Prepare 0.10.0-rc.4 (#691)

The pool tests have been implicitly disabled by testing
for a feature that has been removed.

.github/workflows/test.yml

@@ -7,6 +7,8 @@ on:
       - master
 
 env:
+  RUSTFLAGS: "--cfg lettre_ignore_tls_mismatch"
+  RUSTDOCFLAGS: "--cfg lettre_ignore_tls_mismatch"
   RUST_BACKTRACE: full
 
 jobs:
@@ -79,8 +81,8 @@ jobs:
             rust: stable
           - name: beta
             rust: beta
-          - name: 1.46.0
+          - name: 1.52.1
-            rust: 1.46.0
+            rust: 1.52.1
 
     steps:
       - name: Checkout

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 1.52.1
 * 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.4"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -19,6 +19,7 @@ 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
@@ -27,7 +28,6 @@ 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"] }
 
 # file transport
@@ -36,15 +36,14 @@ 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 }
+nom = { version = "7", optional = true }
-r2d2 = { version = "0.8", optional = true } # feature
 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 }
+rustls = { version = "0.20", features = ["dangerous_configuration"], optional = true }
-webpki = { version = "0.21", optional = true }
+rustls-pemfile = { version = "0.2.1", optional = true }
-webpki-roots = { version = "0.21", optional = true }
+webpki-roots = { version = "0.22", optional = true }
 
 # async
 futures-io = { version = "0.3.7", optional = true }
@@ -54,12 +53,12 @@ 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", "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 }
 
 [dev-dependencies]
 criterion = "0.3"
@@ -76,8 +75,9 @@ harness = false
 name = "transport_smtp"
 
 [features]
-default = ["smtp-transport", "native-tls", "hostname", "r2d2", "builder"]
+default = ["smtp-transport", "pool", "native-tls", "hostname", "builder"]
 builder = ["httpdate", "mime", "base64", "fastrand", "quoted_printable"]
+mime03 = ["mime"]
 
 # transports
 file-transport = ["uuid"]
@@ -85,19 +85,21 @@ 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"]
 
 [package.metadata.docs.rs]
 all-features = true
-rustdoc-args = ["--cfg", "docsrs"]
+rustdoc-args = ["--cfg", "docsrs", "--cfg", "lettre_ignore_tls_mismatch"]
 
 [[example]]
 name = "basic_html"

README.md

@@ -28,8 +28,8 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0-rc.1">
+  <a href="https://deps.rs/crate/lettre/0.10.0-rc.4">
-    <img src="https://deps.rs/crate/lettre/0.10.0-rc.1/status.svg"
+    <img src="https://deps.rs/crate/lettre/0.10.0-rc.4/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 1.52.1 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.4"
 ```
 
 ```rust,no_run
@@ -103,7 +103,8 @@ 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`.
 

src/address/types.rs

@@ -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 at_start = check_address(val)?;
-        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(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;
+#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
+use futures_util::future::BoxFuture;
 
 use std::fmt::Debug;
+#[cfg(feature = "smtp-transport")]
+use std::future::Future;
 #[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;
 
 #[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 =
+        let mut conn = AsyncSmtpConnection::connect_tokio1(
-            AsyncSmtpConnection::connect_tokio1(hostname, port, hello_name, tls_parameters).await?;
+            (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 =
+        let mut conn = AsyncSmtpConnection::connect_asyncstd1(
-            AsyncSmtpConnection::connect_asyncstd1(hostname, port, hello_name, tls_parameters)
+            (hostname, port),
-                .await?;
+            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 1.52.1 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 +84,21 @@
 //!
 //! * **serde**: Serialization/Deserialization of entities
 //! * **tracing**: Logging using the `tracing` crate
+//! * **mime03**: Allow creating a [`ContentType`] from an existing [mime 0.3] `Mime` struct
 //!
 //! [`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
 
-#![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.4")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -109,6 +112,56 @@
 )]
 #![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"))]

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::{Attachment, header::ContentType};
+    ///
+    /// # 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::{Attachment, header::ContentType};
+    ///
+    /// # 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::SevenBit => is_7bit_encoded(self),
-            ContentTransferEncoding::EightBit => is_8bit_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/header/content.rs

@@ -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,

src/message/header/content_type.rs

@@ -11,8 +11,13 @@ 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 {
@@ -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);
@@ -78,6 +91,60 @@ impl Display for ContentTypeErr {
     }
 }
 
+// -- Serialization and Deserialization --
+#[cfg(feature = "serde")]
+mod serde {
+    use serde::de::{self, Deserialize, Deserializer, Visitor};
+    use serde::ser::{Serialize, Serializer};
+
+    use std::fmt;
+
+    use 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;

src/message/header/mod.rs

@@ -26,6 +26,9 @@ 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;
 
@@ -160,7 +163,7 @@ impl Display for Headers {
         for (name, value) in &self.headers {
             Display::fmt(name, f)?;
             f.write_str(": ")?;
-            HeaderValueEncoder::encode(&name, &value, f)?;
+            HeaderValueEncoder::encode(name, value, f)?;
             f.write_str("\r\n")?;
         }
 
@@ -237,7 +240,7 @@ impl HeaderName {
 
 impl Display for HeaderName {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        f.write_str(&self)
+        f.write_str(self)
     }
 }
 
@@ -408,13 +411,14 @@ 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;
@@ -797,4 +801,21 @@ mod tests {
             )
         );
     }
+
+    #[test]
+    fn issue_653() {
+        let mut headers = Headers::new();
+        headers.insert_raw(
+            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

@@ -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 {

src/message/mailbox/types.rs

@@ -70,7 +70,7 @@ impl Display for Mailbox {
         if let Some(ref name) = self.name {
             let name = name.trim();
             if !name.is_empty() {
-                f.write_str(&name)?;
+                f.write_str(name)?;
                 f.write_str(" <")?;
                 self.email.fmt(f)?;
                 return f.write_char('>');

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>
 //!
 //!
@@ -401,7 +394,7 @@ 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 +417,10 @@ 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,
@@ -503,7 +500,7 @@ 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 +556,7 @@ mod test {
 
         let email = Message::builder()
             .date(date)
+            .bcc("hidden@example.com".parse().unwrap())
             .header(header::From(
                 vec![Mailbox::new(
                     Some("Каи".into()),

src/transport/mod.rs

@@ -152,7 +152,7 @@ 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/mod.rs

@@ -92,7 +92,7 @@ use std::{
 
 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 +113,10 @@ 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 +150,10 @@ 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 +266,7 @@ 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 +298,7 @@ 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> {
+pub struct AsyncSmtpTransport<E: Executor> {
-    // TODO: pool
+    #[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

@@ -4,6 +4,7 @@ use crate::transport::smtp::error::{self, Error};
 use std::fmt::{self, Debug, Display, Formatter};
 
 /// 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

@@ -11,7 +11,7 @@ use crate::{
     Envelope,
 };
 use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
-use std::fmt::Display;
+use std::{fmt::Display, time::Duration};
 
 #[cfg(feature = "tracing")]
 use super::escape_crlf;
@@ -48,13 +48,13 @@ impl AsyncSmtpConnection {
     ///
     /// Sends EHLO and parses server information
     #[cfg(feature = "tokio1")]
-    pub async fn connect_tokio1(
+    pub async fn connect_tokio1<T: tokio1_crate::net::ToSocketAddrs>(
-        hostname: &str,
+        server: T,
-        port: u16,
+        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 +62,13 @@ impl AsyncSmtpConnection {
     ///
     /// Sends EHLO and parses server information
     #[cfg(feature = "async-std1")]
-    pub async fn connect_asyncstd1(
+    pub async fn connect_asyncstd1<T: async_std::net::ToSocketAddrs>(
-        hostname: &str,
+        server: T,
-        port: u16,
+        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 +161,7 @@ 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.stream.get_mut().upgrade_tls(tls_parameters).await,
-                self
-            );
             #[cfg(feature = "tracing")]
             tracing::debug!("connection encrypted");
             // Send EHLO again
@@ -298,7 +295,7 @@ impl AsyncSmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code))
+                        Err(error::code(response.code()))
                     }
                 }
                 Err(nom::Err::Failure(e)) => {

src/transport/smtp/client/async_net.rs

@@ -1,8 +1,9 @@
 use std::{
-    mem,
+    io, mem,
     net::SocketAddr,
     pin::Pin,
     task::{Context, Poll},
+    time::Duration,
 };
 
 use futures_io::{
@@ -13,9 +14,9 @@ use futures_io::{
 use tokio1_crate::io::{AsyncRead as _, AsyncWrite as _, ReadBuf as Tokio1ReadBuf};
 
 #[cfg(feature = "async-std1")]
-use async_std::net::TcpStream as AsyncStd1TcpStream;
+use async_std::net::{TcpStream as AsyncStd1TcpStream, ToSocketAddrs as AsyncStd1ToSocketAddrs};
 #[cfg(feature = "tokio1")]
-use tokio1_crate::net::TcpStream as Tokio1TcpStream;
+use tokio1_crate::net::{TcpStream as Tokio1TcpStream, ToSocketAddrs as Tokio1ToSocketAddrs};
 
 #[cfg(feature = "async-std1-native-tls")]
 use async_native_tls::TlsStream as AsyncStd1TlsStream;
@@ -23,7 +24,7 @@ use async_native_tls::TlsStream as AsyncStd1TlsStream;
 use tokio1_native_tls_crate::TlsStream as Tokio1TlsStream;
 
 #[cfg(feature = "async-std1-rustls-tls")]
-use async_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
+use futures_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
 #[cfg(feature = "tokio1-rustls-tls")]
 use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;
 
@@ -107,14 +108,47 @@ impl AsyncNetworkStream {
     }
 
     #[cfg(feature = "tokio1")]
-    pub async fn connect_tokio1(
+    pub async fn connect_tokio1<T: Tokio1ToSocketAddrs>(
-        hostname: &str,
+        server: T,
-        port: u16,
+        timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = Tokio1TcpStream::connect((hostname, port))
+        async fn try_connect_timeout<T: Tokio1ToSocketAddrs>(
-            .await
+            server: T,
-            .map_err(error::connection)?;
+            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 +158,45 @@ impl AsyncNetworkStream {
     }
 
     #[cfg(feature = "async-std1")]
-    pub async fn connect_asyncstd1(
+    pub async fn connect_asyncstd1<T: AsyncStd1ToSocketAddrs>(
-        hostname: &str,
+        server: T,
-        port: u16,
+        timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = AsyncStd1TcpStream::connect((hostname, port))
+        async fn try_connect_timeout<T: AsyncStd1ToSocketAddrs>(
-            .await
+            server: T,
-            .map_err(error::connection)?;
+            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 +261,9 @@ 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 +290,13 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "tokio1-rustls-tls")]
                 return {
-                    use tokio1_rustls::{webpki::DNSNameRef, TlsConnector};
+                    use std::convert::TryFrom;
 
-                    let domain =
+                    use rustls::ServerName;
-                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
+                    use tokio1_rustls::TlsConnector;
+
+                    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 +345,13 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "async-std1-rustls-tls")]
                 return {
-                    use async_rustls::{webpki::DNSNameRef, TlsConnector};
+                    use std::convert::TryFrom;
 
-                    let domain =
+                    use futures_rustls::TlsConnector;
-                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
+                    use rustls::ServerName;
+
+                    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

src/transport/smtp/client/connection.rs

@@ -145,7 +145,7 @@ 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 +276,7 @@ impl SmtpConnection {
                     return if response.is_positive() {
                         Ok(response)
                     } else {
-                        Err(error::code(response.code))
+                        Err(error::code(response.code()))
                     };
                 }
                 Err(nom::Err::Failure(e)) => {

src/transport/smtp/client/mod.rs

@@ -78,7 +78,15 @@ 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 +119,7 @@ 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 +130,7 @@ 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

@@ -1,3 +1,5 @@
+#[cfg(feature = "rustls-tls")]
+use std::convert::TryFrom;
 use std::{
     io::{self, Read, Write},
     mem,
@@ -9,7 +11,7 @@ use std::{
 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 +35,7 @@ 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 +92,20 @@ 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) {
+                match TcpStream::connect_timeout(&addr, timeout) {
-                    return Ok(result);
+                    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 +159,11 @@ impl NetworkStream {
             }
             #[cfg(feature = "rustls-tls")]
             InnerTlsParameters::RustlsTls(connector) => {
-                use webpki::DNSNameRef;
+                let domain = ServerName::try_from(tls_parameters.domain())
-
+                    .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
-                let domain = DNSNameRef::try_from_ascii_str(tls_parameters.domain())
+                let connection =
-                    .map_err(error::connection)?;
+                    ClientConnection::new(connector.clone(), domain).map_err(error::connection)?;
-                let stream = StreamOwned::new(ClientSession::new(&connector, domain), tcp_stream);
+                let stream = StreamOwned::new(connection, tcp_stream);
-
                 InnerNetworkStream::RustlsTls(stream)
             }
         })

src/transport/smtp/client/tls.rs

@@ -3,12 +3,13 @@ use crate::transport::smtp::{error, Error};
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls-tls")]
-use rustls::{ClientConfig, RootCertStore, ServerCertVerified, ServerCertVerifier, TLSError};
+use rustls::{
+    client::{ServerCertVerified, ServerCertVerifier, WebPkiVerifier},
+    ClientConfig, Error as TlsError, OwnedTrustAnchor, RootCertStore, ServerName,
+};
 use std::fmt::{self, Debug};
 #[cfg(feature = "rustls-tls")]
-use std::sync::Arc;
+use std::{sync::Arc, time::SystemTime};
-#[cfg(feature = "rustls-tls")]
-use webpki::DNSNameRef;
 
 /// Accepted protocols by default.
 /// This removes TLS 1.0 and 1.1 compared to tls-native defaults.
@@ -163,21 +164,35 @@ 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();
+        let tls = if self.accept_invalid_certs {
-
+            tls.with_custom_certificate_verifier(Arc::new(InvalidCertsVerifier {}))
-        for cert in self.root_certs {
+        } else {
-            for rustls_cert in cert.rustls {
+            let mut root_cert_store = RootCertStore::empty();
-                tls.root_store.add(&rustls_cert).map_err(error::tls)?;
+            for cert in self.root_certs {
+                for rustls_cert in cert.rustls {
+                    root_cert_store.add(&rustls_cert).map_err(error::tls)?;
+                }
             }
-        }
+            root_cert_store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(
-        if self.accept_invalid_certs {
+                |ta| {
-            tls.dangerous()
+                    OwnedTrustAnchor::from_subject_spki_name_constraints(
-                .set_certificate_verifier(Arc::new(InvalidCertsVerifier {}));
+                        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 +272,14 @@ 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 +304,13 @@ struct InvalidCertsVerifier;
 impl ServerCertVerifier for InvalidCertsVerifier {
     fn verify_server_cert(
         &self,
-        _roots: &RootCertStore,
+        _end_entity: &rustls::Certificate,
-        _presented_certs: &[rustls::Certificate],
+        _intermediates: &[rustls::Certificate],
-        _dns_name: DNSNameRef<'_>,
+        _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/extension.rs

@@ -72,6 +72,7 @@ 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 +108,11 @@ 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 +136,7 @@ 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 +198,11 @@ impl ServerInfo {
         }
         None
     }
+
+    /// The name given in the server banner
+    pub fn name(&self) -> &str {
+        self.name.as_ref()
+    }
 }
 
 /// A `MAIL FROM` extension parameter

src/transport/smtp/mod.rs

@@ -118,10 +118,8 @@
 
 #[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},
@@ -144,11 +142,11 @@ 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 +162,7 @@ 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 +1,299 @@
+use std::fmt::{self, Debug};
+use std::mem;
+use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+use futures_util::lock::Mutex;
+use futures_util::stream::{self, StreamExt};
+use once_cell::sync::OnceCell;
+
+use crate::executor::SpawnHandle;
+use crate::transport::smtp::async_transport::AsyncSmtpClient;
+use crate::Executor;
+
+use super::super::client::AsyncSmtpConnection;
+use super::super::Error;
+use super::PoolConfig;
+
+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 +1,256 @@
+use std::fmt::{self, Debug};
+use std::ops::{Deref, DerefMut};
+use std::sync::{Arc, Mutex, TryLockError};
+use std::time::{Duration, Instant};
+use std::{mem, thread};
+
+use crate::transport::smtp::transport::SmtpClient;
+
+use super::super::client::SmtpConnection;
+use super::super::Error;
+use super::PoolConfig;
+
+pub struct Pool {
+    config: PoolConfig,
+    connections: Mutex<Vec<ParkedConnection>>,
+    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

@@ -137,10 +137,10 @@ 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 +180,16 @@ 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")]
+#[cfg(feature = "pool")]
-use r2d2::Pool;
+use super::pool::sync_impl::Pool;
-
+#[cfg(feature = "pool")]
-#[cfg(feature = "r2d2")]
 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")]
+    #[cfg(feature = "pool")]
-    inner: Pool<SmtpClient>,
+    inner: Arc<Pool>,
-    #[cfg(not(feature = "r2d2"))]
+    #[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(feature = "pool")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "r2d2")))]
+    #[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")]
+        #[cfg(feature = "pool")]
-            inner: self.pool_config.build(client),
+        let client = Pool::new(self.pool_config, client);
-            #[cfg(not(feature = "r2d2"))]
+
-            inner: 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> {

tests/transport_smtp_pool.rs

@@ -1,4 +1,4 @@
-#[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};