Prepare v0.11.12 (#1034)

.github/workflows/test.yml

@@ -75,8 +75,8 @@ jobs:
             rust: stable
           - name: beta
             rust: beta
-          - name: '1.70'
-            rust: '1.70'
+          - name: '1.71'
+            rust: '1.71'
 
     steps:
       - name: Checkout

CHANGELOG.md

@@ -1,3 +1,65 @@
+<a name="v0.11.12"></a>
+### v0.11.12 (2025-02-02)
+
+#### Misc
+
+* Warn against manually configuring `port` and `tls` on SMTP transport builder ([#1014])
+* Document variants of `Tls` enum ([#1015])
+* Fix rustdoc warnings ([#1016])
+* Add `ContentType::TEXT_PLAIN` to `Message` builder examples ([#1017])
+* Document `SmtpTransport` and `AsyncSmtpTransport` ([#1018])
+* Fix typo in transport builder `credentials` method ([#1019])
+* Document required system dependencies for OpenSSL ([#1030])
+* Improve docs for the `transport::smtp` module ([#1031])
+* Improve docs for smtp transport builder `from_url` ([#1032])
+* Replace `assert!` with `?` on `send` examples ([#1033])
+* Warn on more pedantic clippy lints and fix them ([#1035], [#1036])
+
+[#1014]: https://github.com/lettre/lettre/pull/1014
+[#1015]: https://github.com/lettre/lettre/pull/1015
+[#1016]: https://github.com/lettre/lettre/pull/1016
+[#1017]: https://github.com/lettre/lettre/pull/1017
+[#1018]: https://github.com/lettre/lettre/pull/1018
+[#1019]: https://github.com/lettre/lettre/pull/1019
+[#1030]: https://github.com/lettre/lettre/pull/1030
+[#1031]: https://github.com/lettre/lettre/pull/1031
+[#1032]: https://github.com/lettre/lettre/pull/1032
+[#1033]: https://github.com/lettre/lettre/pull/1033
+[#1035]: https://github.com/lettre/lettre/pull/1035
+[#1036]: https://github.com/lettre/lettre/pull/1036
+
+<a name="v0.11.11"></a>
+### v0.11.11 (2024-12-05)
+
+#### Upgrade notes
+
+* MSRV is now 1.71 ([#1008])
+
+#### Bug fixes
+
+* Fix off-by-one error reaching the minimum number of configured pooled connections ([#1012])
+
+#### Misc
+
+* Fix clippy warnings ([#1009])
+* Fix `-Zminimal-versions` build ([#1007])
+
+[#1007]: https://github.com/lettre/lettre/pull/1007
+[#1008]: https://github.com/lettre/lettre/pull/1008
+[#1009]: https://github.com/lettre/lettre/pull/1009
+[#1012]: https://github.com/lettre/lettre/pull/1012
+
+<a name="v0.11.10"></a>
+### v0.11.10 (2024-10-23)
+
+#### Bug fixes
+
+* Ignore disconnect errors when `pool` feature of SMTP transport is disabled ([#999])
+* Use case insensitive comparisons for matching login challenge requests ([#1000])
+
+[#999]: https://github.com/lettre/lettre/pull/999
+[#1000]: https://github.com/lettre/lettre/pull/1000
+
 <a name="v0.11.9"></a>
 ### v0.11.9 (2024-09-13)
 
@@ -508,6 +570,6 @@ Several breaking changes were made between 0.9 and 0.10, but changes should be s
 
 *  multipart support
 *  add non-consuming methods for Email builders
-* `add_header` does not return the builder anymore, 
+* `add_header` does not return the builder anymore,
   for consistency with other methods. Use the `header`
   method instead

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 = "0.11.9"
+version = "0.11.12"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -11,7 +11,7 @@ authors = ["Alexis Mousset <contact@amousset.me>", "Paolo Barbolini <paolo@paolo
 categories = ["email", "network-programming"]
 keywords = ["email", "smtp", "mailer", "message", "sendmail"]
 edition = "2021"
-rust-version = "1.70"
+rust-version = "1.71"
 
 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -44,7 +44,7 @@ url = { version = "2.4", optional = true }
 percent-encoding = { version = "2.3", optional = true }
 
 ## tls
-native-tls = { version = "0.2.5", optional = true } # feature
+native-tls = { version = "0.2.9", optional = true } # feature
 rustls = { version = "0.23.5", default-features = false, features = ["ring", "logging", "std", "tls12"], optional = true }
 rustls-pemfile = { version = "2", optional = true }
 rustls-native-certs = { version = "0.8", optional = true }

README.md

@@ -28,8 +28,8 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.11.9">
-    <img src="https://deps.rs/crate/lettre/0.11.9/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.11.12">
+    <img src="https://deps.rs/crate/lettre/0.11.12/status.svg"
       alt="dependency status" />
   </a>
 </div>
@@ -53,12 +53,12 @@ Lettre does not provide (for now):
 ## Supported Rust Versions
 
 Lettre supports all Rust versions released in the last 6 months. At the time of writing
-the minimum supported Rust version is 1.70, but this could change at any time either from
+the minimum supported Rust version is 1.71, but this could change at any time either from
 one of our dependencies bumping their MSRV or by a new patch release of lettre.
 
 ## Example
 
-This library requires Rust 1.70 or newer.
+This library requires Rust 1.71 or newer.
 To use this library, add the following to your `Cargo.toml`:
 
 ```toml
@@ -107,7 +107,7 @@ cargo run --example autoconfigure SMTP_HOST
 
 ## Testing
 
-The `lettre` tests require an open mail server listening locally on port 2525 and the `sendmail` command. If you have python installed 
+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 127.0.0.1:2525`
 
 Alternatively only unit tests can be run by doing `cargo test --lib`.

benches/transport_smtp.rs

@@ -1,5 +1,5 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use lettre::{Message, SmtpTransport, Transport};
+use lettre::{message::header::ContentType, Message, SmtpTransport, Transport};
 
 fn bench_simple_send(c: &mut Criterion) {
     let sender = SmtpTransport::builder_dangerous("127.0.0.1")
@@ -13,6 +13,7 @@ fn bench_simple_send(c: &mut Criterion) {
                 .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
                 .to("Hei <hei@domain.tld>".parse().unwrap())
                 .subject("Happy new year")
+                .header(ContentType::TEXT_PLAIN)
                 .body(String::from("Be happy!"))
                 .unwrap();
             let result = black_box(sender.send(&email));
@@ -32,6 +33,7 @@ fn bench_reuse_send(c: &mut Criterion) {
                 .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
                 .to("Hei <hei@domain.tld>".parse().unwrap())
                 .subject("Happy new year")
+                .header(ContentType::TEXT_PLAIN)
                 .body(String::from("Be happy!"))
                 .unwrap();
             let result = black_box(sender.send(&email));

src/address/envelope.rs

@@ -29,7 +29,7 @@ pub struct Envelope {
 mod serde_forward_path {
     use super::Address;
     /// dummy type required for serde
-    /// see example: https://serde.rs/deserialize-map.html
+    /// see example: <https://serde.rs/deserialize-map.html>
     struct CustomVisitor;
     impl<'de> serde::de::Visitor<'de> for CustomVisitor {
         type Value = Vec<Address>;

src/address/serde.rs

@@ -36,7 +36,7 @@ impl<'de> Deserialize<'de> for Address {
             {
                 struct FieldVisitor;
 
-                impl<'de> Visitor<'de> for FieldVisitor {
+                impl Visitor<'_> for FieldVisitor {
                     type Value = Field;
 
                     fn expecting(&self, formatter: &mut Formatter<'_>) -> FmtResult {

src/lib.rs

@@ -6,7 +6,7 @@
 //! * Secure defaults
 //! * Async support
 //!
-//! Lettre requires Rust 1.70 or newer.
+//! Lettre requires Rust 1.71 or newer.
 //!
 //! ## Features
 //!
@@ -34,13 +34,25 @@
 //!
 //! _Secure SMTP connections using TLS from the `native-tls` crate_
 //!
-//! Uses schannel on Windows, Security-Framework on macOS, and OpenSSL on Linux.
+//! Uses schannel on Windows, Security-Framework on macOS, and OpenSSL
+//! on all other platforms.
 //!
 //! * **native-tls** 📫: TLS support for the synchronous version of the API
 //! * **tokio1-native-tls**: TLS support for the `tokio1` async version of the API
 //!
 //! NOTE: native-tls isn't supported with `async-std`
 //!
+//! ##### Building lettre with OpenSSL
+//!
+//! When building lettre with native-tls on a system that makes
+//! use of OpenSSL, the following packages will need to be installed
+//! in order for the build and the compiled program to run properly.
+//!
+//! | Distro       | Build-time packages        | Runtime packages             |
+//! | ------------ | -------------------------- | ---------------------------- |
+//! | Debian       | `pkg-config`, `libssl-dev` | `libssl3`, `ca-certificates` |
+//! | Alpine Linux | `pkgconf`, `openssl-dev`   | `libssl3`, `ca-certificates` |
+//!
 //! #### SMTP over TLS via the boring crate (Boring TLS)
 //!
 //! _Secure SMTP connections using TLS from the `boring-tls` crate_
@@ -109,7 +121,7 @@
 //! [mime 0.3]: https://docs.rs/mime/0.3
 //! [DKIM]: https://datatracker.ietf.org/doc/html/rfc6376
 
-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.11.9")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.11.12")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -137,7 +149,14 @@
     clippy::wildcard_imports,
     clippy::str_to_string,
     clippy::empty_structs_with_brackets,
-    clippy::zero_sized_map_values
+    clippy::zero_sized_map_values,
+    clippy::manual_let_else,
+    clippy::semicolon_if_nothing_returned,
+    clippy::unnecessary_wraps,
+    clippy::doc_markdown,
+    clippy::explicit_iter_loop,
+    clippy::redundant_closure_for_method_calls,
+    // Rust 1.86: clippy::unnecessary_semicolon,
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 

src/message/dkim.rs

@@ -69,7 +69,7 @@ impl Display for DkimSigningAlgorithm {
     }
 }
 
-/// Describe DkimSigning key error
+/// Describe [`DkimSigningKey`] key error
 #[derive(Debug)]
 pub struct DkimSigningKeyError(InnerDkimSigningKeyError);
 
@@ -100,7 +100,7 @@ impl StdError for DkimSigningKeyError {
     }
 }
 
-/// Describe a signing key to be carried by DkimConfig struct
+/// Describe a signing key to be carried by [`DkimConfig`] struct
 #[derive(Debug)]
 pub struct DkimSigningKey(InnerDkimSigningKey);
 
@@ -183,7 +183,7 @@ impl DkimConfig {
         }
     }
 
-    /// Create a DkimConfig
+    /// Create a [`DkimConfig`]
     pub fn new(
         selector: String,
         domain: String,
@@ -283,19 +283,19 @@ fn dkim_canonicalize_headers_relaxed(headers: &str) -> String {
             // End of header.
             [b'\r', b'\n', ..] => {
                 *out += "\r\n";
-                name(&h[2..], out)
+                name(&h[2..], out);
             }
             // Sequential whitespace.
             [b' ' | b'\t', b' ' | b'\t' | b'\r', ..] => value(&h[1..], out),
             // All whitespace becomes spaces.
             [b'\t', ..] => {
                 out.push(' ');
-                value(&h[1..], out)
+                value(&h[1..], out);
             }
             [_, ..] => {
                 let mut chars = h.chars();
                 out.push(chars.next().unwrap());
-                value(chars.as_str(), out)
+                value(chars.as_str(), out);
             }
             [] => {}
         }
@@ -317,7 +317,7 @@ fn dkim_canonicalize_header_tag(
     }
 }
 
-/// Canonicalize signed headers passed as headers_list among mail_headers using canonicalization
+/// 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,
@@ -344,10 +344,9 @@ fn dkim_canonicalize_headers<'a>(
 }
 
 /// Sign with Dkim a message by adding Dkim-Signature header created with configuration expressed by
-/// dkim_config
-
+/// `dkim_config`
 pub fn dkim_sign(message: &mut Message, dkim_config: &DkimConfig) {
-    dkim_sign_fixed_time(message, dkim_config, SystemTime::now())
+    dkim_sign_fixed_time(message, dkim_config, SystemTime::now());
 }
 
 fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timestamp: SystemTime) {
@@ -378,7 +377,7 @@ fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timesta
     }
     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()),
+        dkim_config.headers.iter().map(AsRef::as_ref),
         headers,
         dkim_config.canonicalization.header,
     );
@@ -488,14 +487,14 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
     fn test_headers_simple_canonicalize() {
         let message = test_message();
         dbg!(message.headers.to_string());
-        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Simple), "From: =?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\nTest: test  test very very long with spaces and extra spaces   \twill be\r\n folded to several lines \r\n")
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Simple), "From: =?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\nTest: test  test very very long with spaces and extra spaces   \twill be\r\n folded to several lines \r\n");
     }
 
     #[test]
     fn test_headers_relaxed_canonicalize() {
         let message = test_message();
         dbg!(message.headers.to_string());
-        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Relaxed),"from:=?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\ntest:test test very very long with spaces and extra spaces will be folded to several lines\r\n")
+        assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Relaxed),"from:=?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\ntest:test test very very long with spaces and extra spaces will be folded to several lines\r\n");
     }
 
     #[test]

src/message/header/content_type.rs

@@ -119,7 +119,7 @@ mod serde {
         {
             struct ContentTypeVisitor;
 
-            impl<'de> Visitor<'de> for ContentTypeVisitor {
+            impl Visitor<'_> for ContentTypeVisitor {
                 type Value = ContentType;
 
                 // The error message which states what the Visitor expects to

src/message/header/mailbox.rs

@@ -110,7 +110,7 @@ mailbox_header! {
 
     `Sender` header
 
-    This header contains [`Mailbox`][self::Mailbox] associated with sender.
+    This header contains [`Mailbox`] associated with sender.
 
     ```no_test
     header::Sender("Mr. Sender <sender@example.com>".parse().unwrap())
@@ -124,7 +124,7 @@ mailboxes_header! {
 
     `From` header
 
-    This header contains [`Mailboxes`][self::Mailboxes].
+    This header contains [`Mailboxes`].
 
      */
     (From, "From")
@@ -135,7 +135,7 @@ mailboxes_header! {
 
     `Reply-To` header
 
-    This header contains [`Mailboxes`][self::Mailboxes].
+    This header contains [`Mailboxes`].
 
      */
     (ReplyTo, "Reply-To")
@@ -146,7 +146,7 @@ mailboxes_header! {
 
     `To` header
 
-    This header contains [`Mailboxes`][self::Mailboxes].
+    This header contains [`Mailboxes`].
 
      */
     (To, "To")
@@ -157,7 +157,7 @@ mailboxes_header! {
 
     `Cc` header
 
-    This header contains [`Mailboxes`][self::Mailboxes].
+    This header contains [`Mailboxes`].
 
      */
     (Cc, "Cc")
@@ -168,7 +168,7 @@ mailboxes_header! {
 
     `Bcc` header
 
-    This header contains [`Mailboxes`][self::Mailboxes].
+    This header contains [`Mailboxes`].
 
      */
     (Bcc, "Bcc")

src/message/header/mod.rs

@@ -415,7 +415,7 @@ mod tests {
 
     #[test]
     fn empty_headername() {
-        assert!(HeaderName::new_from_ascii(String::from("")).is_err());
+        assert!(HeaderName::new_from_ascii("".to_owned()).is_err());
     }
 
     #[test]

src/message/mailbox/serde.rs

@@ -36,7 +36,7 @@ impl<'de> Deserialize<'de> for Mailbox {
             {
                 struct FieldVisitor;
 
-                impl<'de> Visitor<'de> for FieldVisitor {
+                impl Visitor<'_> for FieldVisitor {
                     type Value = Field;
 
                     fn expecting(&self, formatter: &mut Formatter<'_>) -> FmtResult {

src/message/mailbox/types.rs

@@ -174,7 +174,7 @@ impl Mailboxes {
         self
     }
 
-    /// Adds a new [`Mailbox`] to the list, in a Vec::push style pattern.
+    /// Adds a new [`Mailbox`] to the list, in a `Vec::push` style pattern.
     ///
     /// # Examples
     ///
@@ -351,7 +351,7 @@ impl FromStr for Mailboxes {
         })?;
 
         for (name, (user, domain)) in parsed_mailboxes {
-            mailboxes.push(Mailbox::new(name, Address::new(user, domain)?))
+            mailboxes.push(Mailbox::new(name, Address::new(user, domain)?));
         }
 
         Ok(Mailboxes(mailboxes))
@@ -531,7 +531,7 @@ mod test {
         assert_eq!(
             format!(
                 "{}",
-                Mailbox::new(Some("".into()), "kayo@example.com".parse().unwrap())
+                Mailbox::new(Some("".to_owned()), "kayo@example.com".parse().unwrap())
             ),
             "kayo@example.com"
         );

src/message/mod.rs

@@ -345,7 +345,7 @@ impl MessageBuilder {
                 let hostname = hostname::get()
                     .map_err(|_| ())
                     .and_then(|s| s.into_string().map_err(|_| ()))
-                    .unwrap_or_else(|_| DEFAULT_MESSAGE_ID_DOMAIN.to_owned());
+                    .unwrap_or_else(|()| DEFAULT_MESSAGE_ID_DOMAIN.to_owned());
                 #[cfg(not(feature = "hostname"))]
                 let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_owned();
 
@@ -457,12 +457,12 @@ impl MessageBuilder {
         self.build(MessageBody::Raw(body.into_vec()))
     }
 
-    /// Create message using mime body ([`MultiPart`][self::MultiPart])
+    /// Create message using mime body ([`MultiPart`])
     pub fn multipart(self, part: MultiPart) -> Result<Message, EmailError> {
         self.mime_1_0().build(MessageBody::Mime(Part::Multi(part)))
     }
 
-    /// Create message using mime body ([`SinglePart`][self::SinglePart])
+    /// Create message using mime body ([`SinglePart`])
     pub fn singlepart(self, part: SinglePart) -> Result<Message, EmailError> {
         self.mime_1_0().build(MessageBody::Mime(Part::Single(part)))
     }
@@ -527,7 +527,7 @@ impl Message {
         match &self.body {
             MessageBody::Mime(p) => p.format_body(&mut out),
             MessageBody::Raw(r) => out.extend_from_slice(r),
-        };
+        }
         out.extend_from_slice(b"\r\n");
         out
     }
@@ -537,7 +537,10 @@ impl Message {
     /// Example:
     /// ```rust
     /// use lettre::{
-    ///     message::dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
+    ///     message::{
+    ///         dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
+    ///         header::ContentType,
+    ///     },
     ///     Message,
     /// };
     ///
@@ -546,6 +549,7 @@ impl Message {
     ///     .reply_to("Bob <bob@example.org>".parse().unwrap())
     ///     .to("Carla <carla@example.net>".parse().unwrap())
     ///     .subject("Hello")
+    ///     .header(ContentType::TEXT_PLAIN)
     ///     .body("Hi there, it's a test email, with utf-8 chars ë!\n\n\n".to_owned())
     ///     .unwrap();
     /// let key = "-----BEGIN RSA PRIVATE KEY-----
@@ -601,7 +605,7 @@ 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);
             }
         }
     }
@@ -765,7 +769,7 @@ mod test {
                 continue;
             }
 
-            assert_eq!(line.0, line.1)
+            assert_eq!(line.0, line.1);
         }
     }
 

src/transport/file/error.rs

@@ -68,7 +68,7 @@ impl fmt::Display for Error {
             Kind::Io => f.write_str("response error")?,
             #[cfg(feature = "file-transport-envelope")]
             Kind::Envelope => f.write_str("internal client error")?,
-        };
+        }
 
         if let Some(e) = &self.inner.source {
             write!(f, ": {e}")?;

src/transport/file/mod.rs

@@ -11,7 +11,7 @@
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{FileTransport, Message, Transport};
+//! use lettre::{message::header::ContentType, FileTransport, Message, Transport};
 //!
 //! // Write to the local temp directory
 //! let sender = FileTransport::new(temp_dir());
@@ -20,10 +20,10 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! # Ok(())
 //! # }
 //!
@@ -44,7 +44,7 @@
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{FileTransport, Message, Transport};
+//! use lettre::{message::header::ContentType, FileTransport, Message, Transport};
 //!
 //! // Write to the local temp directory
 //! let sender = FileTransport::with_envelope(temp_dir());
@@ -53,10 +53,10 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! # Ok(())
 //! # }
 //!
@@ -73,7 +73,9 @@
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
+//! use lettre::{
+//!     message::header::ContentType, AsyncFileTransport, AsyncTransport, Message, Tokio1Executor,
+//! };
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -82,10 +84,10 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let result = sender.send(email).await;
-//! assert!(result.is_ok());
+//! sender.send(email).await?;
 //! # Ok(())
 //! # }
 //! ```
@@ -99,7 +101,10 @@
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
+//! use lettre::{
+//!     message::header::ContentType, AsyncFileTransport, AsyncStd1Executor, AsyncTransport,
+//!     Message,
+//! };
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
@@ -108,10 +113,10 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let result = sender.send(email).await;
-//! assert!(result.is_ok());
+//! sender.send(email).await?;
 //! # Ok(())
 //! # }
 //! ```
@@ -125,6 +130,7 @@
 //! Reply-To: Yuin <yuin@domain.tld>
 //! To: Hei <hei@domain.tld>
 //! Subject: Happy new year
+//! Content-Type: text/plain; charset=utf-8
 //! Date: Tue, 18 Aug 2020 22:50:17 GMT
 //!
 //! Be happy!

src/transport/mod.rs

@@ -56,13 +56,17 @@
 //! #
 //! # #[cfg(all(feature = "builder", feature = "smtp-transport"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
+//! use lettre::{
+//!     message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+//!     SmtpTransport, Transport,
+//! };
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());

src/transport/sendmail/error.rs

@@ -65,7 +65,7 @@ impl fmt::Display for Error {
         match self.inner.kind {
             Kind::Response => f.write_str("response error")?,
             Kind::Client => f.write_str("internal client error")?,
-        };
+        }
 
         if let Some(e) = &self.inner.source {
             write!(f, ": {e}")?;

src/transport/sendmail/mod.rs

@@ -7,18 +7,18 @@
 //! #
 //! # #[cfg(all(feature = "sendmail-transport", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, SendmailTransport, Transport};
+//! use lettre::{message::header::ContentType, Message, SendmailTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let sender = SendmailTransport::new();
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! # Ok(())
 //! # }
 //!
@@ -34,7 +34,8 @@
 //! # #[cfg(all(feature = "tokio1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use lettre::{
-//!     AsyncSendmailTransport, AsyncTransport, Message, SendmailTransport, Tokio1Executor,
+//!     message::header::ContentType, AsyncSendmailTransport, AsyncTransport, Message,
+//!     SendmailTransport, Tokio1Executor,
 //! };
 //!
 //! let email = Message::builder()
@@ -42,11 +43,11 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let sender = AsyncSendmailTransport::<Tokio1Executor>::new();
-//! let result = sender.send(email).await;
-//! assert!(result.is_ok());
+//! sender.send(email).await?;
 //! # Ok(())
 //! # }
 //! ```
@@ -58,18 +59,17 @@
 //! #
 //! # #[cfg(all(feature = "async-std1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, AsyncTransport, AsyncStd1Executor, AsyncSendmailTransport};
+//! use lettre::{Message, AsyncTransport, AsyncStd1Executor,message::header::ContentType, AsyncSendmailTransport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
-//!     .subject("Happy new year")
+//!     .subject("Happy new year").header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let sender = AsyncSendmailTransport::<AsyncStd1Executor>::new();
-//! let result = sender.send(email).await;
-//! assert!(result.is_ok());
+//! sender.send(email).await?;
 //! # Ok(())
 //! # }
 //! ```
@@ -120,7 +120,7 @@ impl SendmailTransport {
     /// 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].
+    /// use [`SendmailTransport::new_with_command`].
     pub fn new() -> SendmailTransport {
         SendmailTransport {
             command: DEFAULT_SENDMAIL.into(),
@@ -157,7 +157,7 @@ where
     /// 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].
+    /// use [`AsyncSendmailTransport::new_with_command`].
     pub fn new() -> Self {
         Self {
             inner: SendmailTransport::new(),

src/transport/smtp/async_transport.rs

@@ -12,6 +12,12 @@ use async_trait::async_trait;
 use super::pool::async_impl::Pool;
 #[cfg(feature = "pool")]
 use super::PoolConfig;
+#[cfg(any(
+    feature = "tokio1-native-tls",
+    feature = "tokio1-rustls-tls",
+    feature = "async-std1-rustls-tls"
+))]
+use super::Tls;
 use super::{
     client::AsyncSmtpConnection, ClientId, Credentials, Error, Mechanism, Response, SmtpInfo,
 };
@@ -24,6 +30,30 @@ use crate::Tokio1Executor;
 use crate::{Envelope, Executor};
 
 /// Asynchronously sends emails using the SMTP protocol
+///
+/// `AsyncSmtpTransport` is the primary way for communicating
+/// with SMTP relay servers to send email messages. It holds the
+/// client connect configuration and creates new connections
+/// as necessary.
+///
+/// # Connection pool
+///
+/// When the `pool` feature is enabled (default), `AsyncSmtpTransport` maintains a
+/// connection pool to manage SMTP connections. The pool:
+///
+/// - Establishes a new connection when sending a message.
+/// - Recycles connections internally after a message is sent.
+/// - Reuses connections for subsequent messages, reducing connection setup overhead.
+///
+/// The connection pool can grow to hold multiple SMTP connections if multiple
+/// emails are sent concurrently, as SMTP does not support multiplexing within a
+/// single connection.
+///
+/// However, **connection reuse is not possible** if the `SyncSmtpTransport` instance
+/// is dropped after every email send operation. You must reuse the instance
+/// of this struct for the connection pool to be of any use.
+///
+/// To customize connection pool settings, use [`AsyncSmtpTransportBuilder::pool_config`].
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
 pub struct AsyncSmtpTransport<E: Executor> {
     #[cfg(feature = "pool")]
@@ -45,7 +75,7 @@ impl AsyncTransport for AsyncSmtpTransport<Tokio1Executor> {
         let result = conn.send(envelope, email).await?;
 
         #[cfg(not(feature = "pool"))]
-        conn.quit().await?;
+        conn.abort().await;
 
         Ok(result)
     }
@@ -161,54 +191,72 @@ where
 
     /// Creates a `AsyncSmtpTransportBuilder` from a connection URL
     ///
-    /// The protocol, credentials, host and port can be provided in a single URL.
-    /// Use the scheme `smtp` for an unencrypted relay (optionally in combination with the
-    /// `tls` parameter to allow/require STARTTLS) or `smtps` for SMTP over TLS.
-    /// The path section of the url can be used to set an alternative name for
-    /// the HELO / EHLO command.
-    /// For example `smtps://username:password@smtp.example.com/client.example.com:465`
-    /// will set the HELO / EHLO name `client.example.com`.
+    /// The protocol, credentials, host, port and EHLO name can be provided
+    /// in a single URL. This may be simpler than having to configure SMTP
+    /// through multiple configuration parameters and then having to pass
+    /// those options to lettre.
     ///
-    /// <table>
-    ///   <thead>
-    ///     <tr>
-    ///       <th>scheme</th>
-    ///       <th>tls parameter</th>
-    ///       <th>example</th>
-    ///       <th>remarks</th>
-    ///     </tr>
-    ///   </thead>
-    ///   <tbody>
-    ///     <tr>
-    ///      <td>smtps</td>
-    ///      <td>-</td>
-    ///      <td>smtps://smtp.example.com</td>
-    ///      <td>SMTP over TLS, recommended method</td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>required</td>
-    ///      <td>smtp://smtp.example.com?tls=required</td>
-    ///      <td>SMTP with STARTTLS required, when SMTP over TLS is not available</td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>opportunistic</td>
-    ///      <td>smtp://smtp.example.com?tls=opportunistic</td>
-    ///      <td>
-    ///         SMTP with optionally STARTTLS when supported by the server.
-    ///         Caution: this method is vulnerable to a man-in-the-middle attack.
-    ///         Not recommended for production use.
-    ///       </td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>-</td>
-    ///      <td>smtp://smtp.example.com</td>
-    ///      <td>Unencrypted SMTP, not recommended for production use.</td>
-    ///     </tr>
-    ///   </tbody>
-    /// </table>
+    /// The URL is created in the following way:
+    /// `scheme://user:pass@hostname:port/ehlo-name?tls=TLS`.
+    ///
+    /// `user` (Username) and `pass` (Password) are optional in case the
+    /// SMTP relay doesn't require authentication. When `port` is not
+    /// configured it is automatically determined based on the `scheme`.
+    /// `ehlo-name` optionally overwrites the hostname sent for the EHLO
+    /// command. `TLS` controls whether STARTTLS is simply enabled
+    /// (`opportunistic` - not enough to prevent man-in-the-middle attacks)
+    /// or `required` (require the server to upgrade the connection to
+    /// STARTTLS, otherwise fail on suspicion of main-in-the-middle attempt).
+    ///
+    /// Use the following table to construct your SMTP url:
+    ///
+    /// | scheme  | `tls` query parameter | example                                            | default port | remarks                                                                                                                               |
+    /// | ------- | --------------------- | -------------------------------------------------- | ------------ | ------------------------------------------------------------------------------------------------------------------------------------- |
+    /// | `smtps` | unset                 | `smtps://user:pass@hostname:port`                  | 465          | SMTP over TLS, recommended method                                                                                                     |
+    /// | `smtp`  | `required`            | `smtp://user:pass@hostname:port?tls=required`      | 587          | SMTP with STARTTLS required, when SMTP over TLS is not available                                                                      |
+    /// | `smtp`  | `opportunistic`       | `smtp://user:pass@hostname:port?tls=opportunistic` | 587          | SMTP with optionally STARTTLS when supported by the server. Not suitable for production use: vulnerable to a man-in-the-middle attack |
+    /// | `smtp`  | unset                 | `smtp://user:pass@hostname:port`                   | 587          | Always unencrypted SMTP. Not suitable for production use: sends all data unencrypted                                                  |
+    ///
+    /// IMPORTANT: some parameters like `user` and `pass` cannot simply
+    /// be concatenated to construct the final URL because special characters
+    /// contained within the parameter may confuse the URL decoder.
+    /// Manually URL encode the parameters before concatenating them or use
+    /// a proper URL encoder, like the following cargo script:
+    ///
+    /// ```rust
+    /// # let _ = r#"
+    /// #!/usr/bin/env cargo
+    ///
+    /// //! ```cargo
+    /// //! [dependencies]
+    /// //! url = "2"
+    /// //! ```
+    /// # "#;
+    ///
+    /// use url::Url;
+    ///
+    /// fn main() {
+    ///     // don't touch this line
+    ///     let mut url = Url::parse("foo://bar").unwrap();
+    ///
+    ///     // configure the scheme (`smtp` or `smtps`) here.
+    ///     url.set_scheme("smtps").unwrap();
+    ///     // configure the username and password.
+    ///     // remove the following two lines if unauthenticated.
+    ///     url.set_username("username").unwrap();
+    ///     url.set_password(Some("password")).unwrap();
+    ///     // configure the hostname
+    ///     url.set_host(Some("smtp.example.com")).unwrap();
+    ///     // configure the port - only necessary if using a non-default port
+    ///     url.set_port(Some(465)).unwrap();
+    ///     // configure the EHLO name
+    ///     url.set_path("ehlo-name");
+    ///
+    ///     println!("{url}");
+    /// }
+    /// ```
+    ///
+    /// The connection URL can then be used in the following way:
     ///
     /// ```rust,no_run
     /// use lettre::{
@@ -232,15 +280,11 @@ where
     /// let mailer: AsyncSmtpTransport<Tokio1Executor> =
     ///     AsyncSmtpTransport::<Tokio1Executor>::from_url(
     ///         "smtps://username:password@smtp.example.com:465",
-    ///     )
-    ///     .unwrap()
+    ///     )?
     ///     .build();
     ///
     /// // Send the email
-    /// match mailer.send(email).await {
-    ///     Ok(_) => println!("Email sent successfully!"),
-    ///     Err(e) => panic!("Could not send email: {e:?}"),
-    /// }
+    /// mailer.send(email).await?;
     /// # Ok(())
     /// # }
     /// ```
@@ -323,7 +367,7 @@ impl AsyncSmtpTransportBuilder {
         self
     }
 
-    /// Set the authentication mechanism to use
+    /// Set the authentication credentials to use
     pub fn credentials(mut self, credentials: Credentials) -> Self {
         self.info.credentials = Some(credentials);
         self
@@ -336,6 +380,17 @@ impl AsyncSmtpTransportBuilder {
     }
 
     /// Set the port to use
+    ///
+    /// # ⚠️⚠️⚠️ You probably don't need to call this method ⚠️⚠️⚠️
+    ///
+    /// lettre usually picks the correct `port` when building
+    /// [`AsyncSmtpTransport`] using [`AsyncSmtpTransport::relay`] or
+    /// [`AsyncSmtpTransport::starttls_relay`].
+    ///
+    /// # Errors
+    ///
+    /// Using the incorrect `port` and [`Self::tls`] combination may
+    /// lead to hard to debug IO errors coming from the TLS library.
     pub fn port(mut self, port: u16) -> Self {
         self.info.port = port;
         self
@@ -348,6 +403,17 @@ impl AsyncSmtpTransportBuilder {
     }
 
     /// Set the TLS settings to use
+    ///
+    /// # ⚠️⚠️⚠️ You probably don't need to call this method ⚠️⚠️⚠️
+    ///
+    /// By default lettre chooses the correct `tls` configuration when
+    /// building [`AsyncSmtpTransport`] using [`AsyncSmtpTransport::relay`] or
+    /// [`AsyncSmtpTransport::starttls_relay`].
+    ///
+    /// # Errors
+    ///
+    /// Using the incorrect [`Tls`] and [`Self::port`] combination may
+    /// lead to hard to debug IO errors coming from the TLS library.
     #[cfg(any(
         feature = "tokio1-native-tls",
         feature = "tokio1-rustls-tls",
@@ -361,7 +427,7 @@ impl AsyncSmtpTransportBuilder {
             feature = "async-std1-rustls-tls"
         )))
     )]
-    pub fn tls(mut self, tls: super::Tls) -> Self {
+    pub fn tls(mut self, tls: Tls) -> Self {
         self.info.tls = tls;
         self
     }

src/transport/smtp/authentication.rs

@@ -98,13 +98,17 @@ impl Mechanism {
                 let decoded_challenge = challenge
                     .ok_or_else(|| error::client("This mechanism does expect a challenge"))?;
 
-                if ["User Name", "Username:", "Username", "User Name\0"]
-                    .contains(&decoded_challenge)
-                {
+                if contains_ignore_ascii_case(
+                    decoded_challenge,
+                    ["User Name", "Username:", "Username", "User Name\0"],
+                ) {
                     return Ok(credentials.authentication_identity.clone());
                 }
 
-                if ["Password", "Password:", "Password\0"].contains(&decoded_challenge) {
+                if contains_ignore_ascii_case(
+                    decoded_challenge,
+                    ["Password", "Password:", "Password\0"],
+                ) {
                     return Ok(credentials.secret.clone());
                 }
 
@@ -121,6 +125,15 @@ impl Mechanism {
     }
 }
 
+fn contains_ignore_ascii_case<'a>(
+    haystack: &str,
+    needles: impl IntoIterator<Item = &'a str>,
+) -> bool {
+    needles
+        .into_iter()
+        .any(|item| item.eq_ignore_ascii_case(haystack))
+}
+
 #[cfg(test)]
 mod test {
     use super::{Credentials, Mechanism};
@@ -155,6 +168,23 @@ mod test {
         assert!(mechanism.response(&credentials, None).is_err());
     }
 
+    #[test]
+    fn test_login_case_insensitive() {
+        let mechanism = Mechanism::Login;
+
+        let credentials = Credentials::new("alice".to_owned(), "wonderland".to_owned());
+
+        assert_eq!(
+            mechanism.response(&credentials, Some("username")).unwrap(),
+            "alice"
+        );
+        assert_eq!(
+            mechanism.response(&credentials, Some("password")).unwrap(),
+            "wonderland"
+        );
+        assert!(mechanism.response(&credentials, None).is_err());
+    }
+
     #[test]
     fn test_xoauth2() {
         let mechanism = Mechanism::Xoauth2;

src/transport/smtp/client/async_connection.rs

@@ -86,8 +86,7 @@ impl AsyncSmtpConnection {
     ///     Some(TlsParameters::new("example.com".to_owned())?),
     ///     None,
     /// )
-    /// .await
-    /// .unwrap();
+    /// .await?;
     /// # Ok(())
     /// # }
     /// ```

src/transport/smtp/client/async_net.rs

@@ -170,7 +170,7 @@ impl AsyncNetworkStream {
                             last_err = Some(io::Error::new(
                                 io::ErrorKind::TimedOut,
                                 "connection timed out",
-                            ))
+                            ));
                         }
                     }
                 } else {
@@ -222,7 +222,7 @@ impl AsyncNetworkStream {
                         last_err = Some(io::Error::new(
                             io::ErrorKind::TimedOut,
                             "connection timed out",
-                        ))
+                        ));
                     }
                 }
             }
@@ -270,9 +270,8 @@ impl AsyncNetworkStream {
             InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
-                let tcp_stream = match tcp_stream {
-                    InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream) => tcp_stream,
-                    _ => unreachable!(),
+                let InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream) = tcp_stream else {
+                    unreachable!()
                 };
 
                 self.inner = Self::upgrade_tokio1_tls(tcp_stream, tls_parameters)
@@ -290,9 +289,8 @@ impl AsyncNetworkStream {
             InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
-                let tcp_stream = match tcp_stream {
-                    InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream) => tcp_stream,
-                    _ => unreachable!(),
+                let InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream) = tcp_stream else {
+                    unreachable!()
                 };
 
                 self.inner = Self::upgrade_asyncstd1_tls(tcp_stream, tls_parameters)

src/transport/smtp/client/net.rs

@@ -119,12 +119,12 @@ impl NetworkStream {
 
                 if let Some(timeout) = timeout {
                     match socket.connect_timeout(&addr.into(), timeout) {
-                        Ok(_) => return Ok(socket.into()),
+                        Ok(()) => return Ok(socket.into()),
                         Err(err) => last_err = Some(err),
                     }
                 } else {
                     match socket.connect(&addr.into()) {
-                        Ok(_) => return Ok(socket.into()),
+                        Ok(()) => return Ok(socket.into()),
                         Err(err) => last_err = Some(err),
                     }
                 }
@@ -160,9 +160,8 @@ impl NetworkStream {
             InnerNetworkStream::Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerNetworkStream::None);
-                let tcp_stream = match tcp_stream {
-                    InnerNetworkStream::Tcp(tcp_stream) => tcp_stream,
-                    _ => unreachable!(),
+                let InnerNetworkStream::Tcp(tcp_stream) = tcp_stream else {
+                    unreachable!()
                 };
 
                 self.inner = Self::upgrade_tls_impl(tcp_stream, tls_parameters)?;
@@ -369,7 +368,7 @@ impl Write for NetworkStream {
 /// If the local address is set, binds the socket to this address.
 /// If local address is not set, then destination address is required to determine the default
 /// local address on some platforms.
-/// See: https://github.com/hyperium/hyper/blob/faf24c6ad8eee1c3d5ccc9a4d4835717b8e2903f/src/client/connect/http.rs#L560
+/// See: <https://github.com/hyperium/hyper/blob/faf24c6ad8eee1c3d5ccc9a4d4835717b8e2903f/src/client/connect/http.rs#L560>
 fn bind_local_address(
     socket: &socket2::Socket,
     dst_addr: &SocketAddr,

src/transport/smtp/client/tls.rs

@@ -59,27 +59,61 @@ pub enum TlsVersion {
     Tlsv13,
 }
 
-/// How to apply TLS to a client connection
+/// Specifies how to establish a TLS connection
+///
+/// TLDR: Use [`Tls::Wrapper`] or [`Tls::Required`] when
+/// connecting to a remote server, [`Tls::None`] when
+/// connecting to a local server.
 #[derive(Clone)]
 #[allow(missing_copy_implementations)]
 pub enum Tls {
-    /// Insecure connection only (for testing purposes)
+    /// Insecure (plaintext) connection only.
+    ///
+    /// This option **always** uses a plaintext connection and should only
+    /// be used for trusted local relays. It is **highly discouraged**
+    /// for remote servers, as it exposes credentials and emails to potential
+    /// interception.
+    ///
+    /// Note: Servers requiring credentials or emails to be sent over TLS
+    /// may reject connections when this option is used.
     None,
-    /// Start with insecure connection and use `STARTTLS` when available
+    /// Begin with a plaintext connection and attempt to use `STARTTLS` if available.
+    ///
+    /// lettre will try to upgrade to a TLS-secured connection but will fall back
+    /// to plaintext if the server does not support TLS. This option is provided for
+    /// compatibility but is **strongly discouraged**, as it exposes connections to
+    /// potential MITM (man-in-the-middle) attacks.
+    ///
+    /// Warning: A malicious intermediary could intercept the `STARTTLS` flag,
+    /// causing lettre to believe the server only supports plaintext connections.
     #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     #[cfg_attr(
         docsrs,
         doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
     )]
     Opportunistic(TlsParameters),
-    /// Start with insecure connection and require `STARTTLS`
+    /// Begin with a plaintext connection and require `STARTTLS` for security.
+    ///
+    /// lettre will upgrade plaintext TCP connections to TLS before transmitting
+    /// any sensitive data. If the server does not support TLS, the connection
+    /// attempt will fail, ensuring no credentials or emails are sent in plaintext.
+    ///
+    /// Unlike [`Tls::Opportunistic`], this option is secure against MITM attacks.
+    /// For optimal security and performance, consider using [`Tls::Wrapper`] instead,
+    /// as it requires fewer roundtrips to establish a secure connection.
     #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     #[cfg_attr(
         docsrs,
         doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
     )]
     Required(TlsParameters),
-    /// Use TLS wrapped connection
+    /// Establish a connection wrapped in TLS from the start.
+    ///
+    /// lettre connects to the server and immediately performs a TLS handshake.
+    /// If the handshake fails, the connection attempt is aborted without
+    /// transmitting any sensitive data.
+    ///
+    /// This is the fastest and most secure option for establishing a connection.
     #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     #[cfg_attr(
         docsrs,
@@ -389,7 +423,7 @@ impl TlsParametersBuilder {
         let mut root_cert_store = RootCertStore::empty();
 
         #[cfg(feature = "rustls-native-certs")]
-        fn load_native_roots(store: &mut RootCertStore) -> Result<(), Error> {
+        fn load_native_roots(store: &mut RootCertStore) {
             let rustls_native_certs::CertificateResult { certs, errors, .. } =
                 rustls_native_certs::load_native_certs();
             let errors_len = errors.len();
@@ -399,7 +433,6 @@ impl TlsParametersBuilder {
             tracing::debug!(
                 "loaded platform certs with {errors_len} failing to load, {added} valid and {ignored} ignored (invalid) certs"
             );
-            Ok(())
         }
 
         #[cfg(feature = "rustls-tls")]
@@ -410,7 +443,7 @@ impl TlsParametersBuilder {
         match self.cert_store {
             CertificateStore::Default => {
                 #[cfg(feature = "rustls-native-certs")]
-                load_native_roots(&mut root_cert_store)?;
+                load_native_roots(&mut root_cert_store);
                 #[cfg(not(feature = "rustls-native-certs"))]
                 load_webpki_roots(&mut root_cert_store);
             }
@@ -628,10 +661,11 @@ impl Identity {
     #[cfg(feature = "rustls-tls")]
     fn from_pem_rustls_tls(
         pem: &[u8],
-        key: &[u8],
+        mut key: &[u8],
     ) -> Result<(Vec<CertificateDer<'static>>, PrivateKeyDer<'static>), Error> {
-        let mut key = key;
-        let key = rustls_pemfile::private_key(&mut key).unwrap().unwrap();
+        let key = rustls_pemfile::private_key(&mut key)
+            .map_err(error::tls)?
+            .ok_or_else(|| error::tls("no private key found"))?;
         Ok((vec![pem.to_owned().into()], key))
     }
 

src/transport/smtp/connection_url.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use url::Url;
 
 #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
@@ -62,7 +64,7 @@ impl TransportBuilder for AsyncSmtpTransportBuilder {
     }
 }
 
-/// Create a new SmtpTransportBuilder or AsyncSmtpTransportBuilder from a connection URL
+/// Create a new `SmtpTransportBuilder` or `AsyncSmtpTransportBuilder` from a connection URL
 pub(crate) fn from_connection_url<B: TransportBuilder>(connection_url: &str) -> Result<B, Error> {
     let connection_url = Url::parse(connection_url).map_err(error::connection)?;
     let tls: Option<String> = connection_url
@@ -84,26 +86,26 @@ pub(crate) fn from_connection_url<B: TransportBuilder>(connection_url: &str) ->
         ("smtp", Some("required")) => {
             builder = builder
                 .port(connection_url.port().unwrap_or(SUBMISSION_PORT))
-                .tls(Tls::Required(TlsParameters::new(host.into())?))
+                .tls(Tls::Required(TlsParameters::new(host.into())?));
         }
         #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
         ("smtp", Some("opportunistic")) => {
             builder = builder
                 .port(connection_url.port().unwrap_or(SUBMISSION_PORT))
-                .tls(Tls::Opportunistic(TlsParameters::new(host.into())?))
+                .tls(Tls::Opportunistic(TlsParameters::new(host.into())?));
         }
         #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
         ("smtps", _) => {
             builder = builder
                 .port(connection_url.port().unwrap_or(SUBMISSIONS_PORT))
-                .tls(Tls::Wrapper(TlsParameters::new(host.into())?))
+                .tls(Tls::Wrapper(TlsParameters::new(host.into())?));
         }
         (scheme, tls) => {
             return Err(error::connection(format!(
                 "Unknown scheme '{scheme}' or tls parameter '{tls:?}', note that a transport with TLS requires one of the TLS features"
             )))
         }
-    };
+    }
 
     // use the path segment of the URL as name in the name in the HELO / EHLO command
     if connection_url.path().len() > 1 {
@@ -115,7 +117,7 @@ pub(crate) fn from_connection_url<B: TransportBuilder>(connection_url: &str) ->
         let percent_decode = |s: &str| {
             percent_encoding::percent_decode_str(s)
                 .decode_utf8()
-                .map(|cow| cow.into_owned())
+                .map(Cow::into_owned)
                 .map_err(error::connection)
         };
         let credentials = Credentials::new(

src/transport/smtp/error.rs

@@ -142,7 +142,7 @@ impl fmt::Display for Error {
             Kind::Permanent(code) => {
                 write!(f, "permanent error ({code})")?;
             }
-        };
+        }
 
         if let Some(e) = &self.inner.source {
             write!(f, ": {e}")?;

src/transport/smtp/extension.rs

@@ -129,9 +129,8 @@ impl Display for ServerInfo {
 impl ServerInfo {
     /// Parses a EHLO response to create a `ServerInfo`
     pub fn from_response(response: &Response) -> Result<ServerInfo, Error> {
-        let name = match response.first_word() {
-            Some(name) => name,
-            None => return Err(error::response("Could not read server name")),
+        let Some(name) = response.first_word() else {
+            return Err(error::response("Could not read server name"));
         };
 
         let mut features: HashSet<Extension> = HashSet::new();
@@ -169,7 +168,7 @@ impl ServerInfo {
                     }
                 }
                 _ => (),
-            };
+            }
         }
 
         Ok(ServerInfo {

src/transport/smtp/mod.rs

@@ -26,43 +26,17 @@
 //!
 //! The relay server can be the local email server, a specific host or a third-party service.
 //!
-//! #### Simple example
+//! #### Simple example with authentication
 //!
-//! This is the most basic example of usage:
-//!
-//! ```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, SmtpTransport, Transport};
-//!
-//! let email = Message::builder()
-//!     .from("NoBody <nobody@domain.tld>".parse()?)
-//!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
-//!     .to("Hei <hei@domain.tld>".parse()?)
-//!     .subject("Happy new year")
-//!     .body(String::from("Be happy!"))?;
-//!
-//! // Create TLS transport on port 465
-//! let sender = SmtpTransport::relay("smtp.example.com")?.build();
-//! // Send the email via remote relay
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
-//! # Ok(())
-//! # }
-//! ```
-//!
-//! #### Authentication
-//!
-//! Example with authentication and connection pool:
+//! A good starting point for sending emails via SMTP relay is to
+//! do the following:
 //!
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
 //! use lettre::{
-//!     transport::smtp::{
-//!         authentication::{Credentials, Mechanism},
-//!         PoolConfig,
-//!     },
+//!     message::header::ContentType,
+//!     transport::smtp::authentication::{Credentials, Mechanism},
 //!     Message, SmtpTransport, Transport,
 //! };
 //!
@@ -71,35 +45,39 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! // Create TLS transport on port 587 with STARTTLS
-//! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
+//! // Create the SMTPS transport
+//! let sender = SmtpTransport::relay("smtp.example.com")?
 //!     // Add credentials for authentication
 //!     .credentials(Credentials::new(
 //!         "username".to_owned(),
 //!         "password".to_owned(),
 //!     ))
-//!     // Configure expected authentication mechanism
+//!     // Optionally configure expected authentication mechanism
 //!     .authentication(vec![Mechanism::Plain])
-//!     // Connection pool settings
-//!     .pool_config(PoolConfig::new().max_size(20))
 //!     .build();
 //!
 //! // Send the email via remote relay
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! # Ok(())
 //! # }
 //! ```
 //!
-//! You can specify custom TLS settings:
+//! #### Shortening configuration
+//!
+//! It can be very repetitive to ask the user for every SMTP connection parameter.
+//! In some cases this can be simplified by using a connection URI instead.
+//!
+//! For more information take a look at [`SmtpTransport::from_url`] or [`AsyncSmtpTransport::from_url`].
 //!
 //! ```rust,no_run
 //! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
 //! use lettre::{
-//!     transport::smtp::client::{Tls, TlsParameters},
+//!     message::header::ContentType,
+//!     transport::smtp::authentication::{Credentials, Mechanism},
 //!     Message, SmtpTransport, Transport,
 //! };
 //!
@@ -108,25 +86,106 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
-//! // Custom TLS configuration
-//! let tls = TlsParameters::builder("smtp.example.com".to_owned())
-//!     .dangerous_accept_invalid_certs(true)
+//! // Create the SMTPS transport
+//! let sender = SmtpTransport::from_url("smtps://username:password@smtp.example.com")?.build();
+//!
+//! // Send the email via remote relay
+//! sender.send(&email)?;
+//! # Ok(())
+//! # }
+//! ```
+//!
+//! #### Advanced configuration with custom TLS settings
+//!
+//! ```rust,no_run
+//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
+//! # fn test() -> Result<(), Box<dyn std::error::Error>> {
+//! use std::fs;
+//!
+//! use lettre::{
+//!     message::header::ContentType,
+//!     transport::smtp::client::{Certificate, Tls, TlsParameters},
+//!     Message, SmtpTransport, Transport,
+//! };
+//!
+//! let email = Message::builder()
+//!     .from("NoBody <nobody@domain.tld>".parse()?)
+//!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
+//!     .to("Hei <hei@domain.tld>".parse()?)
+//!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
+//!     .body(String::from("Be happy!"))?;
+//!
+//! // Custom TLS configuration - Use a self signed certificate
+//! let cert = fs::read("self-signed.crt")?;
+//! let cert = Certificate::from_pem(&cert)?;
+//! let tls = TlsParameters::builder(/* TLS SNI value */ "smtp.example.com".to_owned())
+//!     .add_root_certificate(cert)
 //!     .build()?;
 //!
-//! // Create TLS transport on port 465
+//! // Create the SMTPS transport
 //! let sender = SmtpTransport::relay("smtp.example.com")?
-//!     // Custom TLS configuration
-//!     .tls(Tls::Required(tls))
+//!     .tls(Tls::Wrapper(tls))
 //!     .build();
 //!
 //! // Send the email via remote relay
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! # Ok(())
 //! # }
 //! ```
+//!
+//! #### Connection pooling
+//!
+//! [`SmtpTransport`] and [`AsyncSmtpTransport`] store connections in
+//! a connection pool by default. This avoids connecting and disconnecting
+//! from the relay server for every message the application tries to send. For the connection pool
+//! to work the instance of the transport **must** be reused.
+//! In a webserver context it may go about this:
+//!
+//! ```rust,no_run
+//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
+//! # fn test() {
+//! use lettre::{
+//!     message::header::ContentType,
+//!     transport::smtp::{authentication::Credentials, PoolConfig},
+//!     Message, SmtpTransport, Transport,
+//! };
+//! #
+//! # type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;
+//!
+//! /// The global application state
+//! #[derive(Debug)]
+//! struct AppState {
+//!     smtp: SmtpTransport,
+//!     // ... other global application parameters
+//! }
+//!
+//! impl AppState {
+//!     pub fn new(smtp_url: &str) -> Result<Self> {
+//!         let smtp = SmtpTransport::from_url(smtp_url)?.build();
+//!         Ok(Self { smtp })
+//!     }
+//! }
+//!
+//! fn handle_request(app_state: &AppState) -> Result<String> {
+//!     let email = Message::builder()
+//!         .from("NoBody <nobody@domain.tld>".parse()?)
+//!         .reply_to("Yuin <yuin@domain.tld>".parse()?)
+//!         .to("Hei <hei@domain.tld>".parse()?)
+//!         .subject("Happy new year")
+//!         .header(ContentType::TEXT_PLAIN)
+//!         .body(String::from("Be happy!"))?;
+//!
+//!     // Send the email via remote relay
+//!     app_state.smtp.send(&email)?;
+//!
+//!     Ok("The email has successfully been sent!".to_owned())
+//! }
+//! # }
+//! ```
 
 use std::time::Duration;
 

src/transport/smtp/pool/async_impl.rs

@@ -78,7 +78,7 @@ impl<E: Executor> Pool<E> {
 
                             #[cfg(feature = "tracing")]
                             let mut created = 0;
-                            for _ in count..=(min_idle as usize) {
+                            for _ in count..(min_idle as usize) {
                                 let conn = match pool.client.connection().await {
                                     Ok(conn) => conn,
                                     Err(err) => {
@@ -109,7 +109,7 @@ impl<E: Executor> Pool<E> {
                                 #[cfg(feature = "tracing")]
                                 tracing::debug!("dropped {} idle connections", dropped.len());
 
-                                abort_concurrent(dropped.into_iter().map(|conn| conn.unpark()))
+                                abort_concurrent(dropped.into_iter().map(ParkedConnection::unpark))
                                     .await;
                             }
                         }
@@ -229,7 +229,7 @@ impl<E: Executor> Drop for Pool<E> {
                 handle.shutdown().await;
             }
 
-            abort_concurrent(connections.into_iter().map(|conn| conn.unpark())).await;
+            abort_concurrent(connections.into_iter().map(ParkedConnection::unpark)).await;
         });
     }
 }

src/transport/smtp/pool/sync_impl.rs

@@ -72,7 +72,7 @@ impl Pool {
 
                         #[cfg(feature = "tracing")]
                         let mut created = 0;
-                        for _ in count..=(min_idle as usize) {
+                        for _ in count..(min_idle as usize) {
                             let conn = match pool.client.connection() {
                                 Ok(conn) => conn,
                                 Err(err) => {

src/transport/smtp/transport.rs

@@ -11,7 +11,31 @@ use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, S
 use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{address::Envelope, Transport};
 
-/// Sends emails using the SMTP protocol
+/// Synchronously send emails using the SMTP protocol
+///
+/// `SmtpTransport` is the primary way for communicating
+/// with SMTP relay servers to send email messages. It holds the
+/// client connect configuration and creates new connections
+/// as necessary.
+///
+/// # Connection pool
+///
+/// When the `pool` feature is enabled (default), `SmtpTransport` maintains a
+/// connection pool to manage SMTP connections. The pool:
+///
+/// - Establishes a new connection when sending a message.
+/// - Recycles connections internally after a message is sent.
+/// - Reuses connections for subsequent messages, reducing connection setup overhead.
+///
+/// The connection pool can grow to hold multiple SMTP connections if multiple
+/// emails are sent concurrently, as SMTP does not support multiplexing within a
+/// single connection.
+///
+/// However, **connection reuse is not possible** if the `SmtpTransport` instance
+/// is dropped after every email send operation. You must reuse the instance
+/// of this struct for the connection pool to be of any use.
+///
+/// To customize connection pool settings, use [`SmtpTransportBuilder::pool_config`].
 #[cfg_attr(docsrs, doc(cfg(feature = "smtp-transport")))]
 #[derive(Clone)]
 pub struct SmtpTransport {
@@ -32,7 +56,7 @@ impl Transport for SmtpTransport {
         let result = conn.send(envelope, email)?;
 
         #[cfg(not(feature = "pool"))]
-        conn.quit()?;
+        conn.abort();
 
         Ok(result)
     }
@@ -115,54 +139,72 @@ impl SmtpTransport {
 
     /// Creates a `SmtpTransportBuilder` from a connection URL
     ///
-    /// The protocol, credentials, host and port can be provided in a single URL.
-    /// Use the scheme `smtp` for an unencrypted relay (optionally in combination with the
-    /// `tls` parameter to allow/require STARTTLS) or `smtps` for SMTP over TLS.
-    /// The path section of the url can be used to set an alternative name for
-    /// the HELO / EHLO command.
-    /// For example `smtps://username:password@smtp.example.com/client.example.com:465`
-    /// will set the HELO / EHLO name `client.example.com`.
+    /// The protocol, credentials, host, port and EHLO name can be provided
+    /// in a single URL. This may be simpler than having to configure SMTP
+    /// through multiple configuration parameters and then having to pass
+    /// those options to lettre.
     ///
-    /// <table>
-    ///   <thead>
-    ///     <tr>
-    ///       <th>scheme</th>
-    ///       <th>tls parameter</th>
-    ///       <th>example</th>
-    ///       <th>remarks</th>
-    ///     </tr>
-    ///   </thead>
-    ///   <tbody>
-    ///     <tr>
-    ///      <td>smtps</td>
-    ///      <td>-</td>
-    ///      <td>smtps://smtp.example.com</td>
-    ///      <td>SMTP over TLS, recommended method</td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>required</td>
-    ///      <td>smtp://smtp.example.com?tls=required</td>
-    ///      <td>SMTP with STARTTLS required, when SMTP over TLS is not available</td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>opportunistic</td>
-    ///      <td>smtp://smtp.example.com?tls=opportunistic</td>
-    ///      <td>
-    ///         SMTP with optionally STARTTLS when supported by the server.
-    ///         Caution: this method is vulnerable to a man-in-the-middle attack.
-    ///         Not recommended for production use.
-    ///       </td>
-    ///     </tr>
-    ///     <tr>
-    ///      <td>smtp</td>
-    ///      <td>-</td>
-    ///      <td>smtp://smtp.example.com</td>
-    ///      <td>Unencrypted SMTP, not recommended for production use.</td>
-    ///     </tr>
-    ///   </tbody>
-    /// </table>
+    /// The URL is created in the following way:
+    /// `scheme://user:pass@hostname:port/ehlo-name?tls=TLS`.
+    ///
+    /// `user` (Username) and `pass` (Password) are optional in case the
+    /// SMTP relay doesn't require authentication. When `port` is not
+    /// configured it is automatically determined based on the `scheme`.
+    /// `ehlo-name` optionally overwrites the hostname sent for the EHLO
+    /// command. `TLS` controls whether STARTTLS is simply enabled
+    /// (`opportunistic` - not enough to prevent man-in-the-middle attacks)
+    /// or `required` (require the server to upgrade the connection to
+    /// STARTTLS, otherwise fail on suspicion of main-in-the-middle attempt).
+    ///
+    /// Use the following table to construct your SMTP url:
+    ///
+    /// | scheme  | `tls` query parameter | example                                            | default port | remarks                                                                                                                               |
+    /// | ------- | --------------------- | -------------------------------------------------- | ------------ | ------------------------------------------------------------------------------------------------------------------------------------- |
+    /// | `smtps` | unset                 | `smtps://user:pass@hostname:port`                  | 465          | SMTP over TLS, recommended method                                                                                                     |
+    /// | `smtp`  | `required`            | `smtp://user:pass@hostname:port?tls=required`      | 587          | SMTP with STARTTLS required, when SMTP over TLS is not available                                                                      |
+    /// | `smtp`  | `opportunistic`       | `smtp://user:pass@hostname:port?tls=opportunistic` | 587          | SMTP with optionally STARTTLS when supported by the server. Not suitable for production use: vulnerable to a man-in-the-middle attack |
+    /// | `smtp`  | unset                 | `smtp://user:pass@hostname:port`                   | 587          | Always unencrypted SMTP. Not suitable for production use: sends all data unencrypted                                                  |
+    ///
+    /// IMPORTANT: some parameters like `user` and `pass` cannot simply
+    /// be concatenated to construct the final URL because special characters
+    /// contained within the parameter may confuse the URL decoder.
+    /// Manually URL encode the parameters before concatenating them or use
+    /// a proper URL encoder, like the following cargo script:
+    ///
+    /// ```rust
+    /// # let _ = r#"
+    /// #!/usr/bin/env cargo
+    ///
+    /// //! ```cargo
+    /// //! [dependencies]
+    /// //! url = "2"
+    /// //! ```
+    /// # "#;
+    ///
+    /// use url::Url;
+    ///
+    /// fn main() {
+    ///     // don't touch this line
+    ///     let mut url = Url::parse("foo://bar").unwrap();
+    ///
+    ///     // configure the scheme (`smtp` or `smtps`) here.
+    ///     url.set_scheme("smtps").unwrap();
+    ///     // configure the username and password.
+    ///     // remove the following two lines if unauthenticated.
+    ///     url.set_username("username").unwrap();
+    ///     url.set_password(Some("password")).unwrap();
+    ///     // configure the hostname
+    ///     url.set_host(Some("smtp.example.com")).unwrap();
+    ///     // configure the port - only necessary if using a non-default port
+    ///     url.set_port(Some(465)).unwrap();
+    ///     // configure the EHLO name
+    ///     url.set_path("ehlo-name");
+    ///
+    ///     println!("{url}");
+    /// }
+    /// ```
+    ///
+    /// The connection URL can then be used in the following way:
     ///
     /// ```rust,no_run
     /// use lettre::{
@@ -170,6 +212,7 @@ impl SmtpTransport {
     ///     SmtpTransport, Transport,
     /// };
     ///
+    /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
     /// let email = Message::builder()
     ///     .from("NoBody <nobody@domain.tld>".parse().unwrap())
     ///     .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
@@ -180,15 +223,12 @@ impl SmtpTransport {
     ///     .unwrap();
     ///
     /// // Open a remote connection to example
-    /// let mailer = SmtpTransport::from_url("smtps://username:password@smtp.example.com:465")
-    ///     .unwrap()
-    ///     .build();
+    /// let mailer = SmtpTransport::from_url("smtps://username:password@smtp.example.com")?.build();
     ///
     /// // Send the email
-    /// match mailer.send(&email) {
-    ///     Ok(_) => println!("Email sent successfully!"),
-    ///     Err(e) => panic!("Could not send email: {e:?}"),
-    /// }
+    /// mailer.send(&email)?;
+    /// # Ok(())
+    /// # }
     /// ```
     #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     #[cfg_attr(
@@ -246,7 +286,7 @@ impl SmtpTransportBuilder {
         self
     }
 
-    /// Set the authentication mechanism to use
+    /// Set the authentication credentials to use
     pub fn credentials(mut self, credentials: Credentials) -> Self {
         self.info.credentials = Some(credentials);
         self
@@ -265,12 +305,34 @@ impl SmtpTransportBuilder {
     }
 
     /// Set the port to use
+    ///
+    /// # ⚠️⚠️⚠️ You probably don't need to call this method ⚠️⚠️⚠️
+    ///
+    /// lettre usually picks the correct `port` when building
+    /// [`SmtpTransport`] using [`SmtpTransport::relay`] or
+    /// [`SmtpTransport::starttls_relay`].
+    ///
+    /// # Errors
+    ///
+    /// Using the incorrect `port` and [`Self::tls`] combination may
+    /// lead to hard to debug IO errors coming from the TLS library.
     pub fn port(mut self, port: u16) -> Self {
         self.info.port = port;
         self
     }
 
     /// Set the TLS settings to use
+    ///
+    /// # ⚠️⚠️⚠️ You probably don't need to call this method ⚠️⚠️⚠️
+    ///
+    /// By default lettre chooses the correct `tls` configuration when
+    /// building [`SmtpTransport`] using [`SmtpTransport::relay`] or
+    /// [`SmtpTransport::starttls_relay`].
+    ///
+    /// # Errors
+    ///
+    /// Using the wrong [`Tls`] and [`Self::port`] combination may
+    /// lead to hard to debug IO errors coming from the TLS library.
     #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     #[cfg_attr(
         docsrs,

src/transport/smtp/util.rs

@@ -6,7 +6,7 @@ use std::fmt::{Display, Formatter, Result as FmtResult};
 #[derive(Debug)]
 pub struct XText<'a>(pub &'a str);
 
-impl<'a> Display for XText<'a> {
+impl Display for XText<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         let mut rest = self.0;
         while let Some(idx) = rest.find(|c| c < '!' || c == '+' || c == '=') {
@@ -38,9 +38,7 @@ mod tests {
             ("bjørn", "bjørn"),
             ("Ø+= ❤️‰", "Ø+2B+3D+20❤️‰"),
             ("+", "+2B"),
-        ]
-        .iter()
-        {
+        ] {
             assert_eq!(format!("{}", XText(input)), (*expect).to_owned());
         }
     }

src/transport/stub/mod.rs

@@ -11,7 +11,9 @@
 //! ```rust
 //! # #[cfg(feature = "builder")]
 //! # {
-//! use lettre::{transport::stub::StubTransport, Message, Transport};
+//! use lettre::{
+//!     message::header::ContentType, transport::stub::StubTransport, Message, Transport,
+//! };
 //!
 //! # use std::error::Error;
 //! # fn try_main() -> Result<(), Box<dyn Error>> {
@@ -20,11 +22,11 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let mut sender = StubTransport::new_ok();
-//! let result = sender.send(&email);
-//! assert!(result.is_ok());
+//! sender.send(&email)?;
 //! assert_eq!(
 //!     sender.messages(),
 //!     vec![(