: Option<Duration>,
         hello_name: &ClientId,
         tls: &Tls,
     ) -> Result<AsyncSmtpConnection, Error> {
@@ -89,8 +141,13 @@ impl Executor for Tokio1Executor {
             _ => None,
         };
         #[allow(unused_mut)]
-        let mut conn =
-            AsyncSmtpConnection::connect_tokio1(hostname, port, hello_name, tls_parameters).await?;
+        let mut conn = AsyncSmtpConnection::connect_tokio1(
+            (hostname, port),
+            timeout,
+            hello_name,
+            tls_parameters,
+        )
+        .await?;
 
         #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
         match tls {
@@ -121,6 +178,14 @@ impl Executor for Tokio1Executor {
     }
 }
 
+#[cfg(all(feature = "smtp-transport", feature = "tokio1"))]
+#[async_trait]
+impl SpawnHandle for tokio1_crate::task::JoinHandle<()> {
+    async fn shutdown(self) {
+        self.abort();
+    }
+}
+
 /// Async [`Executor`] using `async-std` `1.x`
 ///
 /// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
@@ -139,11 +204,34 @@ pub struct AsyncStd1Executor;
 #[async_trait]
 #[cfg(feature = "async-std1")]
 impl Executor for AsyncStd1Executor {
+    #[cfg(feature = "smtp-transport")]
+    type Handle = async_std::task::JoinHandle<()>;
+    #[cfg(feature = "smtp-transport")]
+    type Sleep = BoxFuture<'static, ()>;
+
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    fn spawn<F>(fut: F) -> Self::Handle
+    where
+        F: Future<Output = ()> + Send + 'static,
+        F::Output: Send + 'static,
+    {
+        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,
         port: u16,
+        timeout: Option<Duration>,
         hello_name: &ClientId,
         tls: &Tls,
     ) -> Result<AsyncSmtpConnection, Error> {
@@ -154,9 +242,13 @@ impl Executor for AsyncStd1Executor {
             _ => None,
         };
         #[allow(unused_mut)]
-        let mut conn =
-            AsyncSmtpConnection::connect_asyncstd1(hostname, port, hello_name, tls_parameters)
-                .await?;
+        let mut conn = AsyncSmtpConnection::connect_asyncstd1(
+            (hostname, port),
+            timeout,
+            hello_name,
+            tls_parameters,
+        )
+        .await?;
 
         #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
         match tls {
@@ -187,6 +279,14 @@ impl Executor for AsyncStd1Executor {
     }
 }
 
+#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
+#[async_trait]
+impl SpawnHandle for async_std::task::JoinHandle<()> {
+    async fn shutdown(self) {
+        self.cancel().await;
+    }
+}
+
 mod private {
     use super::*;
 
@@ -197,4 +297,10 @@ mod private {
 
     #[cfg(feature = "async-std1")]
     impl Sealed for AsyncStd1Executor {}
+
+    #[cfg(all(feature = "smtp-transport", feature = "tokio1"))]
+    impl Sealed for tokio1_crate::task::JoinHandle<()> {}
+
+    #[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
+    impl Sealed for async_std::task::JoinHandle<()> {}
 }

src/lib.rs

@@ -6,7 +6,7 @@
 //! * Secure defaults
 //! * Async support
 //!
-//! Lettre requires Rust 1.46 or newer.
+//! Lettre requires Rust 1.56.0 or newer.
 //!
 //! ## Features
 //!
@@ -27,7 +27,7 @@
 //! _Send emails using [`SMTP`]_
 //!
 //! * **smtp-transport** 📫: Enable the SMTP transport
-//! * **r2d2** 📫: Connection pool for SMTP transport
+//! * **pool** 📫: Connection pool for SMTP transport
 //! * **hostname** 📫: Try to use the actual system hostname for the SMTP `CLIENTID`
 //!
 //! #### SMTP over TLS via the native-tls crate
@@ -84,18 +84,23 @@
 //!
 //! * **serde**: Serialization/Deserialization of entities
 //! * **tracing**: Logging using the `tracing` crate
+//! * **mime03**: Allow creating a [`ContentType`] from an existing [mime 0.3] `Mime` struct
+//! * **dkim**: Add support for signing email with DKIM
 //!
 //! [`SMTP`]: crate::transport::smtp
 //! [`sendmail`]: crate::transport::sendmail
 //! [`file`]: crate::transport::file
+//! [`ContentType`]: crate::message::header::ContentType
 //! [tokio]: https://docs.rs/tokio/1
 //! [async-std]: https://docs.rs/async-std/1
 //! [ring]: https://github.com/briansmith/ring#ring
 //! [ring-support]: https://github.com/briansmith/ring#online-automated-testing
 //! [Tokio 1.x]: https://docs.rs/tokio/1
 //! [async-std 1.x]: https://docs.rs/async-std/1
+//! [mime 0.3]: https://docs.rs/mime/0.3
+//! [DKIM]: https://datatracker.ietf.org/doc/html/rfc6376
 
-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0-rc.1")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0-rc.5")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -105,10 +110,62 @@
     trivial_numeric_casts,
     unstable_features,
     unused_import_braces,
-    rust_2018_idioms
+    rust_2018_idioms,
+    clippy::string_add,
+    clippy::string_add_assign
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+#[cfg(not(lettre_ignore_tls_mismatch))]
+mod compiletime_checks {
+    #[cfg(all(
+        feature = "tokio1",
+        feature = "native-tls",
+        not(feature = "tokio1-native-tls")
+    ))]
+    compile_error!("Lettre is being built with the `tokio1` and the `native-tls` features, but the `tokio1-native-tls` feature hasn't been turned on.
+If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+
+    #[cfg(all(
+        feature = "tokio1",
+        feature = "rustls-tls",
+        not(feature = "tokio1-rustls-tls")
+    ))]
+    compile_error!("Lettre is being built with the `tokio1` and the `rustls-tls` features, but the `tokio1-rustls-tls` feature hasn't been turned on.
+If you'd like to use native-tls make sure that the `rustls-tls` hasn't been enabled by mistake.
+If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+
+    /*
+    #[cfg(all(
+        feature = "async-std1",
+        feature = "native-tls",
+        not(feature = "async-std1-native-tls")
+    ))]
+    compile_error!("Lettre is being built with the `async-std1` and the `native-tls` features, but the `async-std1-native-tls` feature hasn't been turned on.
+    If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+    If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+    */
+    #[cfg(all(
+        feature = "async-std1",
+        feature = "native-tls",
+        not(feature = "async-std1-native-tls")
+    ))]
+    compile_error!("Lettre is being built with the `async-std1` and the `native-tls` features, but the async-std integration doesn't support native-tls yet.
+If you'd like to work on the issue please take a look at https://github.com/lettre/lettre/issues/576.
+If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+If you're building a library which depends on lettre import lettre without default features and enable just the features you need.");
+
+    #[cfg(all(
+        feature = "async-std1",
+        feature = "rustls-tls",
+        not(feature = "async-std1-rustls-tls")
+    ))]
+    compile_error!("Lettre is being built with the `async-std1` and the `rustls-tls` features, but the `async-std1-rustls-tls` feature hasn't been turned on.
+If you'd like to use native-tls make sure that the `rustls-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
+If you're building a library which depends on lettre import it without default features and enable just the features you need.");
+}
+
 pub mod address;
 pub mod error;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
@@ -118,6 +175,8 @@ mod executor;
 pub mod message;
 pub mod transport;
 
+use std::error::Error as StdError;
+
 #[cfg(feature = "async-std1")]
 pub use self::executor::AsyncStd1Executor;
 #[cfg(all(any(feature = "tokio1", feature = "async-std1")))]
@@ -154,21 +213,17 @@ pub use crate::transport::sendmail::SendmailTransport;
     any(feature = "tokio1", feature = "async-std1")
 ))]
 pub use crate::transport::smtp::AsyncSmtpTransport;
+#[cfg(feature = "smtp-transport")]
+pub use crate::transport::smtp::SmtpTransport;
 #[doc(inline)]
 pub use crate::transport::Transport;
 use crate::{address::Envelope, error::Error};
 
-#[cfg(feature = "smtp-transport")]
-pub use crate::transport::smtp::SmtpTransport;
-use std::error::Error as StdError;
-
 pub(crate) type BoxError = Box<dyn StdError + Send + Sync>;
 
 #[cfg(test)]
 #[cfg(feature = "builder")]
 mod test {
-    use std::convert::TryFrom;
-
     use super::*;
     use crate::message::{header, header::Headers, Mailbox, Mailboxes};
 

src/message/attachment.rs

@@ -20,21 +20,75 @@ enum Disposition {
 }
 
 impl Attachment {
-    /// Creates a new attachment
+    /// Create a new attachment
+    ///
+    /// This attachment will be displayed as a normal attachment,
+    /// with the chosen `filename` appearing as the file name.
+    ///
+    /// ```rust
+    /// # use std::error::Error;
+    /// use std::fs;
+    ///
+    /// use lettre::message::{header::ContentType, Attachment};
+    ///
+    /// # fn main() -> Result<(), Box<dyn Error>> {
+    /// let filename = String::from("invoice.pdf");
+    /// # if false {
+    /// let filebody = fs::read("invoice.pdf")?;
+    /// # }
+    /// # let filebody = fs::read("docs/lettre.png")?;
+    /// let content_type = ContentType::parse("application/pdf").unwrap();
+    /// let attachment = Attachment::new(filename).body(filebody, content_type);
+    ///
+    /// // The document `attachment` will show up as a normal attachment.
+    /// # Ok(())
+    /// # }
+    /// ```
     pub fn new(filename: String) -> Self {
         Attachment {
             disposition: Disposition::Attached(filename),
         }
     }
 
-    /// Creates a new inline attachment
+    /// Create a new inline attachment
+    ///
+    /// This attachment should be displayed inline into the message
+    /// body:
+    ///
+    /// ```html
+    /// <img src="cid:123">
+    /// ```
+    ///
+    ///
+    /// ```rust
+    /// # use std::error::Error;
+    /// use std::fs;
+    ///
+    /// use lettre::message::{header::ContentType, Attachment};
+    ///
+    /// # fn main() -> Result<(), Box<dyn Error>> {
+    /// let content_id = String::from("123");
+    /// # if false {
+    /// let filebody = fs::read("image.jpg")?;
+    /// # }
+    /// # let filebody = fs::read("docs/lettre.png")?;
+    /// let content_type = ContentType::parse("image/jpeg").unwrap();
+    /// let attachment = Attachment::new_inline(content_id).body(filebody, content_type);
+    ///
+    /// // The image `attachment` will display inline into the email.
+    /// # Ok(())
+    /// # }
+    /// ```
     pub fn new_inline(content_id: String) -> Self {
         Attachment {
             disposition: Disposition::Inline(content_id),
         }
     }
 
-    /// Build the attachment part
+    /// Build the attachment into a [`SinglePart`] which can then be used to build the rest of the email
+    ///
+    /// Look at the [Complex MIME body example](crate::message#complex-mime-body)
+    /// to see how [`SinglePart`] can be put into the email.
     pub fn body<T: IntoBody>(self, content: T, content_type: ContentType) -> SinglePart {
         let mut builder = SinglePart::builder();
         builder = match self.disposition {

src/message/body.rs

@@ -183,8 +183,8 @@ impl MaybeString {
     /// would result into an invalid encoded body.
     fn is_encoding_ok(&self, encoding: ContentTransferEncoding) -> bool {
         match encoding {
-            ContentTransferEncoding::SevenBit => is_7bit_encoded(&self),
-            ContentTransferEncoding::EightBit => is_8bit_encoded(&self),
+            ContentTransferEncoding::SevenBit => is_7bit_encoded(self),
+            ContentTransferEncoding::EightBit => is_8bit_encoded(self),
             ContentTransferEncoding::Binary
             | ContentTransferEncoding::QuotedPrintable
             | ContentTransferEncoding::Base64 => true,
@@ -342,7 +342,7 @@ where
 
 /// In place conversion to CRLF line endings
 fn in_place_crlf_line_endings(string: &mut String) {
-    let indices = find_all_lf_char_indices(&string);
+    let indices = find_all_lf_char_indices(string);
 
     for i in indices {
         // this relies on `indices` being in reverse order

src/message/dkim.rs

@@ -0,0 +1,528 @@
+use std::{
+    borrow::Cow,
+    error::Error as StdError,
+    fmt::{self, Display, Write},
+    iter::IntoIterator,
+    time::SystemTime,
+};
+
+use ed25519_dalek::Signer;
+use once_cell::sync::Lazy;
+use regex::{bytes::Regex as BRegex, Regex};
+use rsa::{pkcs1::DecodeRsaPrivateKey, Hash, PaddingScheme, RsaPrivateKey};
+use sha2::{Digest, Sha256};
+
+use crate::message::{
+    header::{HeaderName, HeaderValue},
+    Headers, Message,
+};
+
+/// Describe Dkim Canonicalization to apply to either body or headers
+#[derive(Copy, Clone, Debug)]
+pub enum DkimCanonicalizationType {
+    Simple,
+    Relaxed,
+}
+
+impl Display for DkimCanonicalizationType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            DkimCanonicalizationType::Simple => "simple",
+            DkimCanonicalizationType::Relaxed => "relaxed",
+        })
+    }
+}
+
+/// Describe Canonicalization to be applied before signing
+#[derive(Copy, Clone, Debug)]
+pub struct DkimCanonicalization {
+    pub header: DkimCanonicalizationType,
+    pub body: DkimCanonicalizationType,
+}
+
+impl Default for DkimCanonicalization {
+    fn default() -> Self {
+        DkimCanonicalization {
+            header: DkimCanonicalizationType::Simple,
+            body: DkimCanonicalizationType::Relaxed,
+        }
+    }
+}
+
+/// Format canonicalization to be shown in Dkim header
+impl Display for DkimCanonicalization {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}/{}", self.header, self.body)
+    }
+}
+
+/// Describe the algorithm used for signing the message
+#[derive(Copy, Clone, Debug)]
+pub enum DkimSigningAlgorithm {
+    Rsa,
+    Ed25519,
+}
+
+impl Display for DkimSigningAlgorithm {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            DkimSigningAlgorithm::Rsa => "rsa",
+            DkimSigningAlgorithm::Ed25519 => "ed25519",
+        })
+    }
+}
+
+/// Describe DkimSigning key error
+#[derive(Debug)]
+pub struct DkimSigningKeyError(InnerDkimSigningKeyError);
+
+#[derive(Debug)]
+enum InnerDkimSigningKeyError {
+    Base64(base64::DecodeError),
+    Rsa(rsa::pkcs1::Error),
+    Ed25519(ed25519_dalek::ed25519::Error),
+}
+
+impl Display for DkimSigningKeyError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match &self.0 {
+            InnerDkimSigningKeyError::Base64(_err) => "base64 decode error",
+            InnerDkimSigningKeyError::Rsa(_err) => "rsa decode error",
+            InnerDkimSigningKeyError::Ed25519(_err) => "ed25519 decode error",
+        })
+    }
+}
+
+impl StdError for DkimSigningKeyError {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        Some(match &self.0 {
+            InnerDkimSigningKeyError::Base64(err) => &*err,
+            InnerDkimSigningKeyError::Rsa(err) => &*err,
+            InnerDkimSigningKeyError::Ed25519(err) => &*err,
+        })
+    }
+}
+
+/// Describe a signing key to be carried by DkimConfig struct
+#[derive(Debug)]
+pub struct DkimSigningKey(InnerDkimSigningKey);
+
+#[derive(Debug)]
+enum InnerDkimSigningKey {
+    Rsa(RsaPrivateKey),
+    Ed25519(ed25519_dalek::Keypair),
+}
+
+impl DkimSigningKey {
+    pub fn new(
+        private_key: String,
+        algorithm: DkimSigningAlgorithm,
+    ) -> Result<DkimSigningKey, DkimSigningKeyError> {
+        Ok(Self(match algorithm {
+            DkimSigningAlgorithm::Rsa => InnerDkimSigningKey::Rsa(
+                RsaPrivateKey::from_pkcs1_pem(&private_key)
+                    .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Rsa(err)))?,
+            ),
+            DkimSigningAlgorithm::Ed25519 => {
+                InnerDkimSigningKey::Ed25519(
+                    ed25519_dalek::Keypair::from_bytes(&base64::decode(private_key).map_err(
+                        |err| DkimSigningKeyError(InnerDkimSigningKeyError::Base64(err)),
+                    )?)
+                    .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Ed25519(err)))?,
+                )
+            }
+        }))
+    }
+    fn get_signing_algorithm(&self) -> DkimSigningAlgorithm {
+        match self.0 {
+            InnerDkimSigningKey::Rsa(_) => DkimSigningAlgorithm::Rsa,
+            InnerDkimSigningKey::Ed25519(_) => DkimSigningAlgorithm::Ed25519,
+        }
+    }
+}
+
+/// A struct to describe Dkim configuration applied when signing a message
+/// selector: the name of the key publied in DNS
+/// domain: the domain for which we sign the message
+/// private_key: private key in PKCS1 string format
+/// headers: a list of headers name to be included in the signature. Signing of more than one
+/// header with same name is not supported
+/// canonicalization: the canonicalization to be applied on the message
+/// pub signing_algorithm: the signing algorithm to be used when signing
+#[derive(Debug)]
+pub struct DkimConfig {
+    selector: String,
+    domain: String,
+    private_key: DkimSigningKey,
+    headers: Vec<HeaderName>,
+    canonicalization: DkimCanonicalization,
+}
+
+impl DkimConfig {
+    /// Create a default signature configuration with a set of headers and "simple/relaxed"
+    /// canonicalization
+    pub fn default_config(
+        selector: String,
+        domain: String,
+        private_key: DkimSigningKey,
+    ) -> DkimConfig {
+        DkimConfig {
+            selector,
+            domain,
+            private_key,
+            headers: vec![
+                HeaderName::new_from_ascii_str("From"),
+                HeaderName::new_from_ascii_str("Subject"),
+                HeaderName::new_from_ascii_str("To"),
+                HeaderName::new_from_ascii_str("Date"),
+            ],
+            canonicalization: DkimCanonicalization {
+                header: DkimCanonicalizationType::Simple,
+                body: DkimCanonicalizationType::Relaxed,
+            },
+        }
+    }
+
+    /// Create a DkimConfig
+    pub fn new(
+        selector: String,
+        domain: String,
+        private_key: DkimSigningKey,
+        headers: Vec<HeaderName>,
+        canonicalization: DkimCanonicalization,
+    ) -> DkimConfig {
+        DkimConfig {
+            selector,
+            domain,
+            private_key,
+            headers,
+            canonicalization,
+        }
+    }
+}
+
+/// Create a Headers struct with a Dkim-Signature Header created from given parameters
+fn dkim_header_format(
+    config: &DkimConfig,
+    timestamp: u64,
+    headers_list: &str,
+    body_hash: &str,
+    signature: &str,
+) -> Headers {
+    let mut headers = Headers::new();
+    let header_name =
+        dkim_canonicalize_header_tag("DKIM-Signature", config.canonicalization.header);
+    let header_name = HeaderName::new_from_ascii(header_name.into()).unwrap();
+    headers.insert_raw(HeaderValue::new(header_name, format!("v=1; a={signing_algorithm}-sha256; d={domain}; s={selector}; c={canon}; q=dns/txt; t={timestamp}; h={headers_list}; bh={body_hash}; b={signature}",domain=config.domain, selector=config.selector,canon=config.canonicalization,timestamp=timestamp,headers_list=headers_list,body_hash=body_hash,signature=signature,signing_algorithm=config.private_key.get_signing_algorithm())));
+    headers
+}
+
+/// Canonicalize the body of an email
+fn dkim_canonicalize_body(
+    body: &[u8],
+    canonicalization: DkimCanonicalizationType,
+) -> Cow<'_, [u8]> {
+    static RE: Lazy<BRegex> = Lazy::new(|| BRegex::new("(\r\n)+$").unwrap());
+    static RE_DOUBLE_SPACE: Lazy<BRegex> = Lazy::new(|| BRegex::new("[\\t ]+").unwrap());
+    static RE_SPACE_EOL: Lazy<BRegex> = Lazy::new(|| BRegex::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),
+            }
+        }
+    }
+}
+
+/// Canonicalize the value of an header
+fn dkim_canonicalize_header_value(
+    value: &str,
+    canonicalization: DkimCanonicalizationType,
+) -> Cow<'_, str> {
+    match canonicalization {
+        DkimCanonicalizationType::Simple => Cow::Borrowed(value),
+        DkimCanonicalizationType::Relaxed => {
+            static RE_EOL: Lazy<Regex> = Lazy::new(|| Regex::new("\r\n").unwrap());
+            static RE_SPACES: Lazy<Regex> = Lazy::new(|| Regex::new("[\\t ]+").unwrap());
+            let value = RE_EOL.replace_all(value, "");
+            Cow::Owned(format!(
+                "{}\r\n",
+                RE_SPACES.replace_all(&value, " ").trim_end()
+            ))
+        }
+    }
+}
+
+/// Canonicalize header tag
+fn dkim_canonicalize_header_tag(
+    name: &str,
+    canonicalization: DkimCanonicalizationType,
+) -> Cow<'_, str> {
+    match canonicalization {
+        DkimCanonicalizationType::Simple => Cow::Borrowed(name),
+        DkimCanonicalizationType::Relaxed => Cow::Owned(name.to_lowercase()),
+    }
+}
+
+/// Canonicalize signed headers passed as headers_list among mail_headers using canonicalization
+fn dkim_canonicalize_headers<'a>(
+    headers_list: impl IntoIterator<Item = &'a str>,
+    mail_headers: &Headers,
+    canonicalization: DkimCanonicalizationType,
+) -> String {
+    match canonicalization {
+        DkimCanonicalizationType::Simple => {
+            let mut signed_headers = Headers::new();
+
+            for h in headers_list {
+                let h = dkim_canonicalize_header_tag(h, canonicalization);
+                if let Some(value) = mail_headers.get_raw(&h) {
+                    signed_headers.insert_raw(HeaderValue::new(
+                        HeaderName::new_from_ascii(h.into()).unwrap(),
+                        dkim_canonicalize_header_value(value, canonicalization).to_string(),
+                    ))
+                }
+            }
+
+            signed_headers.to_string()
+        }
+        DkimCanonicalizationType::Relaxed => {
+            let mut signed_headers = String::new();
+
+            for h in headers_list {
+                let h = dkim_canonicalize_header_tag(h, canonicalization);
+                if let Some(value) = mail_headers.get_raw(&h) {
+                    write!(
+                        signed_headers,
+                        "{}:{}",
+                        h,
+                        dkim_canonicalize_header_value(value, canonicalization)
+                    )
+                    .expect("write implementation returned an error")
+                }
+            }
+
+            signed_headers
+        }
+    }
+}
+
+/// Sign with Dkim a message by adding Dkim-Signture header created with configuration expressed by
+/// dkim_config
+pub(super) fn dkim_sign(message: &mut Message, dkim_config: &DkimConfig) {
+    let timestamp = SystemTime::now()
+        .duration_since(SystemTime::UNIX_EPOCH)
+        .unwrap()
+        .as_secs();
+    let headers = message.headers();
+    let body_hash = Sha256::digest(&dkim_canonicalize_body(
+        &message.body_raw(),
+        dkim_config.canonicalization.body,
+    ));
+    let bh = base64::encode(body_hash);
+    let mut signed_headers_list =
+        dkim_config
+            .headers
+            .iter()
+            .fold(String::new(), |mut list, header| {
+                if !list.is_empty() {
+                    list.push(':');
+                }
+
+                list.push_str(header);
+                list
+            });
+    if let DkimCanonicalizationType::Relaxed = dkim_config.canonicalization.header {
+        signed_headers_list.make_ascii_lowercase();
+    }
+    let dkim_header = dkim_header_format(dkim_config, timestamp, &signed_headers_list, &bh, "");
+    let signed_headers = dkim_canonicalize_headers(
+        dkim_config.headers.iter().map(|h| h.as_ref()),
+        headers,
+        dkim_config.canonicalization.header,
+    );
+    let canonicalized_dkim_header = dkim_canonicalize_headers(
+        ["DKIM-Signature"],
+        &dkim_header,
+        dkim_config.canonicalization.header,
+    );
+    let mut hashed_headers = Sha256::new();
+    hashed_headers.update(signed_headers.as_bytes());
+    hashed_headers.update(canonicalized_dkim_header.trim_end().as_bytes());
+    let hashed_headers = hashed_headers.finalize();
+    let signature = match &dkim_config.private_key.0 {
+        InnerDkimSigningKey::Rsa(private_key) => base64::encode(
+            private_key
+                .sign(
+                    PaddingScheme::new_pkcs1v15_sign(Some(Hash::SHA2_256)),
+                    &hashed_headers,
+                )
+                .unwrap(),
+        ),
+        InnerDkimSigningKey::Ed25519(private_key) => {
+            base64::encode(private_key.sign(&hashed_headers).to_bytes())
+        }
+    };
+    let dkim_header = dkim_header_format(
+        dkim_config,
+        timestamp,
+        &signed_headers_list,
+        &bh,
+        &signature,
+    );
+    message.headers.insert_raw(HeaderValue::new(
+        HeaderName::new_from_ascii_str("DKIM-Signature"),
+        dkim_header.get_raw("DKIM-Signature").unwrap().to_owned(),
+    ));
+}
+
+#[cfg(test)]
+mod test {
+    use std::{
+        io::Write,
+        process::{Command, Stdio},
+    };
+
+    use super::{
+        super::{
+            header::{HeaderName, HeaderValue},
+            Header, Message,
+        },
+        dkim_canonicalize_body, dkim_canonicalize_header_value, dkim_canonicalize_headers,
+        DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm, DkimSigningKey,
+    };
+    use crate::StdError;
+
+    #[derive(Clone)]
+    struct TestHeader(String);
+
+    impl Header for TestHeader {
+        fn name() -> HeaderName {
+            HeaderName::new_from_ascii_str("Test")
+        }
+
+        fn parse(s: &str) -> Result<Self, Box<dyn StdError + Send + Sync>> {
+            Ok(Self(s.into()))
+        }
+
+        fn display(&self) -> HeaderValue {
+            HeaderValue::new(Self::name(), self.0.clone())
+        }
+    }
+
+    #[test]
+    fn test_body_simple_canonicalize() {
+        let body = b"test\r\n\r\ntest   \ttest\r\n\r\n\r\n";
+        let expected: &[u8] = b"test\r\n\r\ntest   \ttest\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Simple),
+            expected
+        )
+    }
+    #[test]
+    fn test_body_relaxed_canonicalize() {
+        let body = b"test\r\n\r\ntest   \ttest\r\n\r\n\r\n";
+        let expected: &[u8] = b"test\r\n\r\ntest test\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Relaxed),
+            expected
+        )
+    }
+    #[test]
+    fn test_header_simple_canonicalize() {
+        let value = "test\r\n\r\ntest   \ttest\r\n";
+        let expected = "test\r\n\r\ntest   \ttest\r\n";
+        assert_eq!(
+            dkim_canonicalize_header_value(value, DkimCanonicalizationType::Simple),
+            expected
+        )
+    }
+    #[test]
+    fn test_header_relaxed_canonicalize() {
+        let value = "test\r\n\r\ntest   \ttest\r\n";
+        let expected = "testtest test\r\n";
+        assert_eq!(
+            dkim_canonicalize_header_value(value, DkimCanonicalizationType::Relaxed),
+            expected
+        )
+    }
+
+    fn test_message() -> Message {
+        Message::builder()
+            .from("Test <test+ezrz@example.net>".parse().unwrap())
+            .to("Test2 <test2@example.org>".parse().unwrap())
+            .header(TestHeader("test  test very very long with spaces and extra spaces   \twill be folded to several lines ".to_string()))
+            .subject("Test with utf-8 ë")
+            .body("test\r\n\r\ntest   \ttest\r\n\r\n\r\n".to_string()).unwrap()
+    }
+
+    #[test]
+    fn test_headers_simple_canonicalize() {
+        let message = test_message();
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Simple),"From: Test <test+ezrz@example.net>\r\nTest: test  test very very long with spaces and extra spaces   \twill be \r\n folded to several lines \r\n")
+    }
+    #[test]
+    fn test_headers_relaxed_canonicalize() {
+        let message = test_message();
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Relaxed),"from:Test <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_signature_rsa() {
+        let mut message = test_message();
+        let key = "-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----";
+        let signing_key = DkimSigningKey::new(key.to_string(), DkimSigningAlgorithm::Rsa).unwrap();
+        message.sign(&DkimConfig::default_config(
+            "dkimtest".to_string(),
+            "example.org".to_string(),
+            signing_key,
+        ));
+        println!("{}", std::str::from_utf8(&message.formatted()).unwrap());
+        let mut verify_command = Command::new("dkimverify")
+            .stdin(Stdio::piped())
+            .spawn()
+            .expect("Fail to verify message signature");
+        let mut stdin = verify_command.stdin.take().expect("Failed to open stdin");
+        std::thread::spawn(move || {
+            stdin
+                .write_all(&message.formatted())
+                .expect("Failed to write to stdin");
+        });
+        assert!(verify_command
+            .wait()
+            .expect("Command did not run")
+            .success());
+    }
+}

src/message/header/content.rs

@@ -3,7 +3,7 @@ use std::{
     str::FromStr,
 };
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Transfer-Encoding` of the body
@@ -11,7 +11,8 @@ use crate::BoxError;
 /// The `Message` builder takes care of choosing the most
 /// efficient encoding based on the chosen body, so in most
 /// use-caches this header shouldn't be set manually.
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum ContentTransferEncoding {
     /// ASCII
     SevenBit,
@@ -34,8 +35,9 @@ impl Header for ContentTransferEncoding {
         Ok(s.parse()?)
     }
 
-    fn display(&self) -> String {
-        self.to_string()
+    fn display(&self) -> HeaderValue {
+        let val = self.to_string();
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -74,7 +76,7 @@ impl Default for ContentTransferEncoding {
 #[cfg(test)]
 mod test {
     use super::ContentTransferEncoding;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_transfer_encoding() {
@@ -93,20 +95,20 @@ mod test {
     fn parse_content_transfer_encoding() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
             "7bit".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentTransferEncoding>(),
             Some(ContentTransferEncoding::SevenBit)
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
             "base64".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentTransferEncoding>(),

src/message/header/content_disposition.rs

@@ -1,29 +1,55 @@
-use super::{Header, HeaderName};
+use std::fmt::Write;
+
+use email_encoding::headers::EmailWriter;
+
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Disposition` of an attachment
 ///
 /// Defined in [RFC2183](https://tools.ietf.org/html/rfc2183)
 #[derive(Debug, Clone, PartialEq)]
-pub struct ContentDisposition(String);
+pub struct ContentDisposition(HeaderValue);
 
 impl ContentDisposition {
     /// An attachment which should be displayed inline into the message
     pub fn inline() -> Self {
-        Self("inline".into())
+        Self(HeaderValue::dangerous_new_pre_encoded(
+            Self::name(),
+            "inline".to_string(),
+            "inline".to_string(),
+        ))
     }
 
     /// An attachment which should be displayed inline into the message, but that also
     /// species the filename in case it were to be downloaded
     pub fn inline_with_name(file_name: &str) -> Self {
-        debug_assert!(!file_name.contains('"'), "file_name shouldn't contain '\"'");
-        Self(format!("inline; filename=\"{}\"", file_name))
+        Self::with_name("inline", file_name)
     }
 
     /// An attachment which is separate from the body of the message, and can be downloaded separately
     pub fn attachment(file_name: &str) -> Self {
-        debug_assert!(!file_name.contains('"'), "file_name shouldn't contain '\"'");
-        Self(format!("attachment; filename=\"{}\"", file_name))
+        Self::with_name("attachment", file_name)
+    }
+
+    fn with_name(kind: &str, file_name: &str) -> Self {
+        let raw_value = format!("{}; filename=\"{}\"", kind, 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();
+
+        email_encoding::headers::rfc2231::encode("filename", file_name, &mut w)
+            .expect("some Write implementation returned an error");
+
+        Self(HeaderValue::dangerous_new_pre_encoded(
+            Self::name(),
+            raw_value,
+            encoded_value,
+        ))
     }
 }
 
@@ -33,10 +59,18 @@ impl Header for ContentDisposition {
     }
 
     fn parse(s: &str) -> Result<Self, BoxError> {
-        Ok(Self(s.into()))
+        match (s.split_once(';'), s) {
+            (_, "inline") => Ok(Self::inline()),
+            (Some((kind @ ("inline" | "attachment"), file_name)), _) => file_name
+                .split_once(" filename=\"")
+                .and_then(|(_, file_name)| file_name.strip_suffix('"'))
+                .map(|file_name| Self::with_name(kind, file_name))
+                .ok_or_else(|| "Unsupported ContentDisposition value".into()),
+            _ => Err("Unsupported ContentDisposition value".into()),
+        }
     }
 
-    fn display(&self) -> String {
+    fn display(&self) -> HeaderValue {
         self.0.clone()
     }
 }
@@ -44,7 +78,7 @@ impl Header for ContentDisposition {
 #[cfg(test)]
 mod test {
     use super::ContentDisposition;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_disposition() {
@@ -66,20 +100,20 @@ mod test {
     fn parse_content_disposition() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
             "inline".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentDisposition>(),
             Some(ContentDisposition::inline())
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
             "attachment; filename=\"something.txt\"".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<ContentDisposition>(),

src/message/header/content_type.rs

@@ -6,13 +6,18 @@ use std::{
 
 use mime::Mime;
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// `Content-Type` of the body
 ///
+/// This struct can represent any valid [mime type], which can be parsed via
+/// [`ContentType::parse`]. Constants are provided for the most-used mime-types.
+///
 /// Defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-5)
-#[derive(Debug, Clone, PartialEq)]
+///
+/// [mime type]: https://www.iana.org/assignments/media-types/media-types.xhtml
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ContentType(Mime);
 
 impl ContentType {
@@ -49,8 +54,8 @@ impl Header for ContentType {
         Ok(Self(s.parse()?))
     }
 
-    fn display(&self) -> String {
-        self.0.to_string()
+    fn display(&self) -> HeaderValue {
+        HeaderValue::new(Self::name(), self.0.to_string())
     }
 }
 
@@ -62,6 +67,14 @@ impl FromStr for ContentType {
     }
 }
 
+#[cfg(feature = "mime03")]
+#[cfg_attr(docsrs, doc(cfg(feature = "mime03")))]
+impl From<Mime> for ContentType {
+    fn from(mime: Mime) -> Self {
+        Self::from_mime(mime)
+    }
+}
+
 /// An error occurred while trying to [`ContentType::parse`].
 #[derive(Debug)]
 pub struct ContentTypeErr(mime::FromStrError);
@@ -78,10 +91,66 @@ impl Display for ContentTypeErr {
     }
 }
 
+// -- Serialization and Deserialization --
+#[cfg(feature = "serde")]
+mod serde {
+    use std::fmt;
+
+    use serde::{
+        de::{self, Deserialize, Deserializer, Visitor},
+        ser::{Serialize, Serializer},
+    };
+
+    use super::ContentType;
+
+    impl Serialize for ContentType {
+        fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        where
+            S: Serializer,
+        {
+            serializer.serialize_newtype_struct("ContentType", &format!("{}", &self.0))
+        }
+    }
+
+    impl<'de> Deserialize<'de> for ContentType {
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            struct ContentTypeVisitor;
+
+            impl<'de> Visitor<'de> for ContentTypeVisitor {
+                type Value = ContentType;
+
+                // The error message which states what the Visitor expects to
+                // receive
+                fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                    formatter.write_str("a ContentType string like `text/plain`")
+                }
+
+                fn visit_str<E>(self, mime: &str) -> Result<ContentType, E>
+                where
+                    E: de::Error,
+                {
+                    match ContentType::parse(mime) {
+                        Ok(content_type) => Ok(content_type),
+                        Err(_) => Err(E::custom(format!(
+                            "Couldn't parse the following MIME-Type: {}",
+                            mime
+                        ))),
+                    }
+                }
+            }
+
+            deserializer.deserialize_str(ContentTypeVisitor)
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::ContentType;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_content_type() {
@@ -106,17 +175,17 @@ mod test {
     fn parse_content_type() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
             "text/plain; charset=utf-8".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_PLAIN));
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
             "text/html; charset=utf-8".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_HTML));
     }

src/message/header/date.rs

@@ -2,7 +2,7 @@ use std::time::SystemTime;
 
 use httpdate::HttpDate;
 
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 /// Message `Date` header
@@ -43,18 +43,18 @@ impl Header for Date {
         Ok(Self(s.parse::<HttpDate>()?))
     }
 
-    fn display(&self) -> String {
-        let mut s = self.0.to_string();
-        if s.ends_with(" GMT") {
+    fn display(&self) -> HeaderValue {
+        let mut val = self.0.to_string();
+        if val.ends_with(" GMT") {
             // The httpdate crate always appends ` GMT` to the end of the string,
             // but this is considered an obsolete date format for email
             // https://tools.ietf.org/html/rfc2822#appendix-A.6.2,
             // so we replace `GMT` with `-0000`
-            s.truncate(s.len() - "GMT".len());
-            s.push_str("-0000");
+            val.truncate(val.len() - "GMT".len());
+            val.push_str("-0000");
         }
 
-        s
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -75,7 +75,7 @@ mod test {
     use std::time::{Duration, SystemTime};
 
     use super::Date;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_date() {
@@ -106,10 +106,10 @@ mod test {
     fn parse_date() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
             "Tue, 15 Nov 1994 08:12:31 -0000".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Date>(),
@@ -118,10 +118,10 @@ mod test {
             ))
         );
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
             "Tue, 15 Nov 1994 08:12:32 -0000".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Date>(),

src/message/header/mailbox.rs

@@ -1,4 +1,6 @@
-use super::{Header, HeaderName};
+use email_encoding::headers::EmailWriter;
+
+use super::{Header, HeaderName, HeaderValue};
 use crate::{
     message::mailbox::{Mailbox, Mailboxes},
     BoxError,
@@ -25,8 +27,13 @@ macro_rules! mailbox_header {
                 Ok(Self(mailbox))
             }
 
-            fn display(&self) -> String {
-                self.0.to_string()
+            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");
+
+                HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
         }
 
@@ -68,8 +75,13 @@ macro_rules! mailboxes_header {
                 Ok(Self(mailbox))
             }
 
-            fn display(&self) -> String {
-                self.0.to_string()
+            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");
+
+                HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
         }
 
@@ -161,7 +173,7 @@ mailboxes_header! {
 #[cfg(test)]
 mod test {
     use super::{From, Mailbox, Mailboxes};
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_single_without_name() {
@@ -175,12 +187,12 @@ mod test {
 
     #[test]
     fn format_single_with_name() {
-        let from = Mailboxes::new().with("K. <kayo@example.com>".parse().unwrap());
+        let from = Mailboxes::new().with("Kayo <kayo@example.com>".parse().unwrap());
 
         let mut headers = Headers::new();
         headers.set(From(from));
 
-        assert_eq!(headers.to_string(), "From: K. <kayo@example.com>\r\n");
+        assert_eq!(headers.to_string(), "From: Kayo <kayo@example.com>\r\n");
     }
 
     #[test]
@@ -201,7 +213,7 @@ mod test {
     #[test]
     fn format_multi_with_name() {
         let from = vec![
-            "K. <kayo@example.com>".parse().unwrap(),
+            "Kayo <kayo@example.com>".parse().unwrap(),
             "Pony P. <pony@domain.tld>".parse().unwrap(),
         ];
 
@@ -210,7 +222,7 @@ mod test {
 
         assert_eq!(
             headers.to_string(),
-            "From: K. <kayo@example.com>, Pony P. <pony@domain.tld>\r\n"
+            "From: Kayo <kayo@example.com>, \"Pony P.\" <pony@domain.tld>\r\n"
         );
     }
 
@@ -232,10 +244,10 @@ mod test {
         let from = vec!["kayo@example.com".parse().unwrap()].into();
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
             "kayo@example.com".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
     }
@@ -245,10 +257,10 @@ mod test {
         let from = vec!["K. <kayo@example.com>".parse().unwrap()].into();
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
             "K. <kayo@example.com>".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
     }
@@ -261,10 +273,10 @@ mod test {
         ];
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
             "kayo@example.com, pony@domain.tld".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
     }
@@ -277,10 +289,10 @@ mod test {
         ];
 
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
             "K. <kayo@example.com>, Pony P. <pony@domain.tld>".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
     }

src/message/header/mod.rs

@@ -26,18 +26,21 @@ mod mailbox;
 mod special;
 mod textual;
 
+/// Represents an email header
+///
+/// Email header as defined in [RFC5322](https://datatracker.ietf.org/doc/html/rfc5322) and extensions.
 pub trait Header: Clone {
     fn name() -> HeaderName;
 
     fn parse(s: &str) -> Result<Self, BoxError>;
 
-    fn display(&self) -> String;
+    fn display(&self) -> HeaderValue;
 }
 
 /// A set of email headers
 #[derive(Debug, Clone, Default)]
 pub struct Headers {
-    headers: Vec<(HeaderName, String)>,
+    headers: Vec<HeaderValue>,
 }
 
 impl Headers {
@@ -65,13 +68,14 @@ impl Headers {
     ///
     /// Returns `None` if `Header` isn't present in `Headers`.
     pub fn get<H: Header>(&self) -> Option<H> {
-        self.get_raw(&H::name()).and_then(|raw| H::parse(raw).ok())
+        self.get_raw(&H::name())
+            .and_then(|raw_value| H::parse(raw_value).ok())
     }
 
     /// Sets `Header` into `Headers`, overriding `Header` if it
     /// was already present in `Headers`
     pub fn set<H: Header>(&mut self, header: H) {
-        self.insert_raw(H::name(), header.display());
+        self.insert_raw(header.display());
     }
 
     /// Remove `Header` from `Headers`, returning it
@@ -79,7 +83,7 @@ impl Headers {
     /// Returns `None` if `Header` isn't in `Headers`.
     pub fn remove<H: Header>(&mut self) -> Option<H> {
         self.remove_raw(&H::name())
-            .and_then(|(_name, raw)| H::parse(&raw).ok())
+            .and_then(|value| H::parse(&value.raw_value).ok())
     }
 
     /// Clears `Headers`, removing all headers from it
@@ -94,62 +98,46 @@ impl Headers {
     ///
     /// Returns `None` if `name` isn't present in `Headers`.
     pub fn get_raw(&self, name: &str) -> Option<&str> {
-        self.find_header(name).map(|(_name, value)| value)
+        self.find_header(name).map(|value| value.raw_value.as_str())
     }
 
     /// Inserts a raw header into `Headers`, overriding `value` if it
     /// was already present in `Headers`.
-    pub fn insert_raw(&mut self, name: HeaderName, value: String) {
-        match self.find_header_mut(&name) {
-            Some((_, current_value)) => {
+    pub fn insert_raw(&mut self, value: HeaderValue) {
+        match self.find_header_mut(&value.name) {
+            Some(current_value) => {
                 *current_value = value;
             }
             None => {
-                self.headers.push((name, value));
+                self.headers.push(value);
             }
         }
     }
 
-    /// Appends a raw header into `Headers`
-    ///
-    /// If a header with a name of `name` is already present,
-    /// appends `, ` + `value` to it's current value.
-    pub fn append_raw(&mut self, name: HeaderName, value: String) {
-        match self.find_header_mut(&name) {
-            Some((_name, prev_value)) => {
-                prev_value.push_str(", ");
-                prev_value.push_str(&value);
-            }
-            None => self.headers.push((name, value)),
-        }
-    }
-
     /// Remove a raw header from `Headers`, returning it
     ///
     /// Returns `None` if `name` isn't present in `Headers`.
-    pub fn remove_raw(&mut self, name: &str) -> Option<(HeaderName, String)> {
+    pub fn remove_raw(&mut self, name: &str) -> Option<HeaderValue> {
         self.find_header_index(name).map(|i| self.headers.remove(i))
     }
 
-    fn find_header(&self, name: &str) -> Option<(&HeaderName, &str)> {
+    fn find_header(&self, name: &str) -> Option<&HeaderValue> {
         self.headers
             .iter()
-            .find(|&(name_, _value)| name.eq_ignore_ascii_case(name_))
-            .map(|t| (&t.0, t.1.as_str()))
+            .find(|value| name.eq_ignore_ascii_case(&value.name))
     }
 
-    fn find_header_mut(&mut self, name: &str) -> Option<(&HeaderName, &mut String)> {
+    fn find_header_mut(&mut self, name: &str) -> Option<&mut HeaderValue> {
         self.headers
             .iter_mut()
-            .find(|(name_, _value)| name.eq_ignore_ascii_case(name_))
-            .map(|t| (&t.0, &mut t.1))
+            .find(|value| name.eq_ignore_ascii_case(&value.name))
     }
 
     fn find_header_index(&self, name: &str) -> Option<usize> {
         self.headers
             .iter()
             .enumerate()
-            .find(|&(_i, (name_, _value))| name.eq_ignore_ascii_case(name_))
+            .find(|(_i, value)| name.eq_ignore_ascii_case(&value.name))
             .map(|(i, _)| i)
     }
 }
@@ -157,10 +145,10 @@ impl Headers {
 impl Display for Headers {
     /// Formats `Headers`, ready to put them into an email
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        for (name, value) in &self.headers {
-            Display::fmt(name, f)?;
+        for value in &self.headers {
+            f.write_str(&value.name)?;
             f.write_str(": ")?;
-            HeaderValueEncoder::encode(&name, &value, f)?;
+            f.write_str(&value.encoded_value)?;
             f.write_str("\r\n")?;
         }
 
@@ -237,7 +225,7 @@ impl HeaderName {
 
 impl Display for HeaderName {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        f.write_str(&self)
+        f.write_str(self)
     }
 }
 
@@ -287,6 +275,38 @@ impl PartialEq<HeaderName> for &str {
     }
 }
 
+#[derive(Debug, Clone, PartialEq)]
+pub struct HeaderValue {
+    name: HeaderName,
+    raw_value: String,
+    encoded_value: String,
+}
+
+impl HeaderValue {
+    pub fn new(name: HeaderName, raw_value: String) -> Self {
+        let mut encoded_value = String::with_capacity(raw_value.len());
+        HeaderValueEncoder::encode(&name, &raw_value, &mut encoded_value).unwrap();
+
+        Self {
+            name,
+            raw_value,
+            encoded_value,
+        }
+    }
+
+    pub fn dangerous_new_pre_encoded(
+        name: HeaderName,
+        raw_value: String,
+        encoded_value: String,
+    ) -> Self {
+        Self {
+            name,
+            raw_value,
+            encoded_value,
+        }
+    }
+}
+
 const ENCODING_START_PREFIX: &str = "=?utf-8?b?";
 const ENCODING_END_SUFFIX: &str = "?=";
 const MAX_LINE_LEN: usize = 76;
@@ -298,7 +318,7 @@ struct HeaderValueEncoder {
 }
 
 impl HeaderValueEncoder {
-    fn encode(name: &str, value: &str, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    fn encode(name: &str, value: &str, f: &mut impl fmt::Write) -> fmt::Result {
         let (words_iter, encoder) = Self::new(name, value);
         encoder.format(words_iter, f)
     }
@@ -316,7 +336,7 @@ impl HeaderValueEncoder {
     fn format(
         mut self,
         words_iter: WordsPlusFillIterator<'_>,
-        f: &mut fmt::Formatter<'_>,
+        f: &mut impl fmt::Write,
     ) -> fmt::Result {
         /// Estimate if an encoded string of `len` would fix in an empty line
         fn would_fit_new_line(len: usize) -> bool {
@@ -408,13 +428,14 @@ impl HeaderValueEncoder {
                 let mut next_word = next_word;
 
                 while !next_word.is_empty() {
-                    if self.remaining_line_len() <= base64_len(1) {
+                    let mut len = available_len_to_max_encode_len(self.remaining_line_len())
+                        .min(next_word.len());
+
+                    if len == 0 {
                         self.flush_encode_buf(f, false)?;
                         self.new_line(f)?;
                     }
 
-                    let mut len = available_len_to_max_encode_len(self.remaining_line_len())
-                        .min(next_word.len());
                     // avoid slicing on a char boundary
                     while !next_word.is_char_boundary(len) {
                         len += 1;
@@ -444,11 +465,9 @@ impl HeaderValueEncoder {
 
     fn flush_encode_buf(
         &mut self,
-        f: &mut fmt::Formatter<'_>,
+        f: &mut impl fmt::Write,
         switching_to_allowed: bool,
     ) -> fmt::Result {
-        use std::fmt::Write;
-
         if self.encode_buf.is_empty() {
             // nothing to encode
             return Ok(());
@@ -473,7 +492,7 @@ impl HeaderValueEncoder {
             self.encode_buf.as_bytes(),
             base64::STANDARD,
         );
-        Display::fmt(&encoded, f)?;
+        write!(f, "{}", encoded)?;
         f.write_str(ENCODING_END_SUFFIX)?;
 
         self.line_len += ENCODING_START_PREFIX.len();
@@ -489,7 +508,7 @@ impl HeaderValueEncoder {
         Ok(())
     }
 
-    fn new_line(&mut self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    fn new_line(&mut self, f: &mut impl fmt::Write) -> fmt::Result {
         f.write_str("\r\n ")?;
         self.line_len = 1;
 
@@ -519,7 +538,7 @@ impl<'a> Iterator for WordsPlusFillIterator<'a> {
             .find(|&(_i, c)| !is_space_like(c))
             .map(|(i, _)| i);
 
-        let word = &self.s[..next_word.unwrap_or_else(|| self.s.len())];
+        let word = &self.s[..next_word.unwrap_or(self.s.len())];
         self.s = &self.s[word.len()..];
         Some(word)
     }
@@ -542,7 +561,7 @@ const fn allowed_char(c: char) -> bool {
 
 #[cfg(test)]
 mod tests {
-    use super::{HeaderName, Headers};
+    use super::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn valid_headername() {
@@ -603,10 +622,10 @@ mod tests {
     #[test]
     fn format_ascii() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
             "John Doe <example@example.com>, Jean Dupont <jean@example.com>".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -617,10 +636,10 @@ mod tests {
     #[test]
     fn format_ascii_with_folding() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
             "Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand <jemand@example.com>, Jean Dupont <jean@example.com>".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -635,10 +654,10 @@ mod tests {
     #[test]
     fn format_ascii_with_folding_long_line() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -654,9 +673,10 @@ mod tests {
     fn format_ascii_with_folding_very_long_line() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know".to_string()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -670,10 +690,10 @@ mod tests {
     #[test]
     fn format_ascii_with_folding_giant_word() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_string()
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -688,10 +708,10 @@ mod tests {
     #[test]
     fn format_special() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
             "Seán <sean@example.com>".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -702,10 +722,10 @@ mod tests {
     #[test]
     fn format_special_emoji() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
             "🌎 <world@example.com>".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -716,10 +736,10 @@ mod tests {
     #[test]
     fn format_special_with_folding() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        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(),
-        );
+         ) );
 
         assert_eq!(
             headers.to_string(),
@@ -737,8 +757,9 @@ mod tests {
     fn format_slice_on_char_boundary_bug() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_string(),
+            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_string(),)
         );
 
         assert_eq!(
@@ -750,10 +771,10 @@ mod tests {
     #[test]
     fn format_bad_stuff() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "Hello! \r\n This is \" bad \0. 👋".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -765,21 +786,25 @@ mod tests {
     fn format_everything() {
         let mut headers = Headers::new();
         headers.insert_raw(
+            HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
+            )
         );
         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(),
+            )
         );
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
             "Someone <somewhere@example.com>".to_string(),
-        );
-        headers.insert_raw(
+        ));
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
             "quoted-printable".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.to_string(),
@@ -797,4 +822,21 @@ mod tests {
             )
         );
     }
+
+    #[test]
+    fn issue_653() {
+        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(),
+        ));
+
+        assert_eq!(
+            headers.to_string(),
+            concat!(
+                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go; \r\n",
+                " =?utf-8?b?Ozs7OztzOzs7Ozs7Ozs7Ozs7Ozs7O2ZmZmVpbm1qZ2dnZ2dnZ2dn772G44Gj?=\r\n"
+            )
+        );
+    }
 }

src/message/header/special.rs

@@ -1,5 +1,5 @@
 use crate::{
-    message::header::{Header, HeaderName},
+    message::header::{Header, HeaderName, HeaderValue},
     BoxError,
 };
 
@@ -10,6 +10,9 @@ pub struct MimeVersion {
     minor: u8,
 }
 
+/// MIME version 1.0
+///
+/// Should be used in all MIME messages.
 pub const MIME_VERSION_1_0: MimeVersion = MimeVersion::new(1, 0);
 
 impl MimeVersion {
@@ -47,8 +50,9 @@ impl Header for MimeVersion {
         Ok(MimeVersion::new(major, minor))
     }
 
-    fn display(&self) -> String {
-        format!("{}.{}", self.major, self.minor)
+    fn display(&self) -> HeaderValue {
+        let val = format!("{}.{}", self.major, self.minor);
+        HeaderValue::dangerous_new_pre_encoded(Self::name(), val.clone(), val)
     }
 }
 
@@ -61,7 +65,7 @@ impl Default for MimeVersion {
 #[cfg(test)]
 mod test {
     use super::{MimeVersion, MIME_VERSION_1_0};
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_mime_version() {
@@ -80,17 +84,17 @@ mod test {
     fn parse_mime_version() {
         let mut headers = Headers::new();
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
             "1.0".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MIME_VERSION_1_0));
 
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
             "0.1".to_string(),
-        );
+        ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MimeVersion::new(0, 1)));
     }

src/message/header/textual.rs

@@ -1,4 +1,4 @@
-use super::{Header, HeaderName};
+use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
 
 macro_rules! text_header {
@@ -16,8 +16,8 @@ macro_rules! text_header {
                 Ok(Self(s.into()))
             }
 
-            fn display(&self) -> String {
-                self.0.clone()
+            fn display(&self) -> HeaderValue {
+             HeaderValue::new(Self::name(),   self.0.clone())
             }
         }
 
@@ -86,7 +86,7 @@ text_header! {
 #[cfg(test)]
 mod test {
     use super::Subject;
-    use crate::message::header::{HeaderName, Headers};
+    use crate::message::header::{HeaderName, HeaderValue, Headers};
 
     #[test]
     fn format_ascii() {
@@ -110,10 +110,10 @@ mod test {
     #[test]
     fn parse_ascii() {
         let mut headers = Headers::new();
-        headers.insert_raw(
+        headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
             "Sample subject".to_string(),
-        );
+        ));
 
         assert_eq!(
             headers.get::<Subject>(),

src/message/mailbox/serde.rs

@@ -1,10 +1,12 @@
-use crate::message::{Mailbox, Mailboxes};
+use std::fmt::{Formatter, Result as FmtResult};
+
 use serde::{
     de::{Deserializer, Error as DeError, MapAccess, SeqAccess, Visitor},
     ser::Serializer,
     Deserialize, Serialize,
 };
-use std::fmt::{Formatter, Result as FmtResult};
+
+use crate::message::{Mailbox, Mailboxes};
 
 impl Serialize for Mailbox {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -152,9 +154,10 @@ impl<'de> Deserialize<'de> for Mailboxes {
 
 #[cfg(test)]
 mod test {
+    use serde_json::from_str;
+
     use super::*;
     use crate::address::Address;
-    use serde_json::from_str;
 
     #[test]
     fn parse_address_string() {

src/message/mailbox/types.rs

@@ -1,11 +1,14 @@
-use crate::address::{Address, AddressError};
 use std::{
-    convert::TryFrom,
     fmt::{Display, Formatter, Result as FmtResult, Write},
+    mem,
     slice::Iter,
     str::FromStr,
 };
 
+use email_encoding::headers::EmailWriter;
+
+use crate::address::{Address, AddressError};
+
 /// Represents an email address with an optional name for the sender/recipient.
 ///
 /// This type contains email address and the sender/recipient name (_Some Name \<user@domain.tld\>_ or _withoutname@domain.tld_).
@@ -63,6 +66,22 @@ impl Mailbox {
     pub fn new(name: Option<String>, email: Address) -> Self {
         Mailbox { name, email }
     }
+
+    pub(crate) fn encode(&self, w: &mut EmailWriter<'_>) -> FmtResult {
+        if let Some(name) = &self.name {
+            email_encoding::headers::quoted_string::encode(name, w)?;
+            w.space();
+            w.write_char('<')?;
+        }
+
+        w.write_str(self.email.as_ref())?;
+
+        if self.name.is_some() {
+            w.write_char('>')?;
+        }
+
+        Ok(())
+    }
 }
 
 impl Display for Mailbox {
@@ -70,7 +89,7 @@ impl Display for Mailbox {
         if let Some(ref name) = self.name {
             let name = name.trim();
             if !name.is_empty() {
-                f.write_str(&name)?;
+                write_word(f, name)?;
                 f.write_str(" <")?;
                 self.email.fmt(f)?;
                 return f.write_char('>');
@@ -250,6 +269,20 @@ impl Mailboxes {
     pub fn iter(&self) -> Iter<'_, Mailbox> {
         self.0.iter()
     }
+
+    pub(crate) fn encode(&self, w: &mut EmailWriter<'_>) -> FmtResult {
+        let mut first = true;
+        for mailbox in self.iter() {
+            if !mem::take(&mut first) {
+                w.write_char(',')?;
+                w.space();
+            }
+
+            mailbox.encode(w)?;
+        }
+
+        Ok(())
+    }
 }
 
 impl Default for Mailboxes {
@@ -327,11 +360,93 @@ impl FromStr for Mailboxes {
     }
 }
 
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.6
+fn write_word(f: &mut Formatter<'_>, s: &str) -> FmtResult {
+    if s.as_bytes().iter().copied().all(is_valid_atom_char) {
+        f.write_str(s)
+    } else {
+        // Quoted string: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
+        f.write_char('"')?;
+        for &c in s.as_bytes() {
+            write_quoted_string_char(f, c)?;
+        }
+        f.write_char('"')?;
+
+        Ok(())
+    }
+}
+
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.4
+fn is_valid_atom_char(c: u8) -> bool {
+    matches!(c,
+		// Not really allowed but can be inserted between atoms.
+		b'\t' |
+		b' ' |
+
+		b'!' |
+		b'#' |
+		b'$' |
+		b'%' |
+		b'&' |
+		b'\'' |
+		b'*' |
+		b'+' |
+		b'-' |
+		b'/' |
+		b'0'..=b'8' |
+		b'=' |
+		b'?' |
+		b'A'..=b'Z' |
+		b'^' |
+		b'_' |
+		b'`' |
+		b'a'..=b'z' |
+		b'{' |
+		b'|' |
+		b'}' |
+		b'~' |
+
+		// Not techically allowed but will be escaped into allowed characters.
+		128..=255)
+}
+
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
+fn write_quoted_string_char(f: &mut Formatter<'_>, c: u8) -> FmtResult {
+    match c {
+		// NO-WS-CTL: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
+		1..=8 | 11 | 12 | 14..=31 | 127 |
+
+		// Note, not qcontent but can be put before or after any qcontent.
+		b'\t' |
+		b' ' |
+
+		// The rest of the US-ASCII except \ and "
+		33 |
+		35..=91 |
+		93..=126 |
+
+		// Non-ascii characters will be escaped separately later.
+		128..=255
+
+		=> f.write_char(c.into()),
+
+		// 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())
+		}
+	}
+}
+
 #[cfg(test)]
 mod test {
-    use super::Mailbox;
     use std::convert::TryInto;
 
+    use super::Mailbox;
+
     #[test]
     fn mailbox_format_address_only() {
         assert_eq!(
@@ -350,7 +465,35 @@ mod test {
                 "{}",
                 Mailbox::new(Some("K.".into()), "kayo@example.com".parse().unwrap())
             ),
-            "K. <kayo@example.com>"
+            "\"K.\" <kayo@example.com>"
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_comma() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Last, First".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Last, First" <kayo@example.com>"#
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_color() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Chris's Wiki :: blog".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Chris's Wiki :: blog" <kayo@example.com>"#
         );
     }
 
@@ -372,7 +515,7 @@ mod test {
                 "{}",
                 Mailbox::new(Some(" K. ".into()), "kayo@example.com".parse().unwrap())
             ),
-            "K. <kayo@example.com>"
+            "\"K.\" <kayo@example.com>"
         );
     }
 

src/message/mimebody.rs

@@ -1,11 +1,11 @@
-use std::io::Write;
+use std::{io::Write, iter::repeat_with};
+
+use mime::Mime;
 
 use crate::message::{
     header::{self, ContentTransferEncoding, ContentType, Header, Headers},
     EmailFormat, IntoBody,
 };
-use mime::Mime;
-use std::iter::repeat_with;
 
 /// MIME part variants
 #[derive(Debug, Clone)]

src/message/mod.rs

@@ -4,15 +4,8 @@
 //!
 //! This section demonstrates how to build messages.
 //!
-//! <!--
-//! style for <details><summary>Blablabla</summary> Lots of stuff</details>
-//! borrowed from https://docs.rs/time/0.2.23/src/time/lib.rs.html#49-54
-//! -->
 //! <style>
 //! summary, details:not([open]) { cursor: pointer; }
-//! summary { display: list-item; }
-//! summary::marker { content: '▶ '; }
-//! details[open] summary::marker { content: '▼ '; }
 //! </style>
 //!
 //!
@@ -113,9 +106,10 @@
 //!
 //! ```rust
 //! # use std::error::Error;
-//! use lettre::message::{header, Attachment, Body, Message, MultiPart, SinglePart};
 //! use std::fs;
 //!
+//! use lettre::message::{header, Attachment, Body, Message, MultiPart, SinglePart};
+//!
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! let image = fs::read("docs/lettre.png")?;
 //! // this image_body can be cloned and reused between emails.
@@ -202,15 +196,19 @@
 //! ```
 //! </details>
 
-use std::{convert::TryFrom, io::Write, iter, time::SystemTime};
+use std::{io::Write, iter, time::SystemTime};
 
 pub use attachment::Attachment;
 pub use body::{Body, IntoBody, MaybeString};
+#[cfg(feature = "dkim")]
+pub use dkim::*;
 pub use mailbox::*;
 pub use mimebody::*;
 
 mod attachment;
 mod body;
+#[cfg(feature = "dkim")]
+pub mod dkim;
 pub mod header;
 mod mailbox;
 mod mimebody;
@@ -401,7 +399,7 @@ impl MessageBuilder {
         // https://tools.ietf.org/html/rfc5322#section-3.6
 
         // Insert Date if missing
-        let res = if self.headers.get::<header::Date>().is_none() {
+        let mut res = if self.headers.get::<header::Date>().is_none() {
             self.date_now()
         } else {
             self
@@ -424,6 +422,10 @@ impl MessageBuilder {
             Some(e) => e,
             None => Envelope::try_from(&res.headers)?,
         };
+
+        // Remove `Bcc` headers now the envelope is set
+        res.headers.remove::<header::Bcc>();
+
         Ok(Message {
             headers: res.headers,
             body,
@@ -492,6 +494,77 @@ impl Message {
         self.format(&mut out);
         out
     }
+
+    #[cfg(feature = "dkim")]
+    /// Format body for signing
+    pub(crate) fn body_raw(&self) -> Vec<u8> {
+        let mut out = Vec::new();
+        match &self.body {
+            MessageBody::Mime(p) => p.format(&mut out),
+            MessageBody::Raw(r) => out.extend_from_slice(r),
+        };
+        out.extend_from_slice(b"\r\n");
+        out
+    }
+
+    /// Sign the message using Dkim
+    ///
+    /// Example:
+    /// ```rust
+    /// use lettre::{
+    ///     message::dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
+    ///     Message,
+    /// };
+    ///
+    /// let mut message = Message::builder()
+    ///     .from("Alice <alice@example.org>".parse().unwrap())
+    ///     .reply_to("Bob <bob@example.org>".parse().unwrap())
+    ///     .to("Carla <carla@example.net>".parse().unwrap())
+    ///     .subject("Hello")
+    ///     .body("Hi there, it's a test email, with utf-8 chars ë!\n\n\n".to_string())
+    ///     .unwrap();
+    /// let key = "-----BEGIN RSA PRIVATE KEY-----
+    /// MIIEowIBAAKCAQEAt2gawjoybf0mAz0mSX0cq1ah5F9cPazZdCwLnFBhRufxaZB8
+    /// NLTdc9xfPIOK8l/xGrN7Nd63J4cTATqZukumczkA46O8YKHwa53pNT6NYwCNtDUL
+    /// eBu+7xUW18GmDzkIFkxGO2R5kkTeWPlKvKpEiicIMfl0OmyW/fI3AbtM7e/gmqQ4
+    /// kEYIO0mTjPT+jTgWE4JIi5KUTHudUBtfMKcSFyM2HkUOExl1c9+A4epjRFQwEXMA
+    /// hM5GrqZoOdUm4fIpvGpLIGIxFgHPpZYbyq6yJZzH3+5aKyCHrsHawPuPiCD45zsU
+    /// re31zCE6b6k1sDiiBR4CaRHnbL7hxFp0aNLOVQIDAQABAoIBAGMK3gBrKxaIcUGo
+    /// gQeIf7XrJ6vK72YC9L8uleqI4a9Hy++E7f4MedZ6eBeWta8jrnEL4Yp6xg+beuDc
+    /// A24+Mhng+6Dyp+TLLqj+8pQlPnbrMprRVms7GIXFrrs+wO1RkBNyhy7FmH0roaMM
+    /// pJZzoGW2pE9QdbqjL3rdlWTi/60xRX9eZ42nNxYnbc+RK03SBd46c3UBha6Y9iQX
+    /// 562yWilDnB5WCX2tBoSN39bEhJvuZDzMwOuGw68Q96Hdz82Iz1xVBnRhH+uNStjR
+    /// VnAssSHVxPSpwWrm3sHlhjBHWPnNIaOKIKl1lbL+qWfVQCj/6a5DquC+vYAeYR6L
+    /// 3mA0z0ECgYEA5YkNYcILSXyE0hZ8eA/t58h8eWvYI5iqt3nT4fznCoYJJ74Vukeg
+    /// 6BTlq/CsanwT1lDtvDKrOaJbA7DPTES/bqT0HoeIdOvAw9w/AZI5DAqYp61i6RMK
+    /// xfAQL/Ik5MDFN8gEMLLXRVMe/aR27f6JFZpShJOK/KCzHqikKfYVJ+UCgYEAzI2F
+    /// ZlTyittWSyUSl5UKyfSnFOx2+6vNy+lu5DeMJu8Wh9rqBk388Bxq98CfkCseWESN
+    /// pTCGdYltz9DvVNBdBLwSMdLuYJAI6U+Zd70MWyuNdHFPyWVHUNqMUBvbUtj2w74q
+    /// Hzu0GI0OrRjdX6C63S17PggmT/N2R9X7P4STxbECgYA+AZAD4I98Ao8+0aQ+Ks9x
+    /// 1c8KXf+9XfiAKAD9A3zGcv72JXtpHwBwsXR5xkJNYcdaFfKi7G0k3J8JmDHnwIqW
+    /// MSlhNeu+6hDg2BaNLhsLDbG/Wi9mFybJ4df9m8Qrp4efUgEPxsAwkgvFKTCXijMu
+    /// CspP1iutoxvAJH50d22voQKBgDIsSFtIXNGYaTs3Va8enK3at5zXP3wNsQXiNRP/
+    /// V/44yNL77EktmewfXFF2yuym1uOZtRCerWxpEClYO0wXa6l8pA3aiiPfUIBByQfo
+    /// s/4s2Z6FKKfikrKPWLlRi+NvWl+65kQQ9eTLvJzSq4IIP61+uWsGvrb/pbSLFPyI
+    /// fWKRAoGBALFCStBXvdMptjq4APUzAdJ0vytZzXkOZHxgmc+R0fQn22OiW0huW6iX
+    /// JcaBbL6ZSBIMA3AdaIjtvNRiomueHqh0GspTgOeCE2585TSFnw6vEOJ8RlR4A0Mw
+    /// I45fbR4l+3D/30WMfZlM6bzZbwPXEnr2s1mirmuQpjumY9wLhK25
+    /// -----END RSA PRIVATE KEY-----";
+    /// let signing_key = DkimSigningKey::new(key.to_string(), DkimSigningAlgorithm::Rsa).unwrap();
+    /// message.sign(&DkimConfig::default_config(
+    ///     "dkimtest".to_string(),
+    ///     "example.org".to_string(),
+    ///     signing_key,
+    /// ));
+    /// println!(
+    ///     "message: {}",
+    ///     std::str::from_utf8(&message.formatted()).unwrap()
+    /// );
+    /// ```
+    #[cfg(feature = "dkim")]
+    pub fn sign(&mut self, dkim_config: &DkimConfig) {
+        dkim_sign(self, dkim_config);
+    }
 }
 
 impl EmailFormat for Message {
@@ -503,7 +576,7 @@ impl EmailFormat for Message {
             MessageBody::Mime(p) => p.format(out),
             MessageBody::Raw(r) => {
                 out.extend_from_slice(b"\r\n");
-                out.extend_from_slice(&r)
+                out.extend_from_slice(r)
             }
         }
     }
@@ -559,6 +632,7 @@ mod test {
 
         let email = Message::builder()
             .date(date)
+            .bcc("hidden@example.com".parse().unwrap())
             .header(header::From(
                 vec![Mailbox::new(
                     Some("Каи".into()),
@@ -578,7 +652,7 @@ mod test {
             concat!(
                 "Date: Tue, 15 Nov 1994 08:12:31 -0000\r\n",
                 "From: =?utf-8?b?0JrQsNC4?= <kayo@example.com>\r\n",
-                "To: Pony O.P. <pony@domain.tld>\r\n",
+                "To: \"Pony O.P.\" <pony@domain.tld>\r\n",
                 "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvSE=?=\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",

src/transport/file/error.rs

@@ -1,8 +1,9 @@
 //! Error and result type for file transport
 
-use crate::BoxError;
 use std::{error::Error as StdError, fmt};
 
+use crate::BoxError;
+
 /// The Errors that may occur when sending an email over SMTP
 pub struct Error {
     inner: Box<Inner>,

src/transport/file/mod.rs

@@ -9,9 +9,10 @@
 //! #
 //! # #[cfg(all(feature = "file-transport", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
 //!
+//! use lettre::{FileTransport, Message, Transport};
+//!
 //! // Write to the local temp directory
 //! let sender = FileTransport::new(temp_dir());
 //! let email = Message::builder()
@@ -41,9 +42,10 @@
 //! #
 //! # #[cfg(all(feature = "file-transport-envelope", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
 //!
+//! use lettre::{FileTransport, Message, Transport};
+//!
 //! // Write to the local temp directory
 //! let sender = FileTransport::with_envelope(temp_dir());
 //! let email = Message::builder()
@@ -70,7 +72,8 @@
 //! # #[cfg(all(feature = "tokio1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{AsyncTransport, Tokio1Executor, Message, AsyncFileTransport};
+//!
+//! use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -95,7 +98,8 @@
 //! # #[cfg(all(feature = "async-std1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{AsyncTransport, AsyncStd1Executor, Message, AsyncFileTransport};
+//!
+//! use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
@@ -132,20 +136,22 @@
 //! {"forward_path":["hei@domain.tld"],"reverse_path":"nobody@domain.tld"}
 //! ```
 
-pub use self::error::Error;
-use crate::{address::Envelope, Transport};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use crate::{AsyncTransport, Executor};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use async_trait::async_trait;
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
 use std::marker::PhantomData;
 use std::{
     path::{Path, PathBuf},
     str,
 };
+
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use async_trait::async_trait;
 use uuid::Uuid;
 
+pub use self::error::Error;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use crate::{AsyncTransport, Executor};
+
 mod error;
 
 type Id = String;

src/transport/mod.rs

@@ -56,8 +56,7 @@
 //! #
 //! # #[cfg(all(feature = "builder", feature = "smtp-transport"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::transport::smtp::authentication::Credentials;
-//! use lettre::{Message, SmtpTransport, Transport};
+//! use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -152,7 +151,7 @@ pub trait AsyncTransport {
     async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
         let raw = message.formatted();
         let envelope = message.envelope();
-        self.send_raw(&envelope, &raw).await
+        self.send_raw(envelope, &raw).await
     }
 
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;

src/transport/sendmail/error.rs

@@ -1,8 +1,9 @@
 //! Error and result type for sendmail transport
 
-use crate::BoxError;
 use std::{error::Error as StdError, fmt};
 
+use crate::BoxError;
+
 /// The Errors that may occur when sending an email over sendmail
 pub struct Error {
     inner: Box<Inner>,

src/transport/sendmail/mod.rs

@@ -33,7 +33,9 @@
 //! #
 //! # #[cfg(all(feature = "tokio1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, AsyncTransport, Tokio1Executor, AsyncSendmailTransport, SendmailTransport};
+//! use lettre::{
+//!     AsyncSendmailTransport, AsyncTransport, Message, SendmailTransport, Tokio1Executor,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -72,16 +74,6 @@
 //! # }
 //! ```
 
-pub use self::error::Error;
-#[cfg(feature = "async-std1")]
-use crate::AsyncStd1Executor;
-#[cfg(feature = "tokio1")]
-use crate::Tokio1Executor;
-use crate::{address::Envelope, Transport};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use crate::{AsyncTransport, Executor};
-#[cfg(any(feature = "async-std1", feature = "tokio1"))]
-use async_trait::async_trait;
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
 use std::marker::PhantomData;
 use std::{
@@ -90,9 +82,21 @@ use std::{
     process::{Command, Stdio},
 };
 
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use async_trait::async_trait;
+
+pub use self::error::Error;
+#[cfg(feature = "async-std1")]
+use crate::AsyncStd1Executor;
+#[cfg(feature = "tokio1")]
+use crate::Tokio1Executor;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+use crate::{AsyncTransport, Executor};
+
 mod error;
 
-const DEFAULT_SENDMAIL: &str = "/usr/sbin/sendmail";
+const DEFAULT_SENDMAIL: &str = "sendmail";
 
 /// Sends emails using the `sendmail` command
 #[derive(Debug, Clone)]
@@ -113,7 +117,10 @@ pub struct AsyncSendmailTransport<E: Executor> {
 }
 
 impl SendmailTransport {
-    /// Creates a new transport with the default `/usr/sbin/sendmail` command
+    /// Creates a new transport with the `sendmail` command
+    ///
+    /// Note: This uses the `sendmail` command in the current `PATH`. To use another command,
+    /// use [SendmailTransport::new_with_command].
     pub fn new() -> SendmailTransport {
         SendmailTransport {
             command: DEFAULT_SENDMAIL.into(),
@@ -147,7 +154,10 @@ impl<E> AsyncSendmailTransport<E>
 where
     E: Executor,
 {
-    /// Creates a new transport with the default `/usr/sbin/sendmail` command
+    /// Creates a new transport with the `sendmail` command
+    ///
+    /// Note: This uses the `sendmail` command in the current `PATH`. To use another command,
+    /// use [AsyncSendmailTransport::new_with_command].
     pub fn new() -> Self {
         Self {
             inner: SendmailTransport::new(),
@@ -260,7 +270,7 @@ impl AsyncTransport for AsyncSendmailTransport<AsyncStd1Executor> {
             .stdin
             .as_mut()
             .unwrap()
-            .write_all(&email)
+            .write_all(email)
             .await
             .map_err(error::client)?;
         let output = process.output().await.map_err(error::client)?;
@@ -292,7 +302,7 @@ impl AsyncTransport for AsyncSendmailTransport<Tokio1Executor> {
             .stdin
             .as_mut()
             .unwrap()
-            .write_all(&email)
+            .write_all(email)
             .await
             .map_err(error::client)?;
         let output = process.wait_with_output().await.map_err(error::client)?;

src/transport/smtp/async_transport.rs

@@ -1,10 +1,16 @@
 use std::{
     fmt::{self, Debug},
     marker::PhantomData,
+    sync::Arc,
+    time::Duration,
 };
 
 use async_trait::async_trait;
 
+#[cfg(feature = "pool")]
+use super::pool::async_impl::Pool;
+#[cfg(feature = "pool")]
+use super::PoolConfig;
 use super::{
     client::AsyncSmtpConnection, ClientId, Credentials, Error, Mechanism, Response, SmtpInfo,
 };
@@ -18,8 +24,10 @@ use crate::{Envelope, Executor};
 
 /// Asynchronously sends emails using the SMTP protocol
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
-pub struct AsyncSmtpTransport<E> {
-    // TODO: pool
+pub struct AsyncSmtpTransport<E: Executor> {
+    #[cfg(feature = "pool")]
+    inner: Arc<Pool<E>>,
+    #[cfg(not(feature = "pool"))]
     inner: AsyncSmtpClient<E>,
 }
 
@@ -35,6 +43,7 @@ impl AsyncTransport for AsyncSmtpTransport<Tokio1Executor> {
 
         let result = conn.send(envelope, email).await?;
 
+        #[cfg(not(feature = "pool"))]
         conn.quit().await?;
 
         Ok(result)
@@ -141,21 +150,41 @@ where
     ///
     /// * No authentication
     /// * No TLS
+    /// * A 60 seconds timeout for smtp commands
     /// * Port 25
     ///
     /// Consider using [`AsyncSmtpTransport::relay`](#method.relay) or
     /// [`AsyncSmtpTransport::starttls_relay`](#method.starttls_relay) instead,
     /// if possible.
     pub fn builder_dangerous<T: Into<String>>(server: T) -> AsyncSmtpTransportBuilder {
-        let new = SmtpInfo {
+        let info = SmtpInfo {
             server: server.into(),
             ..Default::default()
         };
-        AsyncSmtpTransportBuilder { info: new }
+        AsyncSmtpTransportBuilder {
+            info,
+            #[cfg(feature = "pool")]
+            pool_config: PoolConfig::default(),
+        }
+    }
+
+    /// Tests the SMTP connection
+    ///
+    /// `test_connection()` tests the connection by using the SMTP NOOP command.
+    /// The connection is closed afterwards if a connection pool is not used.
+    pub async fn test_connection(&self) -> Result<bool, Error> {
+        let mut conn = self.inner.connection().await?;
+
+        let is_connected = conn.test_connected().await;
+
+        #[cfg(not(feature = "pool"))]
+        conn.quit().await?;
+
+        Ok(is_connected)
     }
 }
 
-impl<E> Debug for AsyncSmtpTransport<E> {
+impl<E: Executor> Debug for AsyncSmtpTransport<E> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut builder = f.debug_struct("AsyncSmtpTransport");
         builder.field("inner", &self.inner);
@@ -180,6 +209,8 @@ where
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
 pub struct AsyncSmtpTransportBuilder {
     info: SmtpInfo,
+    #[cfg(feature = "pool")]
+    pool_config: PoolConfig,
 }
 
 /// Builder for the SMTP `AsyncSmtpTransport`
@@ -208,6 +239,12 @@ impl AsyncSmtpTransportBuilder {
         self
     }
 
+    /// Set the timeout duration
+    pub fn timeout(mut self, timeout: Option<Duration>) -> Self {
+        self.info.timeout = timeout;
+        self
+    }
+
     /// Set the TLS settings to use
     #[cfg(any(
         feature = "tokio1-native-tls",
@@ -228,6 +265,16 @@ impl AsyncSmtpTransportBuilder {
         self
     }
 
+    /// Use a custom configuration for the connection pool
+    ///
+    /// Defaults can be found at [`PoolConfig`]
+    #[cfg(feature = "pool")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "pool")))]
+    pub fn pool_config(mut self, pool_config: PoolConfig) -> Self {
+        self.pool_config = pool_config;
+        self
+    }
+
     /// Build the transport
     pub fn build<E>(self) -> AsyncSmtpTransport<E>
     where
@@ -237,6 +284,10 @@ impl AsyncSmtpTransportBuilder {
             info: self.info,
             marker_: PhantomData,
         };
+
+        #[cfg(feature = "pool")]
+        let client = Pool::new(self.pool_config, client);
+
         AsyncSmtpTransport { inner: client }
     }
 }
@@ -258,13 +309,14 @@ where
         let mut conn = E::connect(
             &self.info.server,
             self.info.port,
+            self.info.timeout,
             &self.info.hello_name,
             &self.info.tls,
         )
         .await?;
 
         if let Some(credentials) = &self.info.credentials {
-            conn.auth(&self.info.authentication, &credentials).await?;
+            conn.auth(&self.info.authentication, credentials).await?;
         }
         Ok(conn)
     }
@@ -278,6 +330,8 @@ impl<E> Debug for AsyncSmtpClient<E> {
     }
 }
 
+// `clone` is unused when the `pool` feature is on
+#[allow(dead_code)]
 impl<E> AsyncSmtpClient<E>
 where
     E: Executor,

src/transport/smtp/authentication.rs

@@ -1,9 +1,11 @@
 //! Provides limited SASL authentication mechanisms
 
-use crate::transport::smtp::error::{self, Error};
 use std::fmt::{self, Debug, Display, Formatter};
 
+use crate::transport::smtp::error::{self, Error};
+
 /// Accepted authentication mechanisms
+///
 /// Trying LOGIN last as it is deprecated.
 pub const DEFAULT_MECHANISMS: &[Mechanism] = &[Mechanism::Plain, Mechanism::Login];
 

src/transport/smtp/client/async_connection.rs

@@ -1,3 +1,9 @@
+use std::{fmt::Display, time::Duration};
+
+use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+
+#[cfg(feature = "tracing")]
+use super::escape_crlf;
 use super::{AsyncNetworkStream, ClientCodec, TlsParameters};
 use crate::{
     transport::smtp::{
@@ -10,11 +16,6 @@ use crate::{
     },
     Envelope,
 };
-use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
-use std::fmt::Display;
-
-#[cfg(feature = "tracing")]
-use super::escape_crlf;
 
 macro_rules! try_smtp (
     ($err: expr, $client: ident) => ({
@@ -48,13 +49,13 @@ impl AsyncSmtpConnection {
     ///
     /// Sends EHLO and parses server information
     #[cfg(feature = "tokio1")]
-    pub async fn connect_tokio1(
-        hostname: &str,
-        port: u16,
+    pub async fn connect_tokio1<T: tokio1_crate::net::ToSocketAddrs>(
+        server: T,
+        timeout: Option<Duration>,
         hello_name: &ClientId,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncSmtpConnection, Error> {
-        let stream = AsyncNetworkStream::connect_tokio1(hostname, port, tls_parameters).await?;
+        let stream = AsyncNetworkStream::connect_tokio1(server, timeout, tls_parameters).await?;
         Self::connect_impl(stream, hello_name).await
     }
 
@@ -62,13 +63,13 @@ impl AsyncSmtpConnection {
     ///
     /// Sends EHLO and parses server information
     #[cfg(feature = "async-std1")]
-    pub async fn connect_asyncstd1(
-        hostname: &str,
-        port: u16,
+    pub async fn connect_asyncstd1<T: async_std::net::ToSocketAddrs>(
+        server: T,
+        timeout: Option<Duration>,
         hello_name: &ClientId,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncSmtpConnection, Error> {
-        let stream = AsyncNetworkStream::connect_asyncstd1(hostname, port, tls_parameters).await?;
+        let stream = AsyncNetworkStream::connect_asyncstd1(server, timeout, tls_parameters).await?;
         Self::connect_impl(stream, hello_name).await
     }
 
@@ -161,10 +162,7 @@ impl AsyncSmtpConnection {
     ) -> Result<(), Error> {
         if self.server_info.supports_feature(Extension::StartTls) {
             try_smtp!(self.command(Starttls).await, self);
-            try_smtp!(
-                self.stream.get_mut().upgrade_tls(tls_parameters).await,
-                self
-            );
+            self.stream.get_mut().upgrade_tls(tls_parameters).await?;
             #[cfg(feature = "tracing")]
             tracing::debug!("connection encrypted");
             // Send EHLO again
@@ -298,7 +296,10 @@ impl AsyncSmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code))
+                        Err(error::code(
+                            response.code(),
+                            response.first_line().map(|s| s.to_owned()),
+                        ))
                     }
                 }
                 Err(nom::Err::Failure(e)) => {
@@ -313,4 +314,10 @@ impl AsyncSmtpConnection {
 
         Err(error::response("incomplete response"))
     }
+
+    /// The X509 certificate of the server (DER encoded)
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        self.stream.get_ref().peer_certificate()
+    }
 }

src/transport/smtp/client/async_net.rs

@@ -1,29 +1,27 @@
 use std::{
-    mem,
+    io, mem,
     net::SocketAddr,
     pin::Pin,
     task::{Context, Poll},
+    time::Duration,
 };
 
+#[cfg(feature = "async-std1-native-tls")]
+use async_native_tls::TlsStream as AsyncStd1TlsStream;
+#[cfg(feature = "async-std1")]
+use async_std::net::{TcpStream as AsyncStd1TcpStream, ToSocketAddrs as AsyncStd1ToSocketAddrs};
 use futures_io::{
     AsyncRead as FuturesAsyncRead, AsyncWrite as FuturesAsyncWrite, Error as IoError, ErrorKind,
     Result as IoResult,
 };
+#[cfg(feature = "async-std1-rustls-tls")]
+use futures_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
 #[cfg(feature = "tokio1")]
 use tokio1_crate::io::{AsyncRead as _, AsyncWrite as _, ReadBuf as Tokio1ReadBuf};
-
-#[cfg(feature = "async-std1")]
-use async_std::net::TcpStream as AsyncStd1TcpStream;
 #[cfg(feature = "tokio1")]
-use tokio1_crate::net::TcpStream as Tokio1TcpStream;
-
-#[cfg(feature = "async-std1-native-tls")]
-use async_native_tls::TlsStream as AsyncStd1TlsStream;
+use tokio1_crate::net::{TcpStream as Tokio1TcpStream, ToSocketAddrs as Tokio1ToSocketAddrs};
 #[cfg(feature = "tokio1-native-tls")]
 use tokio1_native_tls_crate::TlsStream as Tokio1TlsStream;
-
-#[cfg(feature = "async-std1-rustls-tls")]
-use async_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
 #[cfg(feature = "tokio1-rustls-tls")]
 use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;
 
@@ -107,14 +105,47 @@ impl AsyncNetworkStream {
     }
 
     #[cfg(feature = "tokio1")]
-    pub async fn connect_tokio1(
-        hostname: &str,
-        port: u16,
+    pub async fn connect_tokio1<T: Tokio1ToSocketAddrs>(
+        server: T,
+        timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = Tokio1TcpStream::connect((hostname, port))
-            .await
-            .map_err(error::connection)?;
+        async fn try_connect_timeout<T: Tokio1ToSocketAddrs>(
+            server: T,
+            timeout: Duration,
+        ) -> Result<Tokio1TcpStream, Error> {
+            let addrs = tokio1_crate::net::lookup_host(server)
+                .await
+                .map_err(error::connection)?;
+
+            let mut last_err = None;
+
+            for addr in addrs {
+                let connect_future = Tokio1TcpStream::connect(&addr);
+                match tokio1_crate::time::timeout(timeout, connect_future).await {
+                    Ok(Ok(stream)) => return Ok(stream),
+                    Ok(Err(err)) => last_err = Some(err),
+                    Err(_) => {
+                        last_err = Some(io::Error::new(
+                            io::ErrorKind::TimedOut,
+                            "connection timed out",
+                        ))
+                    }
+                }
+            }
+
+            Err(match last_err {
+                Some(last_err) => error::connection(last_err),
+                None => error::connection("could not resolve to any address"),
+            })
+        }
+
+        let tcp_stream = match timeout {
+            Some(t) => try_connect_timeout(server, t).await?,
+            None => Tokio1TcpStream::connect(server)
+                .await
+                .map_err(error::connection)?,
+        };
 
         let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
@@ -124,14 +155,45 @@ impl AsyncNetworkStream {
     }
 
     #[cfg(feature = "async-std1")]
-    pub async fn connect_asyncstd1(
-        hostname: &str,
-        port: u16,
+    pub async fn connect_asyncstd1<T: AsyncStd1ToSocketAddrs>(
+        server: T,
+        timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = AsyncStd1TcpStream::connect((hostname, port))
-            .await
-            .map_err(error::connection)?;
+        async fn try_connect_timeout<T: AsyncStd1ToSocketAddrs>(
+            server: T,
+            timeout: Duration,
+        ) -> Result<AsyncStd1TcpStream, Error> {
+            let addrs = server.to_socket_addrs().await.map_err(error::connection)?;
+
+            let mut last_err = None;
+
+            for addr in addrs {
+                let connect_future = AsyncStd1TcpStream::connect(&addr);
+                match async_std::future::timeout(timeout, connect_future).await {
+                    Ok(Ok(stream)) => return Ok(stream),
+                    Ok(Err(err)) => last_err = Some(err),
+                    Err(_) => {
+                        last_err = Some(io::Error::new(
+                            io::ErrorKind::TimedOut,
+                            "connection timed out",
+                        ))
+                    }
+                }
+            }
+
+            Err(match last_err {
+                Some(last_err) => error::connection(last_err),
+                None => error::connection("could not resolve to any address"),
+            })
+        }
+
+        let tcp_stream = match timeout {
+            Some(t) => try_connect_timeout(server, t).await?,
+            None => AsyncStd1TcpStream::connect(server)
+                .await
+                .map_err(error::connection)?,
+        };
 
         let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
@@ -196,9 +258,9 @@ impl AsyncNetworkStream {
     #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
     async fn upgrade_tokio1_tls(
         tcp_stream: Tokio1TcpStream,
-        mut tls_parameters: TlsParameters,
+        tls_parameters: TlsParameters,
     ) -> Result<InnerAsyncNetworkStream, Error> {
-        let domain = mem::take(&mut tls_parameters.domain);
+        let domain = tls_parameters.domain().to_string();
 
         match tls_parameters.connector {
             #[cfg(feature = "native-tls")]
@@ -225,10 +287,11 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "tokio1-rustls-tls")]
                 return {
-                    use tokio1_rustls::{webpki::DNSNameRef, TlsConnector};
+                    use rustls::ServerName;
+                    use tokio1_rustls::TlsConnector;
 
-                    let domain =
-                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
+                    let domain = ServerName::try_from(domain.as_str())
+                        .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
 
                     let connector = TlsConnector::from(config);
                     let stream = connector
@@ -277,10 +340,11 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "async-std1-rustls-tls")]
                 return {
-                    use async_rustls::{webpki::DNSNameRef, TlsConnector};
+                    use futures_rustls::TlsConnector;
+                    use rustls::ServerName;
 
-                    let domain =
-                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
+                    let domain = ServerName::try_from(domain.as_str())
+                        .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
 
                     let connector = TlsConnector::from(config);
                     let stream = connector
@@ -310,6 +374,50 @@ impl AsyncNetworkStream {
             InnerAsyncNetworkStream::None => false,
         }
     }
+
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        match &self.inner {
+            #[cfg(feature = "tokio1")]
+            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "tokio1-native-tls")]
+            InnerAsyncNetworkStream::Tokio1NativeTls(stream) => Ok(stream
+                .get_ref()
+                .peer_certificate()
+                .map_err(error::tls)?
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            #[cfg(feature = "tokio1-rustls-tls")]
+            InnerAsyncNetworkStream::Tokio1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            #[cfg(feature = "async-std1")]
+            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "async-std1-native-tls")]
+            InnerAsyncNetworkStream::AsyncStd1NativeTls(t) => panic!("Unsupported"),
+            #[cfg(feature = "async-std1-rustls-tls")]
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            InnerAsyncNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
 }
 
 impl FuturesAsyncRead for AsyncNetworkStream {

src/transport/smtp/client/connection.rs

@@ -5,6 +5,8 @@ use std::{
     time::Duration,
 };
 
+#[cfg(feature = "tracing")]
+use super::escape_crlf;
 use super::{ClientCodec, NetworkStream, TlsParameters};
 use crate::{
     address::Envelope,
@@ -18,9 +20,6 @@ use crate::{
     },
 };
 
-#[cfg(feature = "tracing")]
-use super::escape_crlf;
-
 macro_rules! try_smtp (
     ($err: expr, $client: ident) => ({
         match $err {
@@ -145,7 +144,7 @@ impl SmtpConnection {
             #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
             {
                 try_smtp!(self.command(Starttls), self);
-                try_smtp!(self.stream.get_mut().upgrade_tls(tls_parameters), self);
+                self.stream.get_mut().upgrade_tls(tls_parameters)?;
                 #[cfg(feature = "tracing")]
                 tracing::debug!("connection encrypted");
                 // Send EHLO again
@@ -276,7 +275,10 @@ impl SmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code))
+                        Err(error::code(
+                            response.code(),
+                            response.first_line().map(|s| s.to_owned()),
+                        ))
                     };
                 }
                 Err(nom::Err::Failure(e)) => {
@@ -291,4 +293,10 @@ impl SmtpConnection {
 
         Err(error::response("incomplete response"))
     }
+
+    /// The X509 certificate of the server (DER encoded)
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        self.stream.get_ref().peer_certificate()
+    }
 }

src/transport/smtp/client/mod.rs

@@ -7,16 +7,14 @@
 //!
 //! # #[cfg(feature = "smtp-transport")]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::transport::smtp::{SMTP_PORT, extension::ClientId, commands::*, client::SmtpConnection};
+//! use lettre::transport::smtp::{
+//!     client::SmtpConnection, commands::*, extension::ClientId, SMTP_PORT,
+//! };
 //!
 //! let hello = ClientId::Domain("my_hostname".to_string());
 //! let mut client = SmtpConnection::connect(&("localhost", SMTP_PORT), None, &hello, None)?;
-//! client.command(
-//!         Mail::new(Some("user@example.com".parse()?), vec![])
-//!     )?;
-//! client.command(
-//!         Rcpt::new("user@example.org".parse()?, vec![])
-//!       )?;
+//! client.command(Mail::new(Some("user@example.com".parse()?), vec![]))?;
+//! client.command(Rcpt::new("user@example.org".parse()?, vec![]))?;
 //! client.command(Data)?;
 //! client.message("Test email".as_bytes())?;
 //! client.command(Quit)?;
@@ -28,9 +26,9 @@
 use std::fmt::Debug;
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-pub(crate) use self::async_connection::AsyncSmtpConnection;
+pub use self::async_connection::AsyncSmtpConnection;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-pub(crate) use self::async_net::AsyncNetworkStream;
+pub use self::async_net::AsyncNetworkStream;
 use self::net::NetworkStream;
 #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
 pub(super) use self::tls::InnerTlsParameters;
@@ -78,7 +76,15 @@ impl ClientCodec {
                     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 { 0 },
+                        2 => {
+                            self.escape_count = if *byte == b'.' {
+                                3
+                            } else if *byte == b'\r' {
+                                1
+                            } else {
+                                0
+                            }
+                        }
                         _ => unreachable!(),
                     }
                     if self.escape_count == 3 {
@@ -111,6 +117,7 @@ mod test {
         let mut buf: Vec<u8> = vec![];
 
         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);
         codec.encode(b"\r\ntest", &mut buf);
         codec.encode(b"te\r\n.\r\nst", &mut buf);
@@ -121,7 +128,7 @@ mod test {
         codec.encode(b"test", &mut buf);
         assert_eq!(
             String::from_utf8(buf).unwrap(),
-            "test\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntest"
+            "test\r\ntest\r\n\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntest"
         );
     }
 

src/transport/smtp/client/net.rs

@@ -7,9 +7,8 @@ use std::{
 
 #[cfg(feature = "native-tls")]
 use native_tls::TlsStream;
-
 #[cfg(feature = "rustls-tls")]
-use rustls::{ClientSession, StreamOwned};
+use rustls::{ClientConnection, ServerName, StreamOwned};
 
 #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
 use super::InnerTlsParameters;
@@ -33,7 +32,7 @@ enum InnerNetworkStream {
     NativeTls(TlsStream<TcpStream>),
     /// Encrypted TCP stream
     #[cfg(feature = "rustls-tls")]
-    RustlsTls(StreamOwned<ClientSession, TcpStream>),
+    RustlsTls(StreamOwned<ClientConnection, TcpStream>),
     /// Can't be built
     None,
 }
@@ -90,12 +89,20 @@ impl NetworkStream {
             timeout: Duration,
         ) -> Result<TcpStream, Error> {
             let addrs = server.to_socket_addrs().map_err(error::connection)?;
+
+            let mut last_err = None;
+
             for addr in addrs {
-                if let Ok(result) = TcpStream::connect_timeout(&addr, timeout) {
-                    return Ok(result);
+                match TcpStream::connect_timeout(&addr, timeout) {
+                    Ok(stream) => return Ok(stream),
+                    Err(err) => last_err = Some(err),
                 }
             }
-            Err(error::connection("Could not connect"))
+
+            Err(match last_err {
+                Some(last_err) => error::connection(last_err),
+                None => error::connection("could not resolve to any address"),
+            })
         }
 
         let tcp_stream = match timeout {
@@ -149,12 +156,11 @@ impl NetworkStream {
             }
             #[cfg(feature = "rustls-tls")]
             InnerTlsParameters::RustlsTls(connector) => {
-                use webpki::DNSNameRef;
-
-                let domain = DNSNameRef::try_from_ascii_str(tls_parameters.domain())
-                    .map_err(error::connection)?;
-                let stream = StreamOwned::new(ClientSession::new(&connector, domain), tcp_stream);
-
+                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 stream = StreamOwned::new(connection, tcp_stream);
                 InnerNetworkStream::RustlsTls(stream)
             }
         })
@@ -174,6 +180,30 @@ impl NetworkStream {
         }
     }
 
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
+        match &self.inner {
+            InnerNetworkStream::Tcp(_) => Err(error::client("Connection is not encrypted")),
+            #[cfg(feature = "native-tls")]
+            InnerNetworkStream::NativeTls(stream) => Ok(stream
+                .peer_certificate()
+                .map_err(error::tls)?
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
+            #[cfg(feature = "rustls-tls")]
+            InnerNetworkStream::RustlsTls(stream) => Ok(stream
+                .conn
+                .peer_certificates()
+                .unwrap()
+                .first()
+                .unwrap()
+                .clone()
+                .0),
+            InnerNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
+
     pub fn set_read_timeout(&mut self, duration: Option<Duration>) -> io::Result<()> {
         match self.inner {
             InnerNetworkStream::Tcp(ref mut stream) => stream.set_read_timeout(duration),

src/transport/smtp/client/tls.rs

@@ -1,14 +1,17 @@
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-use crate::transport::smtp::{error, Error};
+use std::fmt::{self, Debug};
+#[cfg(feature = "rustls-tls")]
+use std::{sync::Arc, time::SystemTime};
+
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls-tls")]
-use rustls::{ClientConfig, RootCertStore, ServerCertVerified, ServerCertVerifier, TLSError};
-use std::fmt::{self, Debug};
-#[cfg(feature = "rustls-tls")]
-use std::sync::Arc;
-#[cfg(feature = "rustls-tls")]
-use webpki::DNSNameRef;
+use rustls::{
+    client::{ServerCertVerified, ServerCertVerifier, WebPkiVerifier},
+    ClientConfig, Error as TlsError, OwnedTrustAnchor, RootCertStore, ServerName,
+};
+
+#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+use crate::transport::smtp::{error, Error};
 
 /// Accepted protocols by default.
 /// This removes TLS 1.0 and 1.1 compared to tls-native defaults.
@@ -163,21 +166,35 @@ impl TlsParametersBuilder {
     #[cfg(feature = "rustls-tls")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
     pub fn build_rustls(self) -> Result<TlsParameters, Error> {
-        use webpki_roots::TLS_SERVER_ROOTS;
+        let tls = ClientConfig::builder();
+        let tls = tls.with_safe_defaults();
 
-        let mut tls = ClientConfig::new();
-
-        for cert in self.root_certs {
-            for rustls_cert in cert.rustls {
-                tls.root_store.add(&rustls_cert).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();
+            for cert in self.root_certs {
+                for rustls_cert in cert.rustls {
+                    root_cert_store.add(&rustls_cert).map_err(error::tls)?;
+                }
             }
-        }
-        if self.accept_invalid_certs {
-            tls.dangerous()
-                .set_certificate_verifier(Arc::new(InvalidCertsVerifier {}));
-        }
+            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,
+                None,
+            )))
+        };
+        let tls = tls.with_no_client_auth();
 
-        tls.root_store.add_server_trust_anchors(&TLS_SERVER_ROOTS);
         Ok(TlsParameters {
             connector: InnerTlsParameters::RustlsTls(Arc::new(tls)),
             domain: self.domain,
@@ -257,11 +274,14 @@ impl Certificate {
 
         #[cfg(feature = "rustls-tls")]
         let rustls_cert = {
-            use rustls::internal::pemfile;
             use std::io::Cursor;
 
             let mut pem = Cursor::new(pem);
-            pemfile::certs(&mut pem).map_err(|_| error::tls("invalid certificates"))?
+            rustls_pemfile::certs(&mut pem)
+                .map_err(|_| error::tls("invalid certificates"))?
+                .into_iter()
+                .map(rustls::Certificate)
+                .collect::<Vec<_>>()
         };
 
         Ok(Self {
@@ -286,11 +306,13 @@ struct InvalidCertsVerifier;
 impl ServerCertVerifier for InvalidCertsVerifier {
     fn verify_server_cert(
         &self,
-        _roots: &RootCertStore,
-        _presented_certs: &[rustls::Certificate],
-        _dns_name: DNSNameRef<'_>,
+        _end_entity: &rustls::Certificate,
+        _intermediates: &[rustls::Certificate],
+        _server_name: &ServerName,
+        _scts: &mut dyn Iterator<Item = &[u8]>,
         _ocsp_response: &[u8],
-    ) -> Result<ServerCertVerified, TLSError> {
+        _now: SystemTime,
+    ) -> Result<ServerCertVerified, TlsError> {
         Ok(ServerCertVerified::assertion())
     }
 }

src/transport/smtp/commands.rs

@@ -1,5 +1,7 @@
 //! SMTP commands
 
+use std::fmt::{self, Display, Formatter};
+
 use crate::{
     address::Address,
     transport::smtp::{
@@ -9,7 +11,6 @@ use crate::{
         response::Response,
     },
 };
-use std::fmt::{self, Display, Formatter};
 
 /// EHLO command
 #[derive(PartialEq, Clone, Debug)]
@@ -288,9 +289,10 @@ impl Auth {
 
 #[cfg(test)]
 mod test {
+    use std::str::FromStr;
+
     use super::*;
     use crate::transport::smtp::extension::MailBodyParameter;
-    use std::str::FromStr;
 
     #[test]
     fn test_display() {

src/transport/smtp/error.rs

@@ -1,10 +1,11 @@
 //! Error and result type for SMTP clients
 
+use std::{error::Error as StdError, fmt};
+
 use crate::{
     transport::smtp::response::{Code, Severity},
     BoxError,
 };
-use std::{error::Error as StdError, fmt};
 
 // Inspired by https://github.com/seanmonstar/reqwest/blob/a8566383168c0ef06c21f38cbc9213af6ff6db31/src/error.rs
 
@@ -154,10 +155,10 @@ impl StdError for Error {
     }
 }
 
-pub(crate) fn code(c: Code) -> Error {
+pub(crate) fn code(c: Code, s: Option<String>) -> Error {
     match c.severity {
-        Severity::TransientNegativeCompletion => Error::new::<Error>(Kind::Transient(c), None),
-        Severity::PermanentNegativeCompletion => Error::new::<Error>(Kind::Permanent(c), None),
+        Severity::TransientNegativeCompletion => Error::new(Kind::Transient(c), s),
+        Severity::PermanentNegativeCompletion => Error::new(Kind::Permanent(c), s),
         _ => client("Unknown error code"),
     }
 }

src/transport/smtp/extension.rs

@@ -1,17 +1,18 @@
 //! ESMTP features
 
+use std::{
+    collections::HashSet,
+    fmt::{self, Display, Formatter},
+    net::{Ipv4Addr, Ipv6Addr},
+    result::Result,
+};
+
 use crate::transport::smtp::{
     authentication::Mechanism,
     error::{self, Error},
     response::Response,
     util::XText,
 };
-use std::{
-    collections::HashSet,
-    fmt::{self, Display, Formatter},
-    net::{Ipv4Addr, Ipv6Addr},
-    result::Result,
-};
 
 /// Client identifier, the parameter to `EHLO`
 #[derive(PartialEq, Eq, Clone, Debug)]
@@ -72,6 +73,7 @@ impl ClientId {
 /// Supported ESMTP keywords
 #[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[non_exhaustive]
 pub enum Extension {
     /// 8BITMIME keyword
     ///
@@ -107,11 +109,11 @@ pub struct ServerInfo {
     /// Server name
     ///
     /// The name given in the server banner
-    pub name: String,
+    name: String,
     /// ESMTP features supported by the server
     ///
     /// It contains the features supported by the server and known by the `Extension` module.
-    pub features: HashSet<Extension>,
+    features: HashSet<Extension>,
 }
 
 impl Display for ServerInfo {
@@ -135,7 +137,7 @@ impl ServerInfo {
 
         let mut features: HashSet<Extension> = HashSet::new();
 
-        for line in response.message.as_slice() {
+        for line in response.message() {
             if line.is_empty() {
                 continue;
             }
@@ -197,6 +199,11 @@ impl ServerInfo {
         }
         None
     }
+
+    /// The name given in the server banner
+    pub fn name(&self) -> &str {
+        self.name.as_ref()
+    }
 }
 
 /// A `MAIL FROM` extension parameter
@@ -286,12 +293,13 @@ impl Display for RcptParameter {
 #[cfg(test)]
 mod test {
 
+    use std::collections::HashSet;
+
     use super::*;
     use crate::transport::smtp::{
         authentication::Mechanism,
         response::{Category, Code, Detail, Response, Severity},
     };
-    use std::collections::HashSet;
 
     #[test]
     fn test_clientid_fmt() {

src/transport/smtp/mod.rs

@@ -33,7 +33,7 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport};
+//! use lettre::{Message, SmtpTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -43,8 +43,7 @@
 //!     .body(String::from("Be happy!"))?;
 //!
 //! // Create TLS transport on port 465
-//! let sender = SmtpTransport::relay("smtp.example.com")?
-//!     .build();
+//! let sender = SmtpTransport::relay("smtp.example.com")?.build();
 //! // Send the email via remote relay
 //! let result = sender.send(&email);
 //! assert!(result.is_ok());
@@ -59,7 +58,13 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::{PoolConfig, authentication::{Credentials, Mechanism}}};
+//! use lettre::{
+//!     transport::smtp::{
+//!         authentication::{Credentials, Mechanism},
+//!         PoolConfig,
+//!     },
+//!     Message, SmtpTransport, Transport,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -71,11 +76,14 @@
 //! // Create TLS transport on port 587 with STARTTLS
 //! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
 //!     // Add credentials for authentication
-//!     .credentials(Credentials::new("username".to_string(), "password".to_string()))
+//!     .credentials(Credentials::new(
+//!         "username".to_string(),
+//!         "password".to_string(),
+//!     ))
 //!     // Configure expected authentication mechanism
 //!     .authentication(vec![Mechanism::Plain])
 //!     // Connection pool settings
-//!     .pool_config( PoolConfig::new().max_size(20))
+//!     .pool_config(PoolConfig::new().max_size(20))
 //!     .build();
 //!
 //! // Send the email via remote relay
@@ -90,7 +98,10 @@
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
-//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::client::{TlsParameters, Tls}};
+//! use lettre::{
+//!     transport::smtp::client::{Tls, TlsParameters},
+//!     Message, SmtpTransport, Transport,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -101,7 +112,8 @@
 //!
 //! // Custom TLS configuration
 //! let tls = TlsParameters::builder("smtp.example.com".to_string())
-//!           .dangerous_accept_invalid_certs(true).build()?;
+//!     .dangerous_accept_invalid_certs(true)
+//!     .build()?;
 //!
 //! // Create TLS transport on port 465
 //! let sender = SmtpTransport::relay("smtp.example.com")?
@@ -116,12 +128,14 @@
 //! # }
 //! ```
 
+use std::time::Duration;
+
+use client::Tls;
+
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 pub use self::async_transport::{AsyncSmtpTransport, AsyncSmtpTransportBuilder};
-#[cfg(feature = "r2d2")]
+#[cfg(feature = "pool")]
 pub use self::pool::PoolConfig;
-#[cfg(feature = "r2d2")]
-pub(crate) use self::transport::SmtpClient;
 pub use self::{
     error::Error,
     transport::{SmtpTransport, SmtpTransportBuilder},
@@ -134,8 +148,6 @@ use crate::transport::smtp::{
     extension::ClientId,
     response::Response,
 };
-use client::Tls;
-use std::time::Duration;
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 mod async_transport;
@@ -144,11 +156,11 @@ pub mod client;
 pub mod commands;
 mod error;
 pub mod extension;
-#[cfg(feature = "r2d2")]
+#[cfg(feature = "pool")]
 mod pool;
 pub mod response;
 mod transport;
-pub mod util;
+pub(super) mod util;
 
 // Registered port numbers:
 // https://www.iana.
@@ -164,7 +176,7 @@ pub const SUBMISSION_PORT: u16 = 587;
 pub const SUBMISSIONS_PORT: u16 = 465;
 
 /// Default timeout
-pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(60);
+const DEFAULT_TIMEOUT: Duration = Duration::from_secs(60);
 
 #[derive(Debug, Clone)]
 struct SmtpInfo {

src/transport/smtp/pool.rs

@@ -1,111 +0,0 @@
-use std::time::Duration;
-
-use crate::transport::smtp::{client::SmtpConnection, error, error::Error, SmtpClient};
-
-use r2d2::{CustomizeConnection, ManageConnection, Pool};
-
-/// Configuration for a connection pool
-#[derive(Debug, Clone)]
-#[allow(missing_copy_implementations)]
-#[cfg_attr(docsrs, doc(cfg(feature = "r2d2")))]
-pub struct PoolConfig {
-    min_idle: u32,
-    max_size: u32,
-    connection_timeout: Duration,
-    idle_timeout: Duration,
-}
-
-impl PoolConfig {
-    /// Create a new pool configuration with default values
-    pub fn new() -> Self {
-        Self::default()
-    }
-
-    /// Minimum number of idle connections
-    ///
-    /// Defaults to `0`
-    pub fn min_idle(mut self, min_idle: u32) -> Self {
-        self.min_idle = min_idle;
-        self
-    }
-
-    /// Maximum number of pooled connections
-    ///
-    /// Defaults to `10`
-    pub fn max_size(mut self, max_size: u32) -> Self {
-        self.max_size = max_size;
-        self
-    }
-
-    /// Connection timeout
-    ///
-    /// Defaults to `30 seconds`
-    pub fn connection_timeout(mut self, connection_timeout: Duration) -> Self {
-        self.connection_timeout = connection_timeout;
-        self
-    }
-
-    /// Connection idle timeout
-    ///
-    /// Defaults to `60 seconds`
-    pub fn idle_timeout(mut self, idle_timeout: Duration) -> Self {
-        self.idle_timeout = idle_timeout;
-        self
-    }
-
-    pub(crate) fn build<C: ManageConnection<Connection = SmtpConnection, Error = Error>>(
-        &self,
-        client: C,
-    ) -> Pool<C> {
-        Pool::builder()
-            .min_idle(Some(self.min_idle))
-            .max_size(self.max_size)
-            .connection_timeout(self.connection_timeout)
-            .idle_timeout(Some(self.idle_timeout))
-            .connection_customizer(Box::new(SmtpConnectionQuitter))
-            .build_unchecked(client)
-    }
-}
-
-impl Default for PoolConfig {
-    fn default() -> Self {
-        Self {
-            min_idle: 0,
-            max_size: 10,
-            connection_timeout: Duration::from_secs(30),
-            idle_timeout: Duration::from_secs(60),
-        }
-    }
-}
-
-impl ManageConnection for SmtpClient {
-    type Connection = SmtpConnection;
-    type Error = Error;
-
-    fn connect(&self) -> Result<Self::Connection, Error> {
-        self.connection()
-    }
-
-    fn is_valid(&self, conn: &mut Self::Connection) -> Result<(), Error> {
-        if conn.test_connected() {
-            return Ok(());
-        }
-        Err(error::network("is not connected anymore"))
-    }
-
-    fn has_broken(&self, conn: &mut Self::Connection) -> bool {
-        conn.has_broken()
-    }
-}
-
-#[derive(Copy, Clone, Debug)]
-struct SmtpConnectionQuitter;
-
-impl CustomizeConnection<SmtpConnection, Error> for SmtpConnectionQuitter {
-    fn on_release(&self, conn: SmtpConnection) {
-        let mut conn = conn;
-        if !conn.has_broken() {
-            let _quit = conn.quit();
-        }
-    }
-}

src/transport/smtp/pool/async_impl.rs

@@ -0,0 +1,301 @@
+use std::{
+    fmt::{self, Debug},
+    mem,
+    ops::{Deref, DerefMut},
+    sync::Arc,
+    time::{Duration, Instant},
+};
+
+use futures_util::{
+    lock::Mutex,
+    stream::{self, StreamExt},
+};
+use once_cell::sync::OnceCell;
+
+use super::{
+    super::{client::AsyncSmtpConnection, Error},
+    PoolConfig,
+};
+use crate::{executor::SpawnHandle, transport::smtp::async_transport::AsyncSmtpClient, Executor};
+
+pub struct Pool<E: Executor> {
+    config: PoolConfig,
+    connections: Mutex<Vec<ParkedConnection>>,
+    client: AsyncSmtpClient<E>,
+    handle: OnceCell<E::Handle>,
+}
+
+struct ParkedConnection {
+    conn: AsyncSmtpConnection,
+    since: Instant,
+}
+
+pub struct PooledConnection<E: Executor> {
+    conn: Option<AsyncSmtpConnection>,
+    pool: Arc<Pool<E>>,
+}
+
+impl<E: Executor> Pool<E> {
+    pub fn new(config: PoolConfig, client: AsyncSmtpClient<E>) -> Arc<Self> {
+        let pool = Arc::new(Self {
+            config,
+            connections: Mutex::new(Vec::new()),
+            client,
+            handle: OnceCell::new(),
+        });
+
+        {
+            let pool_ = Arc::clone(&pool);
+
+            let min_idle = pool_.config.min_idle;
+            let idle_timeout = pool_.config.idle_timeout;
+            let pool = Arc::downgrade(&pool_);
+
+            let handle = E::spawn(async move {
+                loop {
+                    #[cfg(feature = "tracing")]
+                    tracing::trace!("running cleanup tasks");
+
+                    match pool.upgrade() {
+                        Some(pool) => {
+                            #[allow(clippy::needless_collect)]
+                            let (count, dropped) = {
+                                let mut connections = pool.connections.lock().await;
+
+                                let to_drop = connections
+                                    .iter()
+                                    .enumerate()
+                                    .rev()
+                                    .filter(|(_, conn)| conn.idle_duration() > idle_timeout)
+                                    .map(|(i, _)| i)
+                                    .collect::<Vec<_>>();
+                                let dropped = to_drop
+                                    .into_iter()
+                                    .map(|i| connections.remove(i))
+                                    .collect::<Vec<_>>();
+
+                                (connections.len(), dropped)
+                            };
+
+                            #[cfg(feature = "tracing")]
+                            let mut created = 0;
+                            for _ in count..=(min_idle as usize) {
+                                let conn = match pool.client.connection().await {
+                                    Ok(conn) => conn,
+                                    Err(err) => {
+                                        #[cfg(feature = "tracing")]
+                                        tracing::warn!("couldn't create idle connection {}", err);
+                                        #[cfg(not(feature = "tracing"))]
+                                        let _ = err;
+
+                                        break;
+                                    }
+                                };
+
+                                let mut connections = pool.connections.lock().await;
+                                connections.push(ParkedConnection::park(conn));
+
+                                #[cfg(feature = "tracing")]
+                                {
+                                    created += 1;
+                                }
+                            }
+
+                            #[cfg(feature = "tracing")]
+                            if created > 0 {
+                                tracing::debug!("created {} idle connections", created);
+                            }
+
+                            if !dropped.is_empty() {
+                                #[cfg(feature = "tracing")]
+                                tracing::debug!("dropped {} idle connections", dropped.len());
+
+                                abort_concurrent(dropped.into_iter().map(|conn| conn.unpark()))
+                                    .await;
+                            }
+                        }
+                        None => {
+                            #[cfg(feature = "tracing")]
+                            tracing::warn!(
+                                "breaking out of task - no more references to Pool are available"
+                            );
+                            break;
+                        }
+                    }
+
+                    E::sleep(idle_timeout).await;
+                }
+            });
+            pool_
+                .handle
+                .set(handle)
+                .expect("handle hasn't been set yet");
+        }
+
+        pool
+    }
+
+    pub async fn connection(self: &Arc<Self>) -> Result<PooledConnection<E>, Error> {
+        loop {
+            let conn = {
+                let mut connections = self.connections.lock().await;
+                connections.pop()
+            };
+
+            match conn {
+                Some(conn) => {
+                    let mut conn = conn.unpark();
+
+                    // TODO: handle the client try another connection if this one isn't good
+                    if !conn.test_connected().await {
+                        #[cfg(feature = "tracing")]
+                        tracing::debug!("dropping a broken connection");
+
+                        conn.abort().await;
+                        continue;
+                    }
+
+                    #[cfg(feature = "tracing")]
+                    tracing::debug!("reusing a pooled connection");
+
+                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                }
+                None => {
+                    #[cfg(feature = "tracing")]
+                    tracing::debug!("creating a new connection");
+
+                    let conn = self.client.connection().await?;
+                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                }
+            }
+        }
+    }
+
+    async fn recycle(&self, mut conn: AsyncSmtpConnection) {
+        if conn.has_broken() {
+            #[cfg(feature = "tracing")]
+            tracing::debug!("dropping a broken connection instead of recycling it");
+
+            conn.abort().await;
+            drop(conn);
+        } else {
+            #[cfg(feature = "tracing")]
+            tracing::debug!("recycling connection");
+
+            let mut connections = self.connections.lock().await;
+            if connections.len() >= self.config.max_size as usize {
+                drop(connections);
+                conn.abort().await;
+            } else {
+                let conn = ParkedConnection::park(conn);
+                connections.push(conn);
+            }
+        }
+    }
+}
+
+impl<E: Executor> Debug for Pool<E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Pool")
+            .field("config", &self.config)
+            .field(
+                "connections",
+                &match self.connections.try_lock() {
+                    Some(connections) => format!("{} connections", connections.len()),
+
+                    None => "LOCKED".to_string(),
+                },
+            )
+            .field("client", &self.client)
+            .field(
+                "handle",
+                &match self.handle.get() {
+                    Some(_) => "Some(JoinHandle)",
+                    None => "None",
+                },
+            )
+            .finish()
+    }
+}
+
+impl<E: Executor> Drop for Pool<E> {
+    fn drop(&mut self) {
+        #[cfg(feature = "tracing")]
+        tracing::debug!("dropping Pool");
+
+        let connections = mem::take(self.connections.get_mut());
+        let handle = self.handle.take();
+        E::spawn(async move {
+            if let Some(handle) = handle {
+                handle.shutdown().await;
+            }
+
+            abort_concurrent(connections.into_iter().map(|conn| conn.unpark())).await;
+        });
+    }
+}
+
+impl ParkedConnection {
+    fn park(conn: AsyncSmtpConnection) -> Self {
+        Self {
+            conn,
+            since: Instant::now(),
+        }
+    }
+
+    fn idle_duration(&self) -> Duration {
+        self.since.elapsed()
+    }
+
+    fn unpark(self) -> AsyncSmtpConnection {
+        self.conn
+    }
+}
+
+impl<E: Executor> PooledConnection<E> {
+    fn wrap(conn: AsyncSmtpConnection, pool: Arc<Pool<E>>) -> Self {
+        Self {
+            conn: Some(conn),
+            pool,
+        }
+    }
+}
+
+impl<E: Executor> Deref for PooledConnection<E> {
+    type Target = AsyncSmtpConnection;
+
+    fn deref(&self) -> &Self::Target {
+        self.conn.as_ref().expect("conn hasn't been dropped yet")
+    }
+}
+
+impl<E: Executor> DerefMut for PooledConnection<E> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.conn.as_mut().expect("conn hasn't been dropped yet")
+    }
+}
+
+impl<E: Executor> Drop for PooledConnection<E> {
+    fn drop(&mut self) {
+        let conn = self
+            .conn
+            .take()
+            .expect("AsyncSmtpConnection hasn't been taken yet");
+        let pool = Arc::clone(&self.pool);
+
+        E::spawn(async move {
+            pool.recycle(conn).await;
+        });
+    }
+}
+
+async fn abort_concurrent<I>(iter: I)
+where
+    I: Iterator<Item = AsyncSmtpConnection>,
+{
+    stream::iter(iter)
+        .for_each_concurrent(8, |mut conn| async move {
+            conn.abort().await;
+        })
+        .await;
+}

src/transport/smtp/pool/mod.rs

@@ -0,0 +1,66 @@
+use std::time::Duration;
+
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+pub mod async_impl;
+pub mod sync_impl;
+
+/// Configuration for a connection pool
+#[derive(Debug, Clone)]
+#[allow(missing_copy_implementations)]
+#[cfg_attr(docsrs, doc(cfg(feature = "pool")))]
+pub struct PoolConfig {
+    min_idle: u32,
+    max_size: u32,
+    idle_timeout: Duration,
+}
+
+impl PoolConfig {
+    /// Create a new pool configuration with default values
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Minimum number of idle connections
+    ///
+    /// Defaults to `0`
+    pub fn min_idle(mut self, min_idle: u32) -> Self {
+        self.min_idle = min_idle;
+        self
+    }
+
+    /// Maximum number of pooled connections
+    ///
+    /// Defaults to `10`
+    pub fn max_size(mut self, max_size: u32) -> Self {
+        self.max_size = max_size;
+        self
+    }
+
+    /// Connection timeout
+    ///
+    /// Defaults to `30 seconds`
+    #[doc(hidden)]
+    #[deprecated(note = "The Connection timeout is already configured on the SMTP transport")]
+    pub fn connection_timeout(self, connection_timeout: Duration) -> Self {
+        let _ = connection_timeout;
+        self
+    }
+
+    /// Connection idle timeout
+    ///
+    /// Defaults to `60 seconds`
+    pub fn idle_timeout(mut self, idle_timeout: Duration) -> Self {
+        self.idle_timeout = idle_timeout;
+        self
+    }
+}
+
+impl Default for PoolConfig {
+    fn default() -> Self {
+        Self {
+            min_idle: 0,
+            max_size: 10,
+            idle_timeout: Duration::from_secs(60),
+        }
+    }
+}

src/transport/smtp/pool/sync_impl.rs

@@ -0,0 +1,259 @@
+use std::{
+    fmt::{self, Debug},
+    mem,
+    ops::{Deref, DerefMut},
+    sync::{Arc, Mutex, TryLockError},
+    thread,
+    time::{Duration, Instant},
+};
+
+use super::{
+    super::{client::SmtpConnection, Error},
+    PoolConfig,
+};
+use crate::transport::smtp::transport::SmtpClient;
+
+pub struct Pool {
+    config: PoolConfig,
+    connections: Mutex<Vec<ParkedConnection>>,
+    client: SmtpClient,
+}
+
+struct ParkedConnection {
+    conn: SmtpConnection,
+    since: Instant,
+}
+
+pub struct PooledConnection {
+    conn: Option<SmtpConnection>,
+    pool: Arc<Pool>,
+}
+
+impl Pool {
+    pub fn new(config: PoolConfig, client: SmtpClient) -> Arc<Self> {
+        let pool = Arc::new(Self {
+            config,
+            connections: Mutex::new(Vec::new()),
+            client,
+        });
+
+        {
+            let pool_ = Arc::clone(&pool);
+
+            let min_idle = pool_.config.min_idle;
+            let idle_timeout = pool_.config.idle_timeout;
+            let pool = Arc::downgrade(&pool_);
+
+            thread::Builder::new()
+                .name("lettre-connection-pool".into())
+                .spawn(move || {
+                    while let Some(pool) = pool.upgrade() {
+                        #[cfg(feature = "tracing")]
+                        tracing::trace!("running cleanup tasks");
+
+                        #[allow(clippy::needless_collect)]
+                        let (count, dropped) = {
+                            let mut connections = pool.connections.lock().unwrap();
+
+                            let to_drop = connections
+                                .iter()
+                                .enumerate()
+                                .rev()
+                                .filter(|(_, conn)| conn.idle_duration() > idle_timeout)
+                                .map(|(i, _)| i)
+                                .collect::<Vec<_>>();
+                            let dropped = to_drop
+                                .into_iter()
+                                .map(|i| connections.remove(i))
+                                .collect::<Vec<_>>();
+
+                            (connections.len(), dropped)
+                        };
+
+                        #[cfg(feature = "tracing")]
+                        let mut created = 0;
+                        for _ in count..=(min_idle as usize) {
+                            let conn = match pool.client.connection() {
+                                Ok(conn) => conn,
+                                Err(err) => {
+                                    #[cfg(feature = "tracing")]
+                                    tracing::warn!("couldn't create idle connection {}", err);
+                                    #[cfg(not(feature = "tracing"))]
+                                    let _ = err;
+
+                                    break;
+                                }
+                            };
+
+                            let mut connections = pool.connections.lock().unwrap();
+                            connections.push(ParkedConnection::park(conn));
+
+                            #[cfg(feature = "tracing")]
+                            {
+                                created += 1;
+                            }
+                        }
+
+                        #[cfg(feature = "tracing")]
+                        if created > 0 {
+                            tracing::debug!("created {} idle connections", created);
+                        }
+
+                        if !dropped.is_empty() {
+                            #[cfg(feature = "tracing")]
+                            tracing::debug!("dropped {} idle connections", dropped.len());
+
+                            for conn in dropped {
+                                let mut conn = conn.unpark();
+                                conn.abort();
+                            }
+                        }
+
+                        thread::sleep(idle_timeout);
+                    }
+                })
+                .expect("couldn't spawn the Pool thread");
+        }
+
+        pool
+    }
+
+    pub fn connection(self: &Arc<Self>) -> Result<PooledConnection, Error> {
+        loop {
+            let conn = {
+                let mut connections = self.connections.lock().unwrap();
+                connections.pop()
+            };
+
+            match conn {
+                Some(conn) => {
+                    let mut conn = conn.unpark();
+
+                    // TODO: handle the client try another connection if this one isn't good
+                    if !conn.test_connected() {
+                        #[cfg(feature = "tracing")]
+                        tracing::debug!("dropping a broken connection");
+
+                        conn.abort();
+                        continue;
+                    }
+
+                    #[cfg(feature = "tracing")]
+                    tracing::debug!("reusing a pooled connection");
+
+                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                }
+                None => {
+                    #[cfg(feature = "tracing")]
+                    tracing::debug!("creating a new connection");
+
+                    let conn = self.client.connection()?;
+                    return Ok(PooledConnection::wrap(conn, self.clone()));
+                }
+            }
+        }
+    }
+
+    fn recycle(&self, mut conn: SmtpConnection) {
+        if conn.has_broken() {
+            #[cfg(feature = "tracing")]
+            tracing::debug!("dropping a broken connection instead of recycling it");
+
+            conn.abort();
+            drop(conn);
+        } else {
+            #[cfg(feature = "tracing")]
+            tracing::debug!("recycling connection");
+
+            let mut connections = self.connections.lock().unwrap();
+            if connections.len() >= self.config.max_size as usize {
+                drop(connections);
+                conn.abort();
+            } else {
+                let conn = ParkedConnection::park(conn);
+                connections.push(conn);
+            }
+        }
+    }
+}
+
+impl Debug for Pool {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Pool")
+            .field("config", &self.config)
+            .field(
+                "connections",
+                &match self.connections.try_lock() {
+                    Ok(connections) => format!("{} connections", connections.len()),
+
+                    Err(TryLockError::WouldBlock) => "LOCKED".to_string(),
+                    Err(TryLockError::Poisoned(_)) => "POISONED".to_string(),
+                },
+            )
+            .field("client", &self.client)
+            .finish()
+    }
+}
+
+impl Drop for Pool {
+    fn drop(&mut self) {
+        #[cfg(feature = "tracing")]
+        tracing::debug!("dropping Pool");
+
+        let connections = mem::take(&mut *self.connections.get_mut().unwrap());
+        for conn in connections {
+            let mut conn = conn.unpark();
+            conn.abort();
+        }
+    }
+}
+
+impl ParkedConnection {
+    fn park(conn: SmtpConnection) -> Self {
+        Self {
+            conn,
+            since: Instant::now(),
+        }
+    }
+
+    fn idle_duration(&self) -> Duration {
+        self.since.elapsed()
+    }
+
+    fn unpark(self) -> SmtpConnection {
+        self.conn
+    }
+}
+
+impl PooledConnection {
+    fn wrap(conn: SmtpConnection, pool: Arc<Pool>) -> Self {
+        Self {
+            conn: Some(conn),
+            pool,
+        }
+    }
+}
+
+impl Deref for PooledConnection {
+    type Target = SmtpConnection;
+
+    fn deref(&self) -> &Self::Target {
+        self.conn.as_ref().expect("conn hasn't been dropped yet")
+    }
+}
+
+impl DerefMut for PooledConnection {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.conn.as_mut().expect("conn hasn't been dropped yet")
+    }
+}
+
+impl Drop for PooledConnection {
+    fn drop(&mut self) {
+        let conn = self
+            .conn
+            .take()
+            .expect("SmtpConnection hasn't been taken yet");
+        self.pool.recycle(conn);
+    }
+}

src/transport/smtp/response.rs

@@ -1,7 +1,13 @@
 //! SMTP response, containing a mandatory return code and an optional text
 //! message
 
-use crate::transport::smtp::{error, Error};
+use std::{
+    fmt::{Display, Formatter, Result},
+    result,
+    str::FromStr,
+    string::ToString,
+};
+
 use nom::{
     branch::alt,
     bytes::streaming::{tag, take_until},
@@ -10,12 +16,8 @@ use nom::{
     sequence::{preceded, tuple},
     IResult,
 };
-use std::{
-    fmt::{Display, Formatter, Result},
-    result,
-    str::FromStr,
-    string::ToString,
-};
+
+use crate::transport::smtp::{error, Error};
 
 /// First digit indicates severity
 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
@@ -137,10 +139,10 @@ impl Code {
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Response {
     /// Response code
-    pub code: Code,
+    code: Code,
     /// Server response string (optional)
     /// Handle multiline responses
-    pub message: Vec<String>,
+    message: Vec<String>,
 }
 
 impl FromStr for Response {
@@ -180,6 +182,16 @@ impl Response {
     pub fn first_line(&self) -> Option<&str> {
         self.message.first().map(String::as_str)
     }
+
+    /// Response code
+    pub fn code(&self) -> Code {
+        self.code
+    }
+
+    /// Server response string (array of lines)
+    pub fn message(&self) -> impl Iterator<Item = &str> {
+        self.message.iter().map(String::as_str)
+    }
 }
 
 // Parsers (originally from tokio-smtp)

src/transport/smtp/transport.rs

@@ -1,22 +1,23 @@
+#[cfg(feature = "pool")]
+use std::sync::Arc;
 use std::time::Duration;
 
-#[cfg(feature = "r2d2")]
-use r2d2::Pool;
-
-#[cfg(feature = "r2d2")]
+#[cfg(feature = "pool")]
+use super::pool::sync_impl::Pool;
+#[cfg(feature = "pool")]
 use super::PoolConfig;
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-use super::{error, Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
+#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{address::Envelope, Transport};
 
 /// Sends emails using the SMTP protocol
 #[cfg_attr(docsrs, doc(cfg(feature = "smtp-transport")))]
 #[derive(Clone)]
 pub struct SmtpTransport {
-    #[cfg(feature = "r2d2")]
-    inner: Pool<SmtpClient>,
-    #[cfg(not(feature = "r2d2"))]
+    #[cfg(feature = "pool")]
+    inner: Arc<Pool>,
+    #[cfg(not(feature = "pool"))]
     inner: SmtpClient,
 }
 
@@ -26,14 +27,11 @@ impl Transport for SmtpTransport {
 
     /// Sends an email
     fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
-        #[cfg(feature = "r2d2")]
-        let mut conn = self.inner.get().map_err(error::client)?;
-        #[cfg(not(feature = "r2d2"))]
         let mut conn = self.inner.connection()?;
 
         let result = conn.send(envelope, email)?;
 
-        #[cfg(not(feature = "r2d2"))]
+        #[cfg(not(feature = "pool"))]
         conn.quit()?;
 
         Ok(result)
@@ -105,10 +103,25 @@ impl SmtpTransport {
 
         SmtpTransportBuilder {
             info: new,
-            #[cfg(feature = "r2d2")]
+            #[cfg(feature = "pool")]
             pool_config: PoolConfig::default(),
         }
     }
+
+    /// Tests the SMTP connection
+    ///
+    /// `test_connection()` tests the connection by using the SMTP NOOP command.
+    /// The connection is closed afterwards if a connection pool is not used.
+    pub fn test_connection(&self) -> Result<bool, Error> {
+        let mut conn = self.inner.connection()?;
+
+        let is_connected = conn.test_connected();
+
+        #[cfg(not(feature = "pool"))]
+        conn.quit()?;
+
+        Ok(is_connected)
+    }
 }
 
 /// Contains client configuration.
@@ -116,7 +129,7 @@ impl SmtpTransport {
 #[derive(Debug, Clone)]
 pub struct SmtpTransportBuilder {
     info: SmtpInfo,
-    #[cfg(feature = "r2d2")]
+    #[cfg(feature = "pool")]
     pool_config: PoolConfig,
 }
 
@@ -163,8 +176,8 @@ impl SmtpTransportBuilder {
     /// Use a custom configuration for the connection pool
     ///
     /// Defaults can be found at [`PoolConfig`]
-    #[cfg(feature = "r2d2")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "r2d2")))]
+    #[cfg(feature = "pool")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "pool")))]
     pub fn pool_config(mut self, pool_config: PoolConfig) -> Self {
         self.pool_config = pool_config;
         self
@@ -172,16 +185,15 @@ impl SmtpTransportBuilder {
 
     /// Build the transport
     ///
-    /// If the `r2d2` feature is enabled an `Arc` wrapped pool is be created.
+    /// If the `pool` feature is enabled an `Arc` wrapped pool is be created.
     /// Defaults can be found at [`PoolConfig`]
     pub fn build(self) -> SmtpTransport {
         let client = SmtpClient { info: self.info };
-        SmtpTransport {
-            #[cfg(feature = "r2d2")]
-            inner: self.pool_config.build(client),
-            #[cfg(not(feature = "r2d2"))]
-            inner: client,
-        }
+
+        #[cfg(feature = "pool")]
+        let client = Pool::new(self.pool_config, client);
+
+        SmtpTransport { inner: client }
     }
 }
 
@@ -225,7 +237,7 @@ impl SmtpClient {
         }
 
         if let Some(credentials) = &self.info.credentials {
-            conn.auth(&self.info.authentication, &credentials)?;
+            conn.auth(&self.info.authentication, credentials)?;
         }
         Ok(conn)
     }

src/transport/smtp/util.rs

@@ -4,7 +4,6 @@ use std::fmt::{Display, Formatter, Result as FmtResult};
 
 /// Encode a string as xtext
 #[derive(Debug)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct XText<'a>(pub &'a str);
 
 impl<'a> Display for XText<'a> {

src/transport/stub/mod.rs

@@ -1,10 +1,12 @@
-//! The stub transport only logs message envelope and drops the content. It can be useful for
-//! testing purposes.
+//! The stub transport logs message envelopes as well as contents. It can be useful for testing
+//! purposes.
 //!
-//! #### Stub Transport
+//! # Stub Transport
 //!
-//! The stub transport returns provided result and drops the content. It can be useful for
-//! testing purposes.
+//! The stub transport logs message envelopes as well as contents. It can be useful for testing
+//! purposes.
+//!
+//! # Examples
 //!
 //! ```rust
 //! # #[cfg(feature = "builder")]
@@ -12,7 +14,7 @@
 //! use lettre::{transport::stub::StubTransport, Message, Transport};
 //!
 //! # use std::error::Error;
-//! # fn main() -> Result<(), Box<dyn Error>> {
+//! # fn try_main() -> Result<(), Box<dyn Error>> {
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
@@ -23,17 +25,33 @@
 //! let mut sender = StubTransport::new_ok();
 //! let result = sender.send(&email);
 //! assert!(result.is_ok());
+//! assert_eq!(
+//!     sender.messages(),
+//!     vec![(
+//!         email.envelope().clone(),
+//!         String::from_utf8(email.formatted()).unwrap()
+//!     )],
+//! );
 //! # Ok(())
 //! # }
+//! # try_main().unwrap();
 //! # }
 //! ```
 
+use std::{
+    error::Error as StdError,
+    fmt,
+    sync::{Arc, Mutex as StdMutex},
+};
+
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+use async_trait::async_trait;
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+use futures_util::lock::Mutex as FuturesMutex;
+
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 use crate::AsyncTransport;
 use crate::{address::Envelope, Transport};
-#[cfg(any(feature = "tokio1", feature = "async-std1"))]
-use async_trait::async_trait;
-use std::{error::Error as StdError, fmt};
 
 #[derive(Debug, Copy, Clone)]
 pub struct Error;
@@ -46,47 +64,113 @@ impl fmt::Display for Error {
 
 impl StdError for Error {}
 
-/// This transport logs the message envelope and returns the given response
-#[derive(Debug, Clone, Copy)]
+/// This transport logs messages and always returns the given response
+#[derive(Debug, Clone)]
 pub struct StubTransport {
     response: Result<(), Error>,
+    message_log: Arc<StdMutex<Vec<(Envelope, String)>>>,
+}
+
+/// This transport logs messages and always returns the given response
+#[derive(Debug, Clone)]
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+#[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
+pub struct AsyncStubTransport {
+    response: Result<(), Error>,
+    message_log: Arc<FuturesMutex<Vec<(Envelope, String)>>>,
 }
 
 impl StubTransport {
     /// Creates a new transport that always returns the given Result
-    pub fn new(response: Result<(), Error>) -> StubTransport {
-        StubTransport { response }
+    pub fn new(response: Result<(), Error>) -> Self {
+        Self {
+            response,
+            message_log: Arc::new(StdMutex::new(vec![])),
+        }
     }
 
     /// Creates a new transport that always returns a success response
-    pub fn new_ok() -> StubTransport {
-        StubTransport { response: Ok(()) }
+    pub fn new_ok() -> Self {
+        Self {
+            response: Ok(()),
+            message_log: Arc::new(StdMutex::new(vec![])),
+        }
     }
 
     /// Creates a new transport that always returns an error
-    pub fn new_error() -> StubTransport {
-        StubTransport {
+    pub fn new_error() -> Self {
+        Self {
             response: Err(Error),
+            message_log: Arc::new(StdMutex::new(vec![])),
         }
     }
+
+    /// Return all logged messages sent using [`Transport::send_raw`]
+    pub fn messages(&self) -> Vec<(Envelope, String)> {
+        self.message_log
+            .lock()
+            .expect("Couldn't acquire lock to write message log")
+            .clone()
+    }
+}
+
+#[cfg(any(feature = "async-std1", feature = "tokio1"))]
+impl AsyncStubTransport {
+    /// Creates a new transport that always returns the given Result
+    pub fn new(response: Result<(), Error>) -> Self {
+        Self {
+            response,
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Creates a new transport that always returns a success response
+    pub fn new_ok() -> Self {
+        Self {
+            response: Ok(()),
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Creates a new transport that always returns an error
+    pub fn new_error() -> Self {
+        Self {
+            response: Err(Error),
+            message_log: Arc::new(FuturesMutex::new(vec![])),
+        }
+    }
+
+    /// Return all logged messages sent using [`AsyncTransport::send_raw`]
+    #[cfg(any(feature = "tokio1", feature = "async-std1"))]
+    pub async fn messages(&self) -> Vec<(Envelope, String)> {
+        self.message_log.lock().await.clone()
+    }
 }
 
 impl Transport for StubTransport {
     type Ok = ();
     type Error = Error;
 
-    fn send_raw(&self, _envelope: &Envelope, _email: &[u8]) -> Result<Self::Ok, Self::Error> {
+    fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        self.message_log
+            .lock()
+            .expect("Couldn't acquire lock to write message log")
+            .push((envelope.clone(), String::from_utf8_lossy(email).into()));
         self.response
     }
 }
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 #[async_trait]
-impl AsyncTransport for StubTransport {
+impl AsyncTransport for AsyncStubTransport {
     type Ok = ();
     type Error = Error;
 
-    async fn send_raw(&self, _envelope: &Envelope, _email: &[u8]) -> Result<Self::Ok, Self::Error> {
+    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        self.message_log
+            .lock()
+            .await
+            .push((envelope.clone(), String::from_utf8_lossy(email).into()));
         self.response
     }
 }

testdata/coredns.conf

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

testdata/db.example.org

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

tests/transport_file.rs

@@ -9,13 +9,15 @@ fn default_date() -> std::time::SystemTime {
 #[cfg(test)]
 #[cfg(all(feature = "file-transport", feature = "builder"))]
 mod sync {
-    use crate::default_date;
-    use lettre::{FileTransport, Message, Transport};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{FileTransport, Message, Transport};
+
+    use crate::default_date;
+
     #[test]
     fn file_transport() {
         let sender = FileTransport::new(temp_dir());
@@ -104,15 +106,16 @@ mod sync {
 #[cfg(test)]
 #[cfg(all(feature = "file-transport", feature = "builder", feature = "tokio1"))]
 mod tokio_1 {
-    use crate::default_date;
-    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
     use tokio1_crate as tokio;
 
+    use crate::default_date;
+
     #[tokio::test]
     async fn file_transport_tokio1() {
         let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -155,13 +158,15 @@ mod tokio_1 {
     feature = "async-std1"
 ))]
 mod asyncstd_1 {
-    use crate::default_date;
-    use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
+    use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
+
+    use crate::default_date;
+
     #[async_std::test]
     async fn file_transport_asyncstd1() {
         let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());

tests/transport_smtp.rs

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

tests/transport_smtp_pool.rs

@@ -1,8 +1,9 @@
-#[cfg(all(test, feature = "smtp-transport", feature = "r2d2"))]
+#[cfg(all(test, feature = "smtp-transport", feature = "pool"))]
 mod sync {
-    use lettre::{address::Envelope, SmtpTransport, Transport};
     use std::{sync::mpsc, thread};
 
+    use lettre::{address::Envelope, SmtpTransport, Transport};
+
     fn envelope() -> Envelope {
         Envelope::new(
             Some("user@localhost".parse().unwrap()),

tests/transport_stub.rs

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