wip

2024-10-22 22:49:25 +02:00
49 changed files with 1013 additions and 1922 deletions
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -57,12 +57,12 @@ jobs:

      - name: Setup cache
        uses: Swatinem/rust-cache@v2
-
+      
      - name: Install cargo hack
        run: cargo install cargo-hack --debug

      - name: Check with cargo hack
-        run: cargo hack check --feature-powerset --depth 3 --at-least-one-of aws-lc-rs,ring --at-least-one-of rustls-native-certs,webpki-roots
+        run: cargo hack check --feature-powerset --depth 3

  test:
    name: test / ${{ matrix.name }}
@@ -75,8 +75,8 @@ jobs:
            rust: stable
          - name: beta
            rust: beta
-          - name: '1.74'
-            rust: '1.74'
+          - name: '1.70'
+            rust: '1.70'

    steps:
      - name: Checkout
@@ -90,7 +90,7 @@ jobs:
      - name: Setup cache
        uses: Swatinem/rust-cache@v2

-      - name: Install postfix
+      - name: Install postfix  
        run: |
          DEBIAN_FRONTEND=noninteractive sudo apt-get update
          DEBIAN_FRONTEND=noninteractive sudo apt-get -y install postfix
@@ -119,10 +119,10 @@ jobs:
        run: cargo test

      - name: Test with all features (-native-tls)
-        run: cargo test --no-default-features --features async-std1,async-std1-rustls,aws-lc-rs,rustls-native-certs,boring-tls,builder,dkim,file-transport,file-transport-envelope,hostname,mime03,pool,rustls-native-certs,rustls,sendmail-transport,smtp-transport,tokio1,tokio1-boring-tls,tokio1-rustls,tracing
-
+        run: cargo test --no-default-features --features async-std1,async-std1-rustls-tls,boring-tls,builder,dkim,file-transport,file-transport-envelope,hostname,mime03,pool,rustls-native-certs,rustls-tls,sendmail-transport,smtp-transport,tokio1,tokio1-boring-tls,tokio1-rustls-tls,tracing
+  
      - name: Test with all features (-boring-tls)
-        run: cargo test --no-default-features --features async-std1,async-std1-rustls,aws-lc-rs,rustls-native-certs,builder,dkim,file-transport,file-transport-envelope,hostname,mime03,native-tls,pool,rustls-native-certs,rustls,sendmail-transport,smtp-transport,tokio1,tokio1-native-tls,tokio1-rustls,tracing
+        run: cargo test --no-default-features --features async-std1,async-std1-rustls-tls,builder,dkim,file-transport,file-transport-envelope,hostname,mime03,native-tls,pool,rustls-native-certs,rustls-tls,sendmail-transport,smtp-transport,tokio1,tokio1-native-tls,tokio1-rustls-tls,tracing

 #  coverage:
 #    name: Coverage
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,159 +1,3 @@
-<a name="v0.11.15"></a>
-### v0.11.15 (2025-03-10)
-
-#### Upgrade notes
-
-* MSRV is now 1.74 ([#1060])
-
-#### Features
-
-* Add controlled shutdown methods ([#1045], [#1068])
-
-#### Misc
-
-* Deny `unreachable_pub` lint ([#1058])
-* Bump minimum supported `rustls` ([#1063])
-* Bump minimum supported `serde` ([#1064])
-* Upgrade semver compatible dependencies ([#1067])
-* Upgrade `email-encoding` to v0.4 ([#1069])
-
-[#1045]: https://github.com/lettre/lettre/pull/1045
-[#1058]: https://github.com/lettre/lettre/pull/1058
-[#1060]: https://github.com/lettre/lettre/pull/1060
-[#1063]: https://github.com/lettre/lettre/pull/1063
-[#1064]: https://github.com/lettre/lettre/pull/1064
-[#1067]: https://github.com/lettre/lettre/pull/1067
-[#1068]: https://github.com/lettre/lettre/pull/1068
-[#1069]: https://github.com/lettre/lettre/pull/1069
-
-<a name="v0.11.14"></a>
-### v0.11.14 (2025-02-23)
-
-This release deprecates the `rustls-tls`, `tokio1-rustls-tls` and `async-std1-rustls-tls`
-features, which will be removed in lettre v0.12.
-
-rustls users should start migrating to the `rustls`, `tokio1-rustls` and
-`async-std1-rustls` features. Unlike the deprecated _*rustls-tls_ features,
-which automatically enabled the `ring` and `webpki-roots` backends, the new
-features do not. To complete the migration, users must either enable the
-`aws-lc-rs` or the `ring` feature. Additionally, those who rely on `webpki-roots`
-for TLS certificate verification must now explicitly enable its feature.
-Users of `rustls-native-certs` do not need to enable `webpki-roots`.
-
-Find out more about the new features via the [lettre rustls docs].
-
-#### Features
-
-* Make it possible to use different `rustls` crypto providers and TLS verifiers ([#1054])
-
-#### Bug fixes
-
-* Use the same `rustls` crypto provider everywhere ([#1055])
-
-#### Misc
-
-* Deprecate `AsyncNetworkStream` being public ([#1059])
-* Upgrade `nom` to v8 ([#1048])
-* Drop `rustls-pemfile` in favor of `rustls-pki-types` APIs ([#1050])
-* Ban direct use of `std::time::SystemTime::now` via clippy ([#1043])
-* Drop direct dependency on `rustls-pki-types` ([#1051])
-* Remove artifact from `web-time` refactor ([#1049])
-* Fix warnings with `rustls-native-certs` when `tracing` is disabled ([#1053])
-* Bump license year ([#1057])
-* Cleanup `Cargo.toml` style ([#1047])
-
-[lettre rustls docs]: https://docs.rs/lettre/0.11.14/lettre/index.html#smtp-over-tls-via-the-rustls-crate
-[#1043]: https://github.com/lettre/lettre/pull/1043
-[#1047]: https://github.com/lettre/lettre/pull/1047
-[#1048]: https://github.com/lettre/lettre/pull/1048
-[#1049]: https://github.com/lettre/lettre/pull/1049
-[#1050]: https://github.com/lettre/lettre/pull/1050
-[#1051]: https://github.com/lettre/lettre/pull/1051
-[#1053]: https://github.com/lettre/lettre/pull/1053
-[#1054]: https://github.com/lettre/lettre/pull/1054
-[#1055]: https://github.com/lettre/lettre/pull/1055
-[#1057]: https://github.com/lettre/lettre/pull/1057
-[#1059]: https://github.com/lettre/lettre/pull/1059
-
-<a name="v0.11.13"></a>
-### v0.11.13 (2025-02-17)
-
-#### Features
-
-* Add WASM support ([#1037], [#1042])
-* Add method to get the TLS verify result with BoringSSL ([#1039])
-
-#### Bug fixes
-
-* Synchronous pool shutdowns being arbitrarily delayed ([#1041])
-
-[#1037]: https://github.com/lettre/lettre/pull/1037
-[#1039]: https://github.com/lettre/lettre/pull/1039
-[#1041]: https://github.com/lettre/lettre/pull/1041
-[#1042]: https://github.com/lettre/lettre/pull/1042
-
-<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)

@@ -664,6 +508,6 @@ 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
--- a/Cargo.lock
+++ b/Cargo.lock
--- a/Cargo.toml
+++ b/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 = "0.11.15"
+version = "0.11.9"
 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 = "1.74"
+rust-version = "1.70"

 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -19,12 +19,9 @@ is-it-maintained-open-issues = { repository = "lettre/lettre" }
 maintenance = { status = "actively-developed" }

 [dependencies]
-email_address = { version = "0.2.1", default-features = false }
 chumsky = "0.9"
 idna = "1"
-
-## tracing support
-tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true }
+tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # feature

 # builder
 httpdate = { version = "1", optional = true }
@@ -32,24 +29,26 @@ mime = { version = "0.3.4", optional = true }
 fastrand = { version = "2.0", optional = true }
 quoted_printable = { version = "0.5", optional = true }
 base64 = { version = "0.22", optional = true }
-email-encoding = { version = "0.4", optional = true }
+email-encoding = { version = "0.3", optional = true }

 # file transport
 uuid = { version = "1", features = ["v4"], optional = true }
-serde = { version = "1.0.110", features = ["derive"], optional = true }
+serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }

 # smtp-transport
-nom = { version = "8", optional = true }
+nom = { version = "7", optional = true }
 hostname = { version = "0.4", optional = true } # feature
 socket2 = { version = "0.5.1", optional = true }
 url = { version = "2.4", optional = true }
 percent-encoding = { version = "2.3", optional = true }

 ## tls
-native-tls = { version = "0.2.9", optional = true } # feature
-rustls = { version = "0.23.18", default-features = false, features = ["logging", "std", "tls12"], optional = true }
+native-tls = { version = "0.2.5", 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 }
+rustls-pki-types = { version = "1.7", optional = true }
 webpki-roots = { version = "0.26", optional = true }
 boring = { version = "4", optional = true }

@@ -60,21 +59,21 @@ async-trait = { version = "0.1", optional = true }

 ## async-std
 async-std = { version = "1.8", optional = true }
-futures-rustls = { version = "0.26", default-features = false, features = ["logging", "tls12"], optional = true }
+futures-rustls = { version = "0.26", default-features = false, features = ["logging", "tls12", "ring"], optional = true }

 ## tokio
 tokio1_crate = { package = "tokio", version = "1", optional = true }
 tokio1_native_tls_crate = { package = "tokio-native-tls", version = "0.3", optional = true }
-tokio1_rustls = { package = "tokio-rustls", version = "0.26", default-features = false, features = ["logging", "tls12"], optional = true }
+tokio1_rustls = { package = "tokio-rustls", version = "0.26", default-features = false, features = ["logging", "tls12", "ring"], optional = true }
 tokio1_boring = { package = "tokio-boring", version = "4", optional = true }

 ## dkim
-sha2 = { version = "0.10", features = ["oid"], optional = true }
+sha2 = { version = "0.10", optional = true, features = ["oid"] }
 rsa = { version = "0.9", optional = true }
 ed25519-dalek = { version = "2", optional = true }

-## web-time for wasm support
-web-time = { version = "1.1.0", optional = true }
+# email formats
+email_address = { version = "0.2.1", default-features = false }

 [dev-dependencies]
 pretty_assertions = "1"
@@ -109,33 +108,20 @@ smtp-transport = ["dep:base64", "dep:nom", "dep:socket2", "dep:url", "dep:percen

 pool = ["dep:futures-util"]

-rustls = ["dep:rustls"]
-aws-lc-rs = ["rustls?/aws-lc-rs"]
-fips = ["aws-lc-rs", "rustls?/fips"]
-ring = ["rustls?/ring"]
-webpki-roots = ["dep:webpki-roots"]
-# deprecated
-rustls-tls = ["webpki-roots", "rustls", "ring"]
+rustls-tls = ["dep:webpki-roots", "dep:rustls", "dep:rustls-pemfile", "dep:rustls-pki-types"]

 boring-tls = ["dep:boring"]

 # async
 async-std1 = ["dep:async-std", "dep:async-trait", "dep:futures-io", "dep:futures-util"]
-async-std1-rustls = ["async-std1", "rustls", "dep:futures-rustls"]
-# deprecated
-async-std1-rustls-tls = ["async-std1-rustls", "rustls-tls"]
+async-std1-rustls-tls = ["async-std1", "rustls-tls", "dep:futures-rustls"]
 tokio1 = ["dep:tokio1_crate", "dep:async-trait", "dep:futures-io", "dep:futures-util"]
 tokio1-native-tls = ["tokio1", "native-tls", "dep:tokio1_native_tls_crate"]
-tokio1-rustls = ["tokio1", "rustls", "dep:tokio1_rustls"]
-# deprecated
-tokio1-rustls-tls = ["tokio1-rustls", "rustls-tls"]
+tokio1-rustls-tls = ["tokio1", "rustls-tls", "dep:tokio1_rustls"]
 tokio1-boring-tls = ["tokio1", "boring-tls", "dep:tokio1_boring"]

 dkim = ["dep:base64", "dep:sha2", "dep:rsa", "dep:ed25519-dalek"]

-# wasm support
-web = ["dep:web-time"]
-
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(lettre_ignore_tls_mismatch)'] }

--- a/2
+++ b/2
@@ -1,5 +1,5 @@
 Copyright (c) 2014-2024 Alexis Mousset <contact@amousset.me>
-Copyright (c) 2019-2025 Paolo Barbolini <paolo@paolo565.org>
+Copyright (c) 2019-2024 Paolo Barbolini <paolo@paolo565.org>
 Copyright (c) 2018 K. <kayo@illumium.org>

 Permission is hereby granted, free of charge, to any
--- a/README.md
+++ b/README.md
@@ -28,8 +28,8 @@
 </div>

 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.11.15">
-    <img src="https://deps.rs/crate/lettre/0.11.15/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.11.9">
+    <img src="https://deps.rs/crate/lettre/0.11.9/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 1.74, but this could change at any time either from
+the minimum supported Rust version is 1.70, 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 1.74 or newer.
+This library requires Rust 1.70 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`.
--- a/benches/transport_smtp.rs
+++ b/benches/transport_smtp.rs
@@ -1,5 +1,5 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use lettre::{message::header::ContentType, Message, SmtpTransport, Transport};
+use lettre::{Message, SmtpTransport, Transport};

 fn bench_simple_send(c: &mut Criterion) {
    let sender = SmtpTransport::builder_dangerous("127.0.0.1")
@@ -13,7 +13,6 @@ 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));
@@ -33,7 +32,6 @@ 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));
--- a/clippy.toml
+++ b/clippy.toml
@@ -1,3 +0,0 @@
-disallowed-methods = [
-    { "path" = "std::time::SystemTime::now", reason = "does not work on WASM environments", replacement = "crate::time::now" }
-]
--- a/src/address/envelope.rs
+++ b/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>;
@@ -53,7 +53,7 @@ mod serde_forward_path {
            }
        }
    }
-    pub(super) fn deserialize<'de, D>(deserializer: D) -> Result<Vec<Address>, D::Error>
+    pub fn deserialize<'de, D>(deserializer: D) -> Result<Vec<Address>, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
--- a/src/address/serde.rs
+++ b/src/address/serde.rs
@@ -36,7 +36,7 @@ impl<'de> Deserialize<'de> for Address {
            {
                struct FieldVisitor;

-                impl Visitor<'_> for FieldVisitor {
+                impl<'de> Visitor<'de> for FieldVisitor {
                    type Value = Field;

                    fn expecting(&self, formatter: &mut Formatter<'_>) -> FmtResult {
--- a/src/executor.rs
+++ b/src/executor.rs
@@ -45,7 +45,6 @@ use crate::transport::smtp::Error;
 #[async_trait]
 pub trait Executor: Debug + Send + Sync + 'static + private::Sealed {
    #[cfg(feature = "smtp-transport")]
-    #[allow(private_bounds)]
    type Handle: SpawnHandle;
    #[cfg(feature = "smtp-transport")]
    type Sleep: Future<Output = ()> + Send + 'static;
@@ -83,8 +82,8 @@ pub trait Executor: Debug + Send + Sync + 'static + private::Sealed {
 #[doc(hidden)]
 #[cfg(feature = "smtp-transport")]
 #[async_trait]
-pub(crate) trait SpawnHandle: Debug + Send + Sync + 'static + private::Sealed {
-    async fn shutdown(&self);
+pub trait SpawnHandle: Debug + Send + Sync + 'static + private::Sealed {
+    async fn shutdown(self);
 }

 /// Async [`Executor`] using `tokio` `1.x`
@@ -134,7 +133,7 @@ impl Executor for Tokio1Executor {
    ) -> Result<AsyncSmtpConnection, Error> {
        #[allow(clippy::match_single_binding)]
        let tls_parameters = match tls {
-            #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls"))]
+            #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
            Tls::Wrapper(tls_parameters) => Some(tls_parameters.clone()),
            _ => None,
        };
@@ -148,7 +147,7 @@ impl Executor for Tokio1Executor {
        )
        .await?;

-        #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls"))]
+        #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
        match tls {
            Tls::Opportunistic(tls_parameters) => {
                if conn.can_starttls() {
@@ -178,7 +177,7 @@ impl Executor for Tokio1Executor {
 #[cfg(all(feature = "smtp-transport", feature = "tokio1"))]
 #[async_trait]
 impl SpawnHandle for tokio1_crate::task::JoinHandle<()> {
-    async fn shutdown(&self) {
+    async fn shutdown(self) {
        self.abort();
    }
 }
@@ -202,7 +201,7 @@ pub struct AsyncStd1Executor;
 #[cfg(feature = "async-std1")]
 impl Executor for AsyncStd1Executor {
    #[cfg(feature = "smtp-transport")]
-    type Handle = futures_util::future::AbortHandle;
+    type Handle = async_std::task::JoinHandle<()>;
    #[cfg(feature = "smtp-transport")]
    type Sleep = BoxFuture<'static, ()>;

@@ -212,9 +211,7 @@ impl Executor for AsyncStd1Executor {
        F: Future<Output = ()> + Send + 'static,
        F::Output: Send + 'static,
    {
-        let (handle, registration) = futures_util::future::AbortHandle::new_pair();
-        async_std::task::spawn(futures_util::future::Abortable::new(fut, registration));
-        handle
+        async_std::task::spawn(fut)
    }

    #[cfg(feature = "smtp-transport")]
@@ -233,7 +230,7 @@ impl Executor for AsyncStd1Executor {
    ) -> Result<AsyncSmtpConnection, Error> {
        #[allow(clippy::match_single_binding)]
        let tls_parameters = match tls {
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            Tls::Wrapper(tls_parameters) => Some(tls_parameters.clone()),
            _ => None,
        };
@@ -246,7 +243,7 @@ impl Executor for AsyncStd1Executor {
        )
        .await?;

-        #[cfg(feature = "async-std1-rustls")]
+        #[cfg(feature = "async-std1-rustls-tls")]
        match tls {
            Tls::Opportunistic(tls_parameters) => {
                if conn.can_starttls() {
@@ -275,9 +272,9 @@ impl Executor for AsyncStd1Executor {

 #[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
 #[async_trait]
-impl SpawnHandle for futures_util::future::AbortHandle {
-    async fn shutdown(&self) {
-        self.abort();
+impl SpawnHandle for async_std::task::JoinHandle<()> {
+    async fn shutdown(self) {
+        self.cancel().await;
    }
 }

@@ -294,5 +291,5 @@ mod private {
    impl Sealed for tokio1_crate::task::JoinHandle<()> {}

    #[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
-    impl Sealed for futures_util::future::AbortHandle {}
+    impl Sealed for async_std::task::JoinHandle<()> {}
 }
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,7 +6,7 @@
 //! * Secure defaults
 //! * Async support
 //!
-//! Lettre requires Rust 1.74 or newer.
+//! Lettre requires Rust 1.70 or newer.
 //!
 //! ## Features
 //!
@@ -34,25 +34,13 @@
 //!
 //! _Secure SMTP connections using TLS from the `native-tls` crate_
 //!
-//! Uses schannel on Windows, Security-Framework on macOS, and OpenSSL
-//! on all other platforms.
+//! Uses schannel on Windows, Security-Framework on macOS, and OpenSSL on Linux.
 //!
 //! * **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_
@@ -64,46 +52,13 @@
 //!
 //! #### SMTP over TLS via the rustls crate
 //!
-//! _Secure SMTP connections using TLS from the `rustls` crate_
+//! _Secure SMTP connections using TLS from the `rustls-tls` crate_
 //!
-//! * **rustls**: TLS support for the synchronous version of the API
-//! * **tokio1-rustls**: TLS support for the `tokio1` async version of the API
-//! * **async-std1-rustls**: TLS support for the `async-std1` async version of the API
+//! Rustls uses [ring] as the cryptography implementation. As a result, [not all Rust's targets are supported][ring-support].
 //!
-//! ##### rustls crypto backends
-//!
-//! _The crypto implementation to use with rustls_
-//!
-//! When the `rustls` feature is enabled, one of the following crypto backends MUST also
-//! be enabled.
-//!
-//! * **aws-lc-rs**: use [AWS-LC] (via [`aws-lc-rs`]) as the `rustls` crypto backend
-//! * **ring**: use [`ring`] as the `rustls` crypto backend
-//!
-//! When enabling `aws-lc-rs`, the `fips` feature can also be enabled to have
-//! rustls use the FIPS certified module of AWS-LC.
-//!
-//! `aws-lc-rs` may require cmake on some platforms to compile.
-//! `fips` always requires cmake and the Go compiler to compile.
-//!
-//! ##### rustls certificate verification backend
-//!
-//! _The TLS certificate verification backend to use with rustls_
-//!
-//! When the `rustls` feature is enabled, one of the following verification backends
-//! MUST also be enabled.
-//!
-//! * **rustls-native-certs**: verify TLS certificates using the platform's native certificate store (see [`rustls-native-certs`])
-//! * **webpki-roots**: verify TLS certificates against Mozilla's root certificates (see [`webpki-roots`])
-//!
-//! For the `rustls-native-certs` backend to work correctly, the following packages
-//! will need to be installed in order for the build stage and the compiled program
-//! to run properly.
-//!
-//! | Distro       | Build-time packages        | Runtime packages             |
-//! | ------------ | -------------------------- | ---------------------------- |
-//! | Debian       | none                       | `ca-certificates`            |
-//! | Alpine Linux | none                       | `ca-certificates`            |
+//! * **rustls-tls**: TLS support for the synchronous version of the API
+//! * **tokio1-rustls-tls**: TLS support for the `tokio1` async version of the API
+//! * **async-std1-rustls-tls**: TLS support for the `async-std1` async version of the API
 //!
 //! ### Sendmail transport
 //!
@@ -140,7 +95,6 @@
 //! * **tracing**: Logging using the `tracing` crate
 //! * **mime03**: Allow creating a [`ContentType`] from an existing [mime 0.3] `Mime` struct
 //! * **dkim**: Add support for signing email with DKIM
-//! * **web**: WebAssembly support using the `web-time` crate for time operations
 //!
 //! [`SMTP`]: crate::transport::smtp
 //! [`sendmail`]: crate::transport::sendmail
@@ -148,22 +102,18 @@
 //! [`ContentType`]: crate::message::header::ContentType
 //! [tokio]: https://docs.rs/tokio/1
 //! [async-std]: https://docs.rs/async-std/1
-//! [AWS-LC]: https://github.com/aws/aws-lc
-//! [`aws-lc-rs`]: https://crates.io/crates/aws-lc-rs
-//! [`ring`]: https://crates.io/crates/ring
-//! [`rustls-native-certs`]: https://crates.io/crates/rustls-native-certs
-//! [`webpki-roots`]: https://crates.io/crates/webpki-roots
+//! [ring]: https://github.com/briansmith/ring#ring
+//! [ring-support]: https://github.com/briansmith/ring#online-automated-testing
 //! [Tokio 1.x]: https://docs.rs/tokio/1
 //! [async-std 1.x]: https://docs.rs/async-std/1
 //! [mime 0.3]: https://docs.rs/mime/0.3
 //! [DKIM]: https://datatracker.ietf.org/doc/html/rfc6376

-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.11.15")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.11.9")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
 #![deny(
-    unreachable_pub,
    missing_copy_implementations,
    trivial_casts,
    trivial_numeric_casts,
@@ -187,35 +137,12 @@
    clippy::wildcard_imports,
    clippy::str_to_string,
    clippy::empty_structs_with_brackets,
-    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,
+    clippy::zero_sized_map_values
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]

 #[cfg(not(lettre_ignore_tls_mismatch))]
 mod compiletime_checks {
-    #[cfg(all(feature = "rustls", not(feature = "aws-lc-rs"), not(feature = "ring")))]
-    compile_error!(
-        "feature `rustls` also requires either the `aws-lc-rs` or the `ring` feature to
-    be enabled"
-    );
-
-    #[cfg(all(
-        feature = "rustls",
-        not(feature = "rustls-native-certs"),
-        not(feature = "webpki-roots")
-    ))]
-    compile_error!(
-        "feature `rustls` also requires either the `rustls-native-certs` or the `webpki-roots` feature to
-    be enabled"
-    );
-
    #[cfg(all(feature = "native-tls", feature = "boring-tls"))]
    compile_error!("feature \"native-tls\" and feature \"boring-tls\" cannot be enabled at the same time, otherwise
    the executable will fail to link.");
@@ -226,12 +153,16 @@ mod compiletime_checks {
        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 were trying to opt into `rustls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
+    If you were trying to opt into `rustls-tls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");

-    #[cfg(all(feature = "tokio1", feature = "rustls", not(feature = "tokio1-rustls")))]
-    compile_error!("Lettre is being built with the `tokio1` and the `rustls` features, but the `tokio1-rustls` feature hasn't been turned on.
-    If you'd like to use `native-tls` make sure that the `rustls` feature hasn't been enabled by mistake.
+    #[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` feature hasn't been enabled by mistake.
    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");

    #[cfg(all(
@@ -240,22 +171,22 @@ mod compiletime_checks {
        not(feature = "tokio1-boring-tls")
    ))]
    compile_error!("Lettre is being built with the `tokio1` and the `boring-tls` features, but the `tokio1-boring-tls` feature hasn't been turned on.
-    If you'd like to use `boring-tls` make sure that the `rustls` feature hasn't been enabled by mistake.
+    If you'd like to use `boring-tls` make sure that the `rustls-tls` feature hasn't been enabled by mistake.
    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");

-    #[cfg(all(feature = "async-std1", feature = "native-tls"))]
+    #[cfg(all(feature = "async-std1", feature = "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 were trying to opt into `rustls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
+If you were trying to opt into `rustls-tls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
 Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");

    #[cfg(all(
        feature = "async-std1",
-        feature = "rustls",
-        not(feature = "async-std1-rustls")
+        feature = "rustls-tls",
+        not(feature = "async-std1-rustls-tls")
    ))]
-    compile_error!("Lettre is being built with the `async-std1` and the `rustls` features, but the `async-std1-rustls` feature hasn't been turned on.
-If you'd like to use `native-tls` make sure that the `rustls` hasn't been enabled by mistake.
+    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.
 Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 }

@@ -268,9 +199,6 @@ mod executor;
 #[cfg(feature = "builder")]
 #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
 pub mod message;
-#[cfg(feature = "rustls")]
-mod rustls_crypto;
-mod time;
 pub mod transport;

 use std::error::Error as StdError;
--- a/src/message/body.rs
+++ b/src/message/body.rs
@@ -1,11 +1,11 @@
-use std::{mem, ops::Deref};
+use std::{mem, ops::Deref, sync::Arc};

 use crate::message::header::ContentTransferEncoding;

 /// A [`Message`][super::Message] or [`SinglePart`][super::SinglePart] body that has already been encoded.
 #[derive(Debug, Clone)]
 pub struct Body {
-    buf: Vec<u8>,
+    buf: Arc<[u8]>,
    encoding: ContentTransferEncoding,
 }

@@ -39,7 +39,7 @@ impl Body {

        let encoding = buf.encoding(false);
        buf.encode_crlf();
-        Self::new_impl(buf.into(), encoding)
+        Self::new_impl(Vec::from(buf).into(), encoding)
    }

    /// Encode the supplied `buf`, using the provided `encoding`.
@@ -77,7 +77,7 @@ impl Body {
        }

        buf.encode_crlf();
-        Ok(Self::new_impl(buf.into(), encoding))
+        Ok(Self::new_impl(Vec::from(buf).into(), encoding))
    }

    /// Builds a new `Body` using a pre-encoded buffer.
@@ -87,11 +87,14 @@ impl Body {
    /// `buf` shouldn't contain non-ascii characters, lines longer than 1000 characters or nul bytes.
    #[inline]
    pub fn dangerous_pre_encoded(buf: Vec<u8>, encoding: ContentTransferEncoding) -> Self {
-        Self { buf, encoding }
+        Self {
+            buf: buf.into(),
+            encoding,
+        }
    }

    /// Encodes the supplied `buf` using the provided `encoding`
-    fn new_impl(buf: Vec<u8>, encoding: ContentTransferEncoding) -> Self {
+    fn new_impl(buf: Arc<[u8]>, encoding: ContentTransferEncoding) -> Self {
        match encoding {
            ContentTransferEncoding::SevenBit
            | ContentTransferEncoding::EightBit
@@ -133,7 +136,16 @@ impl Body {

    /// Consumes `Body` and returns the inner `Vec<u8>`
    #[inline]
+    #[deprecated(
+        note = "The inner memory is not stored into `Vec<u8>` anymore. Consider using `into_inner`"
+    )]
    pub fn into_vec(self) -> Vec<u8> {
+        self.buf.to_vec()
+    }
+
+    /// Consumes `Body` and returns the inner `Arc<[u8]>`
+    #[inline]
+    pub fn into_inner(self) -> Arc<[u8]> {
        self.buf
    }
 }
--- a/src/message/dkim.rs
+++ b/src/message/dkim.rs
@@ -69,7 +69,7 @@ impl Display for DkimSigningAlgorithm {
    }
 }

-/// Describe [`DkimSigningKey`] key error
+/// Describe DkimSigning 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,
@@ -344,9 +344,10 @@ 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, crate::time::now());
+    dkim_sign_fixed_time(message, dkim_config, SystemTime::now())
 }

 fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timestamp: SystemTime) {
@@ -377,7 +378,7 @@ 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(AsRef::as_ref),
+        dkim_config.headers.iter().map(|h| h.as_ref()),
        headers,
        dkim_config.canonicalization.header,
    );
@@ -487,14 +488,14 @@ 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]
--- a/src/message/header/content_type.rs
+++ b/src/message/header/content_type.rs
@@ -119,7 +119,7 @@ mod serde {
        {
            struct ContentTypeVisitor;

-            impl Visitor<'_> for ContentTypeVisitor {
+            impl<'de> Visitor<'de> for ContentTypeVisitor {
                type Value = ContentType;

                // The error message which states what the Visitor expects to
--- a/src/message/header/date.rs
+++ b/src/message/header/date.rs
@@ -21,7 +21,7 @@ impl Date {
    ///
    /// Shortcut for `Date::new(SystemTime::now())`
    pub fn now() -> Self {
-        Self::new(crate::time::now())
+        Self::new(SystemTime::now())
    }
 }

--- a/src/message/header/mailbox.rs
+++ b/src/message/header/mailbox.rs
@@ -110,7 +110,7 @@ mailbox_header! {

    `Sender` header

-    This header contains [`Mailbox`] associated with sender.
+    This header contains [`Mailbox`][self::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`].
+    This header contains [`Mailboxes`][self::Mailboxes].

     */
    (From, "From")
@@ -135,7 +135,7 @@ mailboxes_header! {

    `Reply-To` header

-    This header contains [`Mailboxes`].
+    This header contains [`Mailboxes`][self::Mailboxes].

     */
    (ReplyTo, "Reply-To")
@@ -146,7 +146,7 @@ mailboxes_header! {

    `To` header

-    This header contains [`Mailboxes`].
+    This header contains [`Mailboxes`][self::Mailboxes].

     */
    (To, "To")
@@ -157,7 +157,7 @@ mailboxes_header! {

    `Cc` header

-    This header contains [`Mailboxes`].
+    This header contains [`Mailboxes`][self::Mailboxes].

     */
    (Cc, "Cc")
@@ -168,7 +168,7 @@ mailboxes_header! {

    `Bcc` header

-    This header contains [`Mailboxes`].
+    This header contains [`Mailboxes`][self::Mailboxes].

     */
    (Bcc, "Bcc")
--- a/src/message/header/mod.rs
+++ b/src/message/header/mod.rs
@@ -18,6 +18,7 @@ pub use self::{
    special::*,
    textual::*,
 };
+use super::EmailFormat;
 use crate::BoxError;

 mod content;
@@ -154,6 +155,19 @@ impl Display for Headers {
    }
 }

+impl EmailFormat for Headers {
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
+        for value in &self.headers {
+            out.extend([
+                Cow::Borrowed(value.name.as_bytes()),
+                Cow::Borrowed(b": "),
+                Cow::Borrowed(value.encoded_value.as_bytes()),
+                Cow::Borrowed(b"\r\n"),
+            ]);
+        }
+    }
+}
+
 /// A possible error when converting a `HeaderName` from another type.
 // comes from `http` crate
 #[allow(missing_copy_implementations)]
@@ -415,7 +429,7 @@ mod tests {

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

    #[test]
--- a/src/message/mailbox/parsers/rfc2822.rs
+++ b/src/message/mailbox/parsers/rfc2822.rs
@@ -41,14 +41,14 @@ fn quoted_pair() -> impl Parser<char, char, Error = Cheap<char>> {

 // FWS             =       ([*WSP CRLF] 1*WSP) /   ; Folding white space
 //                         obs-FWS
-pub(super) fn fws() -> impl Parser<char, Option<char>, Error = Cheap<char>> {
+pub fn fws() -> impl Parser<char, Option<char>, Error = Cheap<char>> {
    rfc2234::wsp()
        .or_not()
        .then_ignore(rfc2234::wsp().ignored().repeated())
 }

 // CFWS            =       *([FWS] comment) (([FWS] comment) / FWS)
-pub(super) fn cfws() -> impl Parser<char, Option<char>, Error = Cheap<char>> {
+pub fn cfws() -> impl Parser<char, Option<char>, Error = Cheap<char>> {
    // TODO: comment are not currently supported, so for now a cfws is
    // the same as a fws.
    fws()
@@ -106,12 +106,12 @@ pub(super) fn atom() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
 }

 // dot-atom        =       [CFWS] dot-atom-text [CFWS]
-pub(super) fn dot_atom() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn dot_atom() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    cfws().chain(dot_atom_text())
 }

 // dot-atom-text   =       1*atext *("." 1*atext)
-pub(super) fn dot_atom_text() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn dot_atom_text() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    atext().repeated().at_least(1).chain(
        just('.')
            .chain(atext().repeated().at_least(1))
@@ -204,7 +204,7 @@ pub(crate) fn mailbox_list(
 // https://datatracker.ietf.org/doc/html/rfc2822#section-3.4.1

 // addr-spec       =       local-part "@" domain
-pub(super) fn addr_spec() -> impl Parser<char, (String, String), Error = Cheap<char>> {
+pub fn addr_spec() -> impl Parser<char, (String, String), Error = Cheap<char>> {
    local_part()
        .collect()
        .then_ignore(just('@'))
@@ -212,12 +212,12 @@ pub(super) fn addr_spec() -> impl Parser<char, (String, String), Error = Cheap<c
 }

 // local-part      =       dot-atom / quoted-string / obs-local-part
-pub(super) fn local_part() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn local_part() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    choice((dot_atom(), quoted_string(), obs_local_part()))
 }

 // domain          =       dot-atom / domain-literal / obs-domain
-pub(super) fn domain() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn domain() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    // NOTE: omitting domain-literal since it may never be used
    choice((dot_atom(), obs_domain()))
 }
@@ -240,11 +240,11 @@ fn obs_phrase() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
 // https://datatracker.ietf.org/doc/html/rfc2822#section-4.4

 // obs-local-part  =       word *("." word)
-pub(super) fn obs_local_part() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn obs_local_part() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    word().chain(just('.').chain(word()).repeated().flatten())
 }

 // obs-domain      =       atom *("." atom)
-pub(super) fn obs_domain() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+pub fn obs_domain() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
    atom().chain(just('.').chain(atom()).repeated().flatten())
 }
--- a/src/message/mailbox/serde.rs
+++ b/src/message/mailbox/serde.rs
@@ -36,7 +36,7 @@ impl<'de> Deserialize<'de> for Mailbox {
            {
                struct FieldVisitor;

-                impl Visitor<'_> for FieldVisitor {
+                impl<'de> Visitor<'de> for FieldVisitor {
                    type Value = Field;

                    fn expecting(&self, formatter: &mut Formatter<'_>) -> FmtResult {
--- a/src/message/mailbox/types.rs
+++ b/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("".to_owned()), "kayo@example.com".parse().unwrap())
+                Mailbox::new(Some("".into()), "kayo@example.com".parse().unwrap())
            ),
            "kayo@example.com"
        );
--- a/src/message/mimebody.rs
+++ b/src/message/mimebody.rs
@@ -1,4 +1,4 @@
-use std::{io::Write, iter::repeat_with};
+use std::{borrow::Cow, iter::repeat_with, sync::Arc};

 use mime::Mime;

@@ -28,7 +28,7 @@ impl Part {
 }

 impl EmailFormat for Part {
-    fn format(&self, out: &mut Vec<u8>) {
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
        match self {
            Part::Single(part) => part.format(out),
            Part::Multi(part) => part.format(out),
@@ -71,7 +71,7 @@ impl SinglePartBuilder {

        SinglePart {
            headers: self.headers,
-            body: body.into_vec(),
+            body: body.into_inner(),
        }
    }
 }
@@ -100,7 +100,7 @@ impl Default for SinglePartBuilder {
 #[derive(Debug, Clone)]
 pub struct SinglePart {
    headers: Headers,
-    body: Vec<u8>,
+    body: Arc<[u8]>,
 }

 impl SinglePart {
@@ -138,24 +138,18 @@ impl SinglePart {

    /// Get message content formatted for sending
    pub fn formatted(&self) -> Vec<u8> {
-        let mut out = Vec::new();
-        self.format(&mut out);
-        out
-    }
-
-    /// Format only the signlepart body
-    fn format_body(&self, out: &mut Vec<u8>) {
-        out.extend_from_slice(&self.body);
-        out.extend_from_slice(b"\r\n");
+        self.format_to_vec()
    }
 }

 impl EmailFormat for SinglePart {
-    fn format(&self, out: &mut Vec<u8>) {
-        write!(out, "{}", self.headers)
-            .expect("A Write implementation panicked while formatting headers");
-        out.extend_from_slice(b"\r\n");
-        self.format_body(out);
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
+        self.headers.format(out);
+        out.extend([
+            Cow::Borrowed("\r\n".as_bytes()),
+            Cow::Borrowed(&self.body),
+            Cow::Borrowed(b"\r\n"),
+        ]);
    }
 }

@@ -384,33 +378,36 @@ impl MultiPart {

    /// Get message content formatted for SMTP
    pub fn formatted(&self) -> Vec<u8> {
-        let mut out = Vec::new();
-        self.format(&mut out);
-        out
+        self.format_to_vec()
    }

    /// Format only the multipart body
-    fn format_body(&self, out: &mut Vec<u8>) {
+    fn format_body<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
        let boundary = self.boundary();

        for part in &self.parts {
-            out.extend_from_slice(b"--");
-            out.extend_from_slice(boundary.as_bytes());
-            out.extend_from_slice(b"\r\n");
+            out.extend([
+                Cow::Borrowed("--".as_bytes()),
+                // FIXME: this clone shouldn't exist
+                Cow::Owned(boundary.clone().into()),
+                Cow::Borrowed("\r\n".as_bytes()),
+            ]);
            part.format(out);
        }

-        out.extend_from_slice(b"--");
-        out.extend_from_slice(boundary.as_bytes());
-        out.extend_from_slice(b"--\r\n");
+        out.extend([
+            Cow::Borrowed("--".as_bytes()),
+            Cow::Owned(boundary.into()),
+            Cow::Borrowed("--\r\n".as_bytes()),
+        ]);
    }
 }

 impl EmailFormat for MultiPart {
-    fn format(&self, out: &mut Vec<u8>) {
-        write!(out, "{}", self.headers)
-            .expect("A Write implementation panicked while formatting headers");
-        out.extend_from_slice(b"\r\n");
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
+        self.headers.format(out);
+        out.extend([Cow::Borrowed("\r\n".as_bytes())]);
+
        self.format_body(out);
    }
 }
--- a/src/message/mod.rs
+++ b/src/message/mod.rs
@@ -198,7 +198,7 @@
 //! ```
 //! </details>

-use std::{io::Write, iter, time::SystemTime};
+use std::{borrow::Cow, iter, sync::Arc, time::SystemTime};

 pub use attachment::Attachment;
 pub use body::{Body, IntoBody, MaybeString};
@@ -226,7 +226,23 @@ const DEFAULT_MESSAGE_ID_DOMAIN: &str = "localhost";
 /// Something that can be formatted as an email message
 trait EmailFormat {
    // Use a writer?
-    fn format(&self, out: &mut Vec<u8>);
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>);
+
+    fn format_to_vec(&self) -> Vec<u8> {
+        struct Formatter(Vec<u8>);
+
+        impl<'a> Extend<Cow<'a, [u8]>> for Formatter {
+            fn extend<T: IntoIterator<Item = Cow<'a, [u8]>>>(&mut self, iter: T) {
+                for chunk in iter {
+                    self.0.extend_from_slice(&chunk);
+                }
+            }
+        }
+
+        let mut formatted = Formatter(Vec::new());
+        self.format(&mut formatted);
+        formatted.0
+    }
 }

 /// A builder for messages
@@ -277,7 +293,7 @@ impl MessageBuilder {
    /// Shortcut for `self.date(SystemTime::now())`, it is automatically inserted
    /// if no date has been provided.
    pub fn date_now(self) -> Self {
-        self.date(crate::time::now())
+        self.date(SystemTime::now())
    }

    /// Set or add mailbox to `ReplyTo` header
@@ -345,7 +361,7 @@ 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();

@@ -454,15 +470,15 @@ impl MessageBuilder {
        let body = body.into_body(maybe_encoding);

        self.headers.set(body.encoding());
-        self.build(MessageBody::Raw(body.into_vec()))
+        self.build(MessageBody::Raw(body.into_inner()))
    }

-    /// Create message using mime body ([`MultiPart`])
+    /// Create message using mime body ([`MultiPart`][self::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`])
+    /// Create message using mime body ([`SinglePart`][self::SinglePart])
    pub fn singlepart(self, part: SinglePart) -> Result<Message, EmailError> {
        self.mime_1_0().build(MessageBody::Mime(Part::Single(part)))
    }
@@ -489,7 +505,7 @@ pub struct Message {
 #[derive(Clone, Debug)]
 enum MessageBody {
    Mime(Part),
-    Raw(Vec<u8>),
+    Raw(Arc<[u8]>),
 }

 impl Message {
@@ -515,9 +531,7 @@ impl Message {

    /// Get message content formatted for SMTP
    pub fn formatted(&self) -> Vec<u8> {
-        let mut out = Vec::new();
-        self.format(&mut out);
-        out
+        self.format_to_vec()
    }

    #[cfg(feature = "dkim")]
@@ -527,7 +541,7 @@ 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
    }
@@ -537,10 +551,7 @@ impl Message {
    /// Example:
    /// ```rust
    /// use lettre::{
-    ///     message::{
-    ///         dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
-    ///         header::ContentType,
-    ///     },
+    ///     message::dkim::{DkimConfig, DkimSigningAlgorithm, DkimSigningKey},
    ///     Message,
    /// };
    ///
@@ -549,7 +560,6 @@ 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-----
@@ -597,15 +607,13 @@ impl Message {
 }

 impl EmailFormat for Message {
-    fn format(&self, out: &mut Vec<u8>) {
-        write!(out, "{}", self.headers)
-            .expect("A Write implementation panicked while formatting headers");
+    fn format<'a>(&'a self, out: &mut impl Extend<Cow<'a, [u8]>>) {
+        self.headers.format(out);

        match &self.body {
            MessageBody::Mime(p) => p.format(out),
            MessageBody::Raw(r) => {
-                out.extend_from_slice(b"\r\n");
-                out.extend_from_slice(r);
+                out.extend([Cow::Borrowed("\r\n".as_bytes()), Cow::Borrowed(r)]);
            }
        }
    }
@@ -769,7 +777,7 @@ mod test {
                continue;
            }

-            assert_eq!(line.0, line.1);
+            assert_eq!(line.0, line.1)
        }
    }

--- a/src/rustls_crypto.rs
+++ b/src/rustls_crypto.rs
@@ -1,14 +0,0 @@
-use std::sync::Arc;
-
-use rustls::crypto::CryptoProvider;
-
-pub(crate) fn crypto_provider() -> Arc<CryptoProvider> {
-    CryptoProvider::get_default().cloned().unwrap_or_else(|| {
-        #[cfg(feature = "aws-lc-rs")]
-        let provider = rustls::crypto::aws_lc_rs::default_provider();
-        #[cfg(not(feature = "aws-lc-rs"))]
-        let provider = rustls::crypto::ring::default_provider();
-
-        Arc::new(provider)
-    })
-}
--- a/src/time.rs
+++ b/src/time.rs
@@ -1,26 +0,0 @@
-use std::time::SystemTime;
-
-#[cfg(feature = "web")]
-pub(crate) fn now() -> SystemTime {
-    fn to_std_systemtime(time: web_time::SystemTime) -> std::time::SystemTime {
-        let duration = time
-            .duration_since(web_time::SystemTime::UNIX_EPOCH)
-            .unwrap();
-        SystemTime::UNIX_EPOCH + duration
-    }
-
-    // FIXME: change to:
-    // #[allow(
-    //     clippy::disallowed_methods,
-    //     reason = "`web-time` aliases `std::time::SystemTime::now` on non-WASM platforms"
-    // )]
-    #[allow(clippy::disallowed_methods)]
-    to_std_systemtime(web_time::SystemTime::now())
-}
-
-#[cfg(not(feature = "web"))]
-pub(crate) fn now() -> SystemTime {
-    // FIXME: change to #[expect(clippy::disallowed_methods, reason = "the `web` feature is disabled")]
-    #[allow(clippy::disallowed_methods)]
-    SystemTime::now()
-}
--- a/src/transport/file/error.rs
+++ b/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}")?;
--- a/src/transport/file/mod.rs
+++ b/src/transport/file/mod.rs
@@ -11,7 +11,7 @@
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{message::header::ContentType, FileTransport, Message, Transport};
+//! use lettre::{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!"))?;
 //!
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //!
@@ -44,7 +44,7 @@
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{message::header::ContentType, FileTransport, Message, Transport};
+//! use lettre::{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!"))?;
 //!
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //!
@@ -73,9 +73,7 @@
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{
-//!     message::header::ContentType, AsyncFileTransport, AsyncTransport, Message, Tokio1Executor,
-//! };
+//! use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
@@ -84,10 +82,10 @@
 //!     .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!"))?;
 //!
-//! sender.send(email).await?;
+//! let result = sender.send(email).await;
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //! ```
@@ -101,10 +99,7 @@
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
 //!
-//! use lettre::{
-//!     message::header::ContentType, AsyncFileTransport, AsyncStd1Executor, AsyncTransport,
-//!     Message,
-//! };
+//! use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
 //!
 //! // Write to the local temp directory
 //! let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
@@ -113,10 +108,10 @@
 //!     .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!"))?;
 //!
-//! sender.send(email).await?;
+//! let result = sender.send(email).await;
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //! ```
@@ -130,7 +125,6 @@
 //! 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!
--- a/src/transport/mod.rs
+++ b/src/transport/mod.rs
@@ -56,17 +56,13 @@
 //! #
 //! # #[cfg(all(feature = "builder", feature = "smtp-transport"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! use lettre::{
-//!     message::header::ContentType, transport::smtp::authentication::Credentials, Message,
-//!     SmtpTransport, Transport,
-//! };
+//! use lettre::{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());
@@ -140,10 +136,6 @@ pub trait Transport {
    }

    fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-
-    /// Shuts down the transport. Future calls to [`Self::send`] and
-    /// [`Self::send_raw`] might fail.
-    fn shutdown(&self) {}
 }

 /// Async Transport method for emails
@@ -170,8 +162,4 @@ pub trait AsyncTransport {
    }

    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-
-    /// Shuts down the transport. Future calls to [`Self::send`] and
-    /// [`Self::send_raw`] might fail.
-    async fn shutdown(&self) {}
 }
--- a/src/transport/sendmail/error.rs
+++ b/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}")?;
--- a/src/transport/sendmail/mod.rs
+++ b/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::header::ContentType, Message, SendmailTransport, Transport};
+//! use lettre::{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();
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //!
@@ -34,8 +34,7 @@
 //! # #[cfg(all(feature = "tokio1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use lettre::{
-//!     message::header::ContentType, AsyncSendmailTransport, AsyncTransport, Message,
-//!     SendmailTransport, Tokio1Executor,
+//!     AsyncSendmailTransport, AsyncTransport, Message, SendmailTransport, Tokio1Executor,
 //! };
 //!
 //! let email = Message::builder()
@@ -43,11 +42,11 @@
 //!     .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();
-//! sender.send(email).await?;
+//! let result = sender.send(email).await;
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //! ```
@@ -59,17 +58,18 @@
 //! #
 //! # #[cfg(all(feature = "async-std1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, AsyncTransport, AsyncStd1Executor,message::header::ContentType, AsyncSendmailTransport};
+//! use lettre::{Message, AsyncTransport, AsyncStd1Executor, 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").header(ContentType::TEXT_PLAIN)
+//!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
 //! let sender = AsyncSendmailTransport::<AsyncStd1Executor>::new();
-//! sender.send(email).await?;
+//! let result = sender.send(email).await;
+//! assert!(result.is_ok());
 //! # 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(),
--- a/src/transport/smtp/async_transport.rs
+++ b/src/transport/smtp/async_transport.rs
@@ -12,12 +12,6 @@ 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",
-    feature = "async-std1-rustls"
-))]
-use super::Tls;
 use super::{
    client::AsyncSmtpConnection, ClientId, Credentials, Error, Mechanism, Response, SmtpInfo,
 };
@@ -30,30 +24,6 @@ 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")]
@@ -75,15 +45,10 @@ impl AsyncTransport for AsyncSmtpTransport<Tokio1Executor> {
        let result = conn.send(envelope, email).await?;

        #[cfg(not(feature = "pool"))]
-        conn.abort().await;
+        conn.quit().await?;

        Ok(result)
    }
-
-    async fn shutdown(&self) {
-        #[cfg(feature = "pool")]
-        self.inner.shutdown().await;
-    }
 }

 #[cfg(feature = "async-std1")]
@@ -102,11 +67,6 @@ impl AsyncTransport for AsyncSmtpTransport<AsyncStd1Executor> {

        Ok(result)
    }
-
-    async fn shutdown(&self) {
-        #[cfg(feature = "pool")]
-        self.inner.shutdown().await;
-    }
 }

 impl<E> AsyncSmtpTransport<E>
@@ -121,15 +81,15 @@ where
    /// to validate TLS certificates.
    #[cfg(any(
        feature = "tokio1-native-tls",
-        feature = "tokio1-rustls",
-        feature = "async-std1-rustls"
+        feature = "tokio1-rustls-tls",
+        feature = "async-std1-rustls-tls"
    ))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(
            feature = "tokio1-native-tls",
-            feature = "tokio1-rustls",
-            feature = "async-std1-rustls"
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
        )))
    )]
    pub fn relay(relay: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
@@ -155,15 +115,15 @@ where
    /// or emails will be sent to the server, protecting from downgrade attacks.
    #[cfg(any(
        feature = "tokio1-native-tls",
-        feature = "tokio1-rustls",
-        feature = "async-std1-rustls"
+        feature = "tokio1-rustls-tls",
+        feature = "async-std1-rustls-tls"
    ))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(
            feature = "tokio1-native-tls",
-            feature = "tokio1-rustls",
-            feature = "async-std1-rustls"
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
        )))
    )]
    pub fn starttls_relay(relay: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
@@ -201,72 +161,54 @@ where

    /// Creates a `AsyncSmtpTransportBuilder` from a connection URL
    ///
-    /// 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.
+    /// 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 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:
+    /// <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>
    ///
    /// ```rust,no_run
    /// use lettre::{
@@ -290,18 +232,22 @@ where
    /// let mailer: AsyncSmtpTransport<Tokio1Executor> =
    ///     AsyncSmtpTransport::<Tokio1Executor>::from_url(
    ///         "smtps://username:password@smtp.example.com:465",
-    ///     )?
+    ///     )
+    ///     .unwrap()
    ///     .build();
    ///
    /// // Send the email
-    /// mailer.send(email).await?;
+    /// match mailer.send(email).await {
+    ///     Ok(_) => println!("Email sent successfully!"),
+    ///     Err(e) => panic!("Could not send email: {e:?}"),
+    /// }
    /// # Ok(())
    /// # }
    /// ```
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn from_url(connection_url: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
        super::connection_url::from_connection_url(connection_url)
@@ -377,7 +323,7 @@ impl AsyncSmtpTransportBuilder {
        self
    }

-    /// Set the authentication credentials to use
+    /// Set the authentication mechanism to use
    pub fn credentials(mut self, credentials: Credentials) -> Self {
        self.info.credentials = Some(credentials);
        self
@@ -390,17 +336,6 @@ 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
@@ -413,31 +348,20 @@ 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",
-        feature = "async-std1-rustls"
+        feature = "tokio1-rustls-tls",
+        feature = "async-std1-rustls-tls"
    ))]
    #[cfg_attr(
        docsrs,
        doc(cfg(any(
            feature = "tokio1-native-tls",
-            feature = "tokio1-rustls",
-            feature = "async-std1-rustls"
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
        )))
    )]
-    pub fn tls(mut self, tls: Tls) -> Self {
+    pub fn tls(mut self, tls: super::Tls) -> Self {
        self.info.tls = tls;
        self
    }
@@ -470,7 +394,7 @@ impl AsyncSmtpTransportBuilder {
 }

 /// Build client
-pub(super) struct AsyncSmtpClient<E> {
+pub struct AsyncSmtpClient<E> {
    info: SmtpInfo,
    marker_: PhantomData<E>,
 }
@@ -482,7 +406,7 @@ where
    /// Creates a new connection directly usable to send emails
    ///
    /// Handles encryption and authentication
-    pub(super) async fn connection(&self) -> Result<AsyncSmtpConnection, Error> {
+    pub async fn connection(&self) -> Result<AsyncSmtpConnection, Error> {
        let mut conn = E::connect(
            &self.info.server,
            self.info.port,
--- a/src/transport/smtp/authentication.rs
+++ b/src/transport/smtp/authentication.rs
@@ -98,17 +98,13 @@ impl Mechanism {
                let decoded_challenge = challenge
                    .ok_or_else(|| error::client("This mechanism does expect a challenge"))?;

-                if contains_ignore_ascii_case(
-                    decoded_challenge,
-                    ["User Name", "Username:", "Username", "User Name\0"],
-                ) {
+                if ["User Name", "Username:", "Username", "User Name\0"]
+                    .contains(&decoded_challenge)
+                {
                    return Ok(credentials.authentication_identity.clone());
                }

-                if contains_ignore_ascii_case(
-                    decoded_challenge,
-                    ["Password", "Password:", "Password\0"],
-                ) {
+                if ["Password", "Password:", "Password\0"].contains(&decoded_challenge) {
                    return Ok(credentials.secret.clone());
                }

@@ -125,15 +121,6 @@ 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};
@@ -168,23 +155,6 @@ 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;
--- a/src/transport/smtp/client/async_connection.rs
+++ b/src/transport/smtp/client/async_connection.rs
@@ -6,8 +6,7 @@ use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use super::async_net::AsyncTokioStream;
 #[cfg(feature = "tracing")]
 use super::escape_crlf;
-#[allow(deprecated)]
-use super::{async_net::AsyncNetworkStream, ClientCodec, TlsParameters};
+use super::{AsyncNetworkStream, ClientCodec, TlsParameters};
 use crate::{
    transport::smtp::{
        authentication::{Credentials, Mechanism},
@@ -36,7 +35,6 @@ macro_rules! try_smtp (
 pub struct AsyncSmtpConnection {
    /// TCP stream between client and server
    /// Value is None before connection
-    #[allow(deprecated)]
    stream: BufReader<AsyncNetworkStream>,
    /// Panic state
    panic: bool,
@@ -58,7 +56,6 @@ impl AsyncSmtpConnection {
        stream: Box<dyn AsyncTokioStream>,
        hello_name: &ClientId,
    ) -> Result<AsyncSmtpConnection, Error> {
-        #[allow(deprecated)]
        let stream = AsyncNetworkStream::use_existing_tokio1(stream);
        Self::connect_impl(stream, hello_name).await
    }
@@ -89,7 +86,8 @@ impl AsyncSmtpConnection {
    ///     Some(TlsParameters::new("example.com".to_owned())?),
    ///     None,
    /// )
-    /// .await?;
+    /// .await
+    /// .unwrap();
    /// # Ok(())
    /// # }
    /// ```
@@ -101,7 +99,6 @@ impl AsyncSmtpConnection {
        tls_parameters: Option<TlsParameters>,
        local_address: Option<IpAddr>,
    ) -> Result<AsyncSmtpConnection, Error> {
-        #[allow(deprecated)]
        let stream =
            AsyncNetworkStream::connect_tokio1(server, timeout, tls_parameters, local_address)
                .await?;
@@ -118,12 +115,10 @@ impl AsyncSmtpConnection {
        hello_name: &ClientId,
        tls_parameters: Option<TlsParameters>,
    ) -> Result<AsyncSmtpConnection, Error> {
-        #[allow(deprecated)]
        let stream = AsyncNetworkStream::connect_asyncstd1(server, timeout, tls_parameters).await?;
        Self::connect_impl(stream, hello_name).await
    }

-    #[allow(deprecated)]
    async fn connect_impl(
        stream: AsyncNetworkStream,
        hello_name: &ClientId,
@@ -251,7 +246,6 @@ impl AsyncSmtpConnection {
    }

    /// Sets the underlying stream
-    #[allow(deprecated)]
    pub fn set_stream(&mut self, stream: AsyncNetworkStream) {
        self.stream = BufReader::new(stream);
    }
@@ -375,29 +369,13 @@ impl AsyncSmtpConnection {
    }

    /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
        self.stream.get_ref().peer_certificate()
    }

-    /// Currently this is only avaialable when using Boring TLS and
-    /// returns the result of the verification of the TLS certificate
-    /// presented by the peer, if any. Only the last error encountered
-    /// during verification is presented.
-    /// It can be useful when you don't want to fail outright the TLS
-    /// negotiation, for example when a self-signed certificate is
-    /// encountered, but still want to record metrics or log the fact.
-    /// When using DANE verification, the PKI root of trust moves from
-    /// the CAs to DNS, so self-signed certificates are permitted as long
-    /// as the TLSA records match the leaf or issuer certificates.
-    /// It cannot be called on non Boring TLS streams.
-    #[cfg(feature = "boring-tls")]
-    pub fn tls_verify_result(&self) -> Result<(), Error> {
-        self.stream.get_ref().tls_verify_result()
-    }
-
    /// All the X509 certificates of the chain (DER encoded)
-    #[cfg(any(feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "rustls-tls", feature = "boring-tls"))]
    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
        self.stream.get_ref().certificate_chain()
    }
--- a/src/transport/smtp/client/async_net.rs
+++ b/src/transport/smtp/client/async_net.rs
@@ -12,9 +12,9 @@ use futures_io::{
    AsyncRead as FuturesAsyncRead, AsyncWrite as FuturesAsyncWrite, Error as IoError, ErrorKind,
    Result as IoResult,
 };
-#[cfg(feature = "async-std1-rustls")]
+#[cfg(feature = "async-std1-rustls-tls")]
 use futures_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
-#[cfg(any(feature = "tokio1-rustls", feature = "async-std1-rustls"))]
+#[cfg(any(feature = "tokio1-rustls-tls", feature = "async-std1-rustls-tls"))]
 use rustls::pki_types::ServerName;
 #[cfg(feature = "tokio1-boring-tls")]
 use tokio1_boring::SslStream as Tokio1SslStream;
@@ -27,14 +27,14 @@ use tokio1_crate::net::{
 };
 #[cfg(feature = "tokio1-native-tls")]
 use tokio1_native_tls_crate::TlsStream as Tokio1TlsStream;
-#[cfg(feature = "tokio1-rustls")]
+#[cfg(feature = "tokio1-rustls-tls")]
 use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;

 #[cfg(any(
    feature = "tokio1-native-tls",
-    feature = "tokio1-rustls",
+    feature = "tokio1-rustls-tls",
    feature = "tokio1-boring-tls",
-    feature = "async-std1-rustls"
+    feature = "async-std1-rustls-tls"
 ))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
@@ -44,10 +44,6 @@ use crate::transport::smtp::{error, Error};

 /// A network stream
 #[derive(Debug)]
-#[deprecated(
-    since = "0.11.14",
-    note = "This struct was not meant to be made public"
-)]
 pub struct AsyncNetworkStream {
    inner: InnerAsyncNetworkStream,
 }
@@ -78,7 +74,7 @@ enum InnerAsyncNetworkStream {
    #[cfg(feature = "tokio1-native-tls")]
    Tokio1NativeTls(Tokio1TlsStream<Box<dyn AsyncTokioStream>>),
    /// Encrypted Tokio 1.x TCP stream
-    #[cfg(feature = "tokio1-rustls")]
+    #[cfg(feature = "tokio1-rustls-tls")]
    Tokio1RustlsTls(Tokio1RustlsTlsStream<Box<dyn AsyncTokioStream>>),
    /// Encrypted Tokio 1.x TCP stream
    #[cfg(feature = "tokio1-boring-tls")]
@@ -87,13 +83,12 @@ enum InnerAsyncNetworkStream {
    #[cfg(feature = "async-std1")]
    AsyncStd1Tcp(AsyncStd1TcpStream),
    /// Encrypted Tokio 1.x TCP stream
-    #[cfg(feature = "async-std1-rustls")]
+    #[cfg(feature = "async-std1-rustls-tls")]
    AsyncStd1RustlsTls(AsyncStd1RustlsTlsStream<AsyncStd1TcpStream>),
    /// Can't be built
    None,
 }

-#[allow(deprecated)]
 impl AsyncNetworkStream {
    fn new(inner: InnerAsyncNetworkStream) -> Self {
        if let InnerAsyncNetworkStream::None = inner {
@@ -112,13 +107,13 @@ impl AsyncNetworkStream {
            InnerAsyncNetworkStream::Tokio1NativeTls(s) => {
                s.get_ref().get_ref().get_ref().peer_addr()
            }
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => s.get_ref().0.peer_addr(),
            #[cfg(feature = "tokio1-boring-tls")]
            InnerAsyncNetworkStream::Tokio1BoringTls(s) => s.get_ref().peer_addr(),
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => s.peer_addr(),
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => s.get_ref().0.peer_addr(),
            InnerAsyncNetworkStream::None => {
                debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
@@ -175,7 +170,7 @@ impl AsyncNetworkStream {
                            last_err = Some(io::Error::new(
                                io::ErrorKind::TimedOut,
                                "connection timed out",
-                            ));
+                            ))
                        }
                    }
                } else {
@@ -227,7 +222,7 @@ impl AsyncNetworkStream {
                        last_err = Some(io::Error::new(
                            io::ErrorKind::TimedOut,
                            "connection timed out",
-                        ));
+                        ))
                    }
                }
            }
@@ -258,25 +253,26 @@ impl AsyncNetworkStream {
                feature = "tokio1",
                not(any(
                    feature = "tokio1-native-tls",
-                    feature = "tokio1-rustls",
+                    feature = "tokio1-rustls-tls",
                    feature = "tokio1-boring-tls"
                ))
            ))]
            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                let _ = tls_parameters;
-                panic!("Trying to upgrade an AsyncNetworkStream without having enabled either the tokio1-native-tls or the tokio1-rustls feature");
+                panic!("Trying to upgrade an AsyncNetworkStream without having enabled either the tokio1-native-tls or the tokio1-rustls-tls feature");
            }

            #[cfg(any(
                feature = "tokio1-native-tls",
-                feature = "tokio1-rustls",
+                feature = "tokio1-rustls-tls",
                feature = "tokio1-boring-tls"
            ))]
            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                // get owned TcpStream
                let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
-                let InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream) = tcp_stream else {
-                    unreachable!()
+                let tcp_stream = match tcp_stream {
+                    InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream) => tcp_stream,
+                    _ => unreachable!(),
                };

                self.inner = Self::upgrade_tokio1_tls(tcp_stream, tls_parameters)
@@ -284,18 +280,19 @@ impl AsyncNetworkStream {
                    .map_err(error::connection)?;
                Ok(())
            }
-            #[cfg(all(feature = "async-std1", not(feature = "async-std1-rustls")))]
+            #[cfg(all(feature = "async-std1", not(feature = "async-std1-rustls-tls")))]
            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                let _ = tls_parameters;
-                panic!("Trying to upgrade an AsyncNetworkStream without having enabled the async-std1-rustls feature");
+                panic!("Trying to upgrade an AsyncNetworkStream without having enabled the async-std1-rustls-tls feature");
            }

-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                // get owned TcpStream
                let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
-                let InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream) = tcp_stream else {
-                    unreachable!()
+                let tcp_stream = match tcp_stream {
+                    InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream) => tcp_stream,
+                    _ => unreachable!(),
                };

                self.inner = Self::upgrade_asyncstd1_tls(tcp_stream, tls_parameters)
@@ -310,7 +307,7 @@ impl AsyncNetworkStream {
    #[allow(unused_variables)]
    #[cfg(any(
        feature = "tokio1-native-tls",
-        feature = "tokio1-rustls",
+        feature = "tokio1-rustls-tls",
        feature = "tokio1-boring-tls"
    ))]
    async fn upgrade_tokio1_tls(
@@ -337,12 +334,12 @@ impl AsyncNetworkStream {
                    Ok(InnerAsyncNetworkStream::Tokio1NativeTls(stream))
                };
            }
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerTlsParameters::RustlsTls(config) => {
-                #[cfg(not(feature = "tokio1-rustls"))]
-                panic!("built without the tokio1-rustls feature");
+                #[cfg(not(feature = "tokio1-rustls-tls"))]
+                panic!("built without the tokio1-rustls-tls feature");

-                #[cfg(feature = "tokio1-rustls")]
+                #[cfg(feature = "tokio1-rustls-tls")]
                return {
                    use tokio1_rustls::TlsConnector;

@@ -377,7 +374,7 @@ impl AsyncNetworkStream {
    }

    #[allow(unused_variables)]
-    #[cfg(feature = "async-std1-rustls")]
+    #[cfg(feature = "async-std1-rustls-tls")]
    async fn upgrade_asyncstd1_tls(
        tcp_stream: AsyncStd1TcpStream,
        mut tls_parameters: TlsParameters,
@@ -389,12 +386,12 @@ impl AsyncNetworkStream {
            InnerTlsParameters::NativeTls(connector) => {
                panic!("native-tls isn't supported with async-std yet. See https://github.com/lettre/lettre/pull/531#issuecomment-757893531");
            }
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerTlsParameters::RustlsTls(config) => {
-                #[cfg(not(feature = "async-std1-rustls"))]
-                panic!("built without the async-std1-rustls feature");
+                #[cfg(not(feature = "async-std1-rustls-tls"))]
+                panic!("built without the async-std1-rustls-tls feature");

-                #[cfg(feature = "async-std1-rustls")]
+                #[cfg(feature = "async-std1-rustls-tls")]
                return {
                    use futures_rustls::TlsConnector;

@@ -422,42 +419,18 @@ impl AsyncNetworkStream {
            InnerAsyncNetworkStream::Tokio1Tcp(_) => false,
            #[cfg(feature = "tokio1-native-tls")]
            InnerAsyncNetworkStream::Tokio1NativeTls(_) => true,
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(_) => true,
            #[cfg(feature = "tokio1-boring-tls")]
            InnerAsyncNetworkStream::Tokio1BoringTls(_) => true,
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => false,
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(_) => true,
            InnerAsyncNetworkStream::None => false,
        }
    }

-    #[cfg(feature = "boring-tls")]
-    pub fn tls_verify_result(&self) -> Result<(), Error> {
-        match &self.inner {
-            #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
-                Err(error::client("Connection is not encrypted"))
-            }
-            #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "tokio1-rustls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "tokio1-boring-tls")]
-            InnerAsyncNetworkStream::Tokio1BoringTls(stream) => {
-                stream.ssl().verify_result().map_err(error::tls)
-            }
-            #[cfg(feature = "async-std1")]
-            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
-                Err(error::client("Connection is not encrypted"))
-            }
-            #[cfg(feature = "async-std1-rustls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(_) => panic!("Unsupported"),
-            InnerAsyncNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
-        }
-    }
    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
        match &self.inner {
            #[cfg(feature = "tokio1")]
@@ -466,7 +439,7 @@ impl AsyncNetworkStream {
            }
            #[cfg(feature = "tokio1-native-tls")]
            InnerAsyncNetworkStream::Tokio1NativeTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(stream) => Ok(stream
                .get_ref()
                .1
@@ -487,7 +460,7 @@ impl AsyncNetworkStream {
            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                Err(error::client("Connection is not encrypted"))
            }
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
                .get_ref()
                .1
@@ -514,7 +487,7 @@ impl AsyncNetworkStream {
                .unwrap()
                .to_der()
                .map_err(error::tls)?),
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(stream) => Ok(stream
                .get_ref()
                .1
@@ -534,7 +507,7 @@ impl AsyncNetworkStream {
            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                Err(error::client("Connection is not encrypted"))
            }
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
                .get_ref()
                .1
@@ -548,7 +521,6 @@ impl AsyncNetworkStream {
    }
 }

-#[allow(deprecated)]
 impl FuturesAsyncRead for AsyncNetworkStream {
    fn poll_read(
        mut self: Pin<&mut Self>,
@@ -574,7 +546,7 @@ impl FuturesAsyncRead for AsyncNetworkStream {
                    Poll::Pending => Poll::Pending,
                }
            }
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => {
                let mut b = Tokio1ReadBuf::new(buf);
                match Pin::new(s).poll_read(cx, &mut b) {
@@ -594,7 +566,7 @@ impl FuturesAsyncRead for AsyncNetworkStream {
            }
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_read(cx, buf),
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_read(cx, buf),
            InnerAsyncNetworkStream::None => {
                debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
@@ -604,7 +576,6 @@ impl FuturesAsyncRead for AsyncNetworkStream {
    }
 }

-#[allow(deprecated)]
 impl FuturesAsyncWrite for AsyncNetworkStream {
    fn poll_write(
        mut self: Pin<&mut Self>,
@@ -616,13 +587,13 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_write(cx, buf),
            #[cfg(feature = "tokio1-native-tls")]
            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_write(cx, buf),
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => Pin::new(s).poll_write(cx, buf),
            #[cfg(feature = "tokio1-boring-tls")]
            InnerAsyncNetworkStream::Tokio1BoringTls(s) => Pin::new(s).poll_write(cx, buf),
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_write(cx, buf),
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_write(cx, buf),
            InnerAsyncNetworkStream::None => {
                debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
@@ -637,13 +608,13 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_flush(cx),
            #[cfg(feature = "tokio1-native-tls")]
            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_flush(cx),
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => Pin::new(s).poll_flush(cx),
            #[cfg(feature = "tokio1-boring-tls")]
            InnerAsyncNetworkStream::Tokio1BoringTls(s) => Pin::new(s).poll_flush(cx),
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_flush(cx),
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_flush(cx),
            InnerAsyncNetworkStream::None => {
                debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
@@ -658,13 +629,13 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_shutdown(cx),
            #[cfg(feature = "tokio1-native-tls")]
            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_shutdown(cx),
-            #[cfg(feature = "tokio1-rustls")]
+            #[cfg(feature = "tokio1-rustls-tls")]
            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => Pin::new(s).poll_shutdown(cx),
            #[cfg(feature = "tokio1-boring-tls")]
            InnerAsyncNetworkStream::Tokio1BoringTls(s) => Pin::new(s).poll_shutdown(cx),
            #[cfg(feature = "async-std1")]
            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_close(cx),
-            #[cfg(feature = "async-std1-rustls")]
+            #[cfg(feature = "async-std1-rustls-tls")]
            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_close(cx),
            InnerAsyncNetworkStream::None => {
                debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
--- a/src/transport/smtp/client/connection.rs
+++ b/src/transport/smtp/client/connection.rs
@@ -143,7 +143,7 @@ impl SmtpConnection {
        hello_name: &ClientId,
    ) -> Result<(), Error> {
        if self.server_info.supports_feature(Extension::StartTls) {
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            {
                try_smtp!(self.command(Starttls), self);
                self.stream.get_mut().upgrade_tls(tls_parameters)?;
@@ -153,7 +153,11 @@ impl SmtpConnection {
                try_smtp!(self.ehlo(hello_name), self);
                Ok(())
            }
-            #[cfg(not(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))]
+            #[cfg(not(any(
+                feature = "native-tls",
+                feature = "rustls-tls",
+                feature = "boring-tls"
+            )))]
            // This should never happen as `Tls` can only be created
            // when a TLS library is enabled
            unreachable!("TLS support required but not supported");
@@ -299,29 +303,13 @@ impl SmtpConnection {
    }

    /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
        self.stream.get_ref().peer_certificate()
    }

-    /// Currently this is only avaialable when using Boring TLS and
-    /// returns the result of the verification of the TLS certificate
-    /// presented by the peer, if any. Only the last error encountered
-    /// during verification is presented.
-    /// It can be useful when you don't want to fail outright the TLS
-    /// negotiation, for example when a self-signed certificate is
-    /// encountered, but still want to record metrics or log the fact.
-    /// When using DANE verification, the PKI root of trust moves from
-    /// the CAs to DNS, so self-signed certificates are permitted as long
-    /// as the TLSA records match the leaf or issuer certificates.
-    /// It cannot be called on non Boring TLS streams.
-    #[cfg(feature = "boring-tls")]
-    pub fn tls_verify_result(&self) -> Result<(), Error> {
-        self.stream.get_ref().tls_verify_result()
-    }
-
    /// All the X509 certificates of the chain (DER encoded)
-    #[cfg(any(feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "rustls-tls", feature = "boring-tls"))]
    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
        self.stream.get_ref().certificate_chain()
    }
--- a/src/transport/smtp/client/mod.rs
+++ b/src/transport/smtp/client/mod.rs
@@ -28,14 +28,13 @@ use std::fmt::Debug;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 pub use self::async_connection::AsyncSmtpConnection;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-#[allow(deprecated)]
 pub use self::async_net::AsyncNetworkStream;
 #[cfg(feature = "tokio1")]
 pub use self::async_net::AsyncTokioStream;
 use self::net::NetworkStream;
-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(super) use self::tls::InnerTlsParameters;
-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub use self::tls::TlsVersion;
 pub use self::{
    connection::SmtpConnection,
@@ -58,7 +57,7 @@ struct ClientCodec {

 impl ClientCodec {
    /// Creates a new client codec
-    pub(crate) fn new() -> Self {
+    pub fn new() -> Self {
        Self {
            status: CodecStatus::StartOfNewLine,
        }
--- a/src/transport/smtp/client/net.rs
+++ b/src/transport/smtp/client/net.rs
@@ -1,4 +1,4 @@
-#[cfg(feature = "rustls")]
+#[cfg(feature = "rustls-tls")]
 use std::sync::Arc;
 use std::{
    io::{self, Read, Write},
@@ -11,11 +11,11 @@ use std::{
 use boring::ssl::SslStream;
 #[cfg(feature = "native-tls")]
 use native_tls::TlsStream;
-#[cfg(feature = "rustls")]
+#[cfg(feature = "rustls-tls")]
 use rustls::{pki_types::ServerName, ClientConnection, StreamOwned};
 use socket2::{Domain, Protocol, Type};

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
 use crate::transport::smtp::{error, Error};
@@ -36,7 +36,7 @@ enum InnerNetworkStream {
    #[cfg(feature = "native-tls")]
    NativeTls(TlsStream<TcpStream>),
    /// Encrypted TCP stream
-    #[cfg(feature = "rustls")]
+    #[cfg(feature = "rustls-tls")]
    RustlsTls(StreamOwned<ClientConnection, TcpStream>),
    #[cfg(feature = "boring-tls")]
    BoringTls(SslStream<TcpStream>),
@@ -59,7 +59,7 @@ impl NetworkStream {
            InnerNetworkStream::Tcp(s) => s.peer_addr(),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(s) => s.get_ref().peer_addr(),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(s) => s.get_ref().peer_addr(),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(s) => s.get_ref().peer_addr(),
@@ -79,7 +79,7 @@ impl NetworkStream {
            InnerNetworkStream::Tcp(s) => s.shutdown(how),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(s) => s.get_ref().shutdown(how),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(s) => s.get_ref().shutdown(how),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(s) => s.get_ref().shutdown(how),
@@ -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),
                    }
                }
@@ -146,18 +146,23 @@ impl NetworkStream {

    pub fn upgrade_tls(&mut self, tls_parameters: &TlsParameters) -> Result<(), Error> {
        match &self.inner {
-            #[cfg(not(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))]
+            #[cfg(not(any(
+                feature = "native-tls",
+                feature = "rustls-tls",
+                feature = "boring-tls"
+            )))]
            InnerNetworkStream::Tcp(_) => {
                let _ = tls_parameters;
-                panic!("Trying to upgrade an NetworkStream without having enabled either the `native-tls` or the `rustls` feature");
+                panic!("Trying to upgrade an NetworkStream without having enabled either the native-tls or the rustls-tls feature");
            }

-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            InnerNetworkStream::Tcp(_) => {
                // get owned TcpStream
                let tcp_stream = mem::replace(&mut self.inner, InnerNetworkStream::None);
-                let InnerNetworkStream::Tcp(tcp_stream) = tcp_stream else {
-                    unreachable!()
+                let tcp_stream = match tcp_stream {
+                    InnerNetworkStream::Tcp(tcp_stream) => tcp_stream,
+                    _ => unreachable!(),
                };

                self.inner = Self::upgrade_tls_impl(tcp_stream, tls_parameters)?;
@@ -167,7 +172,7 @@ impl NetworkStream {
        }
    }

-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    fn upgrade_tls_impl(
        tcp_stream: TcpStream,
        tls_parameters: &TlsParameters,
@@ -180,7 +185,7 @@ impl NetworkStream {
                    .map_err(error::connection)?;
                InnerNetworkStream::NativeTls(stream)
            }
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerTlsParameters::RustlsTls(connector) => {
                let domain = ServerName::try_from(tls_parameters.domain())
                    .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
@@ -207,7 +212,7 @@ impl NetworkStream {
            InnerNetworkStream::Tcp(_) => false,
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(_) => true,
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(_) => true,
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(_) => true,
@@ -218,29 +223,13 @@ impl NetworkStream {
        }
    }

-    #[cfg(feature = "boring-tls")]
-    pub fn tls_verify_result(&self) -> Result<(), Error> {
-        match &self.inner {
-            InnerNetworkStream::Tcp(_) => Err(error::client("Connection is not encrypted")),
-            #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "rustls")]
-            InnerNetworkStream::RustlsTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "boring-tls")]
-            InnerNetworkStream::BoringTls(stream) => {
-                stream.ssl().verify_result().map_err(error::tls)
-            }
-            InnerNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
-        }
-    }
-
-    #[cfg(any(feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "rustls-tls", feature = "boring-tls"))]
    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
        match &self.inner {
            InnerNetworkStream::Tcp(_) => Err(error::client("Connection is not encrypted")),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(_) => panic!("Unsupported"),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(stream) => Ok(stream
                .conn
                .peer_certificates()
@@ -260,7 +249,7 @@ impl NetworkStream {
        }
    }

-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
        match &self.inner {
            InnerNetworkStream::Tcp(_) => Err(error::client("Connection is not encrypted")),
@@ -271,7 +260,7 @@ impl NetworkStream {
                .unwrap()
                .to_der()
                .map_err(error::tls)?),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(stream) => Ok(stream
                .conn
                .peer_certificates()
@@ -295,7 +284,7 @@ impl NetworkStream {
            InnerNetworkStream::Tcp(stream) => stream.set_read_timeout(duration),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(stream) => stream.get_ref().set_read_timeout(duration),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(stream) => stream.get_ref().set_read_timeout(duration),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(stream) => stream.get_ref().set_read_timeout(duration),
@@ -313,7 +302,7 @@ impl NetworkStream {

            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(stream) => stream.get_ref().set_write_timeout(duration),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(stream) => stream.get_ref().set_write_timeout(duration),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(stream) => stream.get_ref().set_write_timeout(duration),
@@ -331,7 +320,7 @@ impl Read for NetworkStream {
            InnerNetworkStream::Tcp(s) => s.read(buf),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(s) => s.read(buf),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(s) => s.read(buf),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(s) => s.read(buf),
@@ -349,7 +338,7 @@ impl Write for NetworkStream {
            InnerNetworkStream::Tcp(s) => s.write(buf),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(s) => s.write(buf),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(s) => s.write(buf),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(s) => s.write(buf),
@@ -365,7 +354,7 @@ impl Write for NetworkStream {
            InnerNetworkStream::Tcp(s) => s.flush(),
            #[cfg(feature = "native-tls")]
            InnerNetworkStream::NativeTls(s) => s.flush(),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            InnerNetworkStream::RustlsTls(s) => s.flush(),
            #[cfg(feature = "boring-tls")]
            InnerNetworkStream::BoringTls(s) => s.flush(),
@@ -380,7 +369,7 @@ 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,
--- a/src/transport/smtp/client/tls.rs
+++ b/src/transport/smtp/client/tls.rs
@@ -1,6 +1,6 @@
 use std::fmt::{self, Debug};
-#[cfg(feature = "rustls")]
-use std::sync::Arc;
+#[cfg(feature = "rustls-tls")]
+use std::{io, sync::Arc};

 #[cfg(feature = "boring-tls")]
 use boring::{
@@ -10,22 +10,23 @@ use boring::{
 };
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
-#[cfg(feature = "rustls")]
+#[cfg(feature = "rustls-tls")]
 use rustls::{
    client::danger::{HandshakeSignatureValid, ServerCertVerified, ServerCertVerifier},
-    crypto::{verify_tls12_signature, verify_tls13_signature, CryptoProvider},
-    pki_types::{self, pem::PemObject, CertificateDer, PrivateKeyDer, ServerName, UnixTime},
+    crypto::WebPkiSupportedAlgorithms,
+    crypto::{verify_tls12_signature, verify_tls13_signature},
+    pki_types::{CertificateDer, PrivateKeyDer, ServerName, UnixTime},
    server::ParsedCertificate,
    ClientConfig, DigitallySignedStruct, Error as TlsError, RootCertStore, SignatureScheme,
 };

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::{error, Error};

 /// TLS protocol versions.
 #[derive(Debug, Copy, Clone)]
 #[non_exhaustive]
-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub enum TlsVersion {
    /// TLS 1.0
    ///
@@ -58,65 +59,31 @@ pub enum TlsVersion {
    Tlsv13,
 }

-/// 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.
+/// How to apply TLS to a client connection
 #[derive(Clone)]
 #[allow(missing_copy_implementations)]
 pub enum Tls {
-    /// 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.
+    /// Insecure connection only (for testing purposes)
    None,
-    /// 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", feature = "boring-tls"))]
+    /// Start with insecure connection and use `STARTTLS` when available
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    Opportunistic(TlsParameters),
-    /// 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", feature = "boring-tls"))]
+    /// Start with insecure connection and require `STARTTLS`
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    Required(TlsParameters),
-    /// 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", feature = "boring-tls"))]
+    /// Use TLS wrapped connection
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    Wrapper(TlsParameters),
 }
@@ -125,11 +92,11 @@ impl Debug for Tls {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match &self {
            Self::None => f.pad("None"),
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            Self::Opportunistic(_) => f.pad("Opportunistic"),
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            Self::Required(_) => f.pad("Required"),
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            Self::Wrapper(_) => f.pad("Wrapper"),
        }
    }
@@ -153,7 +120,7 @@ pub enum CertificateStore {
    /// Use a hardcoded set of Mozilla roots via the `webpki-roots` crate.
    ///
    /// This option is only available in the rustls backend.
-    #[cfg(all(feature = "rustls", feature = "webpki-roots"))]
+    #[cfg(feature = "rustls-tls")]
    WebpkiRoots,
    /// Don't use any system certificates.
    None,
@@ -178,7 +145,7 @@ pub struct TlsParametersBuilder {
    identity: Option<Identity>,
    accept_invalid_hostnames: bool,
    accept_invalid_certs: bool,
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    min_tls_version: TlsVersion,
 }

@@ -192,7 +159,7 @@ impl TlsParametersBuilder {
            identity: None,
            accept_invalid_hostnames: false,
            accept_invalid_certs: false,
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            min_tls_version: TlsVersion::Tlsv12,
        }
    }
@@ -230,10 +197,10 @@ impl TlsParametersBuilder {
    /// including those from other sites, are trusted.
    ///
    /// This method introduces significant vulnerabilities to man-in-the-middle attacks.
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn dangerous_accept_invalid_hostnames(mut self, accept_invalid_hostnames: bool) -> Self {
        self.accept_invalid_hostnames = accept_invalid_hostnames;
@@ -243,7 +210,7 @@ impl TlsParametersBuilder {
    /// Controls which minimum TLS version is allowed
    ///
    /// Defaults to [`Tlsv12`][TlsVersion::Tlsv12].
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    pub fn set_min_tls_version(mut self, min_tls_version: TlsVersion) -> Self {
        self.min_tls_version = min_tls_version;
        self
@@ -272,17 +239,17 @@ impl TlsParametersBuilder {

    /// Creates a new `TlsParameters` using native-tls, boring-tls or rustls
    /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn build(self) -> Result<TlsParameters, Error> {
-        #[cfg(feature = "rustls")]
+        #[cfg(feature = "rustls-tls")]
        return self.build_rustls();
-        #[cfg(all(not(feature = "rustls"), feature = "native-tls"))]
+        #[cfg(all(not(feature = "rustls-tls"), feature = "native-tls"))]
        return self.build_native();
-        #[cfg(all(not(feature = "rustls"), feature = "boring-tls"))]
+        #[cfg(all(not(feature = "rustls-tls"), feature = "boring-tls"))]
        return self.build_boring();
    }

@@ -396,8 +363,8 @@ impl TlsParametersBuilder {
    }

    /// Creates a new `TlsParameters` using rustls with the provided configuration
-    #[cfg(feature = "rustls")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "rustls")))]
+    #[cfg(feature = "rustls-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
    pub fn build_rustls(self) -> Result<TlsParameters, Error> {
        let just_version3 = &[&rustls::version::TLS13];
        let supported_versions = match self.min_tls_version {
@@ -411,16 +378,18 @@ impl TlsParametersBuilder {
            TlsVersion::Tlsv13 => just_version3,
        };

-        let crypto_provider = crate::rustls_crypto::crypto_provider();
-        let tls = ClientConfig::builder_with_provider(Arc::clone(&crypto_provider))
-            .with_protocol_versions(supported_versions)
-            .map_err(error::tls)?;
+        let tls = ClientConfig::builder_with_protocol_versions(supported_versions);
+        let provider = rustls::crypto::CryptoProvider::get_default()
+            .cloned()
+            .unwrap_or_else(|| Arc::new(rustls::crypto::ring::default_provider()));

        // Build TLS config
+        let signature_algorithms = provider.signature_verification_algorithms;
+
        let mut root_cert_store = RootCertStore::empty();

        #[cfg(feature = "rustls-native-certs")]
-        fn load_native_roots(store: &mut RootCertStore) {
+        fn load_native_roots(store: &mut RootCertStore) -> Result<(), Error> {
            let rustls_native_certs::CertificateResult { certs, errors, .. } =
                rustls_native_certs::load_native_certs();
            let errors_len = errors.len();
@@ -430,11 +399,10 @@ impl TlsParametersBuilder {
            tracing::debug!(
                "loaded platform certs with {errors_len} failing to load, {added} valid and {ignored} ignored (invalid) certs"
            );
-            #[cfg(not(feature = "tracing"))]
-            let _ = (errors_len, added, ignored);
+            Ok(())
        }

-        #[cfg(all(feature = "rustls", feature = "webpki-roots"))]
+        #[cfg(feature = "rustls-tls")]
        fn load_webpki_roots(store: &mut RootCertStore) {
            store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
        }
@@ -442,11 +410,11 @@ impl TlsParametersBuilder {
        match self.cert_store {
            CertificateStore::Default => {
                #[cfg(feature = "rustls-native-certs")]
-                load_native_roots(&mut root_cert_store);
-                #[cfg(all(not(feature = "rustls-native-certs"), feature = "webpki-roots"))]
+                load_native_roots(&mut root_cert_store)?;
+                #[cfg(not(feature = "rustls-native-certs"))]
                load_webpki_roots(&mut root_cert_store);
            }
-            #[cfg(all(feature = "rustls", feature = "webpki-roots"))]
+            #[cfg(feature = "rustls-tls")]
            CertificateStore::WebpkiRoots => {
                load_webpki_roots(&mut root_cert_store);
            }
@@ -463,7 +431,7 @@ impl TlsParametersBuilder {
                ignore_invalid_hostnames: self.accept_invalid_hostnames,
                ignore_invalid_certs: self.accept_invalid_certs,
                roots: root_cert_store,
-                crypto_provider,
+                signature_algorithms,
            };
            tls.dangerous()
                .with_custom_certificate_verifier(Arc::new(verifier))
@@ -490,10 +458,10 @@ impl TlsParametersBuilder {

 #[derive(Clone)]
 #[allow(clippy::enum_variant_names)]
-pub(crate) enum InnerTlsParameters {
+pub enum InnerTlsParameters {
    #[cfg(feature = "native-tls")]
    NativeTls(TlsConnector),
-    #[cfg(feature = "rustls")]
+    #[cfg(feature = "rustls-tls")]
    RustlsTls(Arc<ClientConfig>),
    #[cfg(feature = "boring-tls")]
    BoringTls(SslConnector),
@@ -502,10 +470,10 @@ pub(crate) enum InnerTlsParameters {
 impl TlsParameters {
    /// Creates a new `TlsParameters` using native-tls or rustls
    /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn new(domain: String) -> Result<Self, Error> {
        TlsParametersBuilder::new(domain).build()
@@ -524,8 +492,8 @@ impl TlsParameters {
    }

    /// Creates a new `TlsParameters` using rustls
-    #[cfg(feature = "rustls")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "rustls")))]
+    #[cfg(feature = "rustls-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
    pub fn new_rustls(domain: String) -> Result<Self, Error> {
        TlsParametersBuilder::new(domain).build_rustls()
    }
@@ -548,13 +516,13 @@ impl TlsParameters {
 pub struct Certificate {
    #[cfg(feature = "native-tls")]
    native_tls: native_tls::Certificate,
-    #[cfg(feature = "rustls")]
+    #[cfg(feature = "rustls-tls")]
    rustls: Vec<CertificateDer<'static>>,
    #[cfg(feature = "boring-tls")]
    boring_tls: boring::x509::X509,
 }

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 impl Certificate {
    /// Create a `Certificate` from a DER encoded certificate
    pub fn from_der(der: Vec<u8>) -> Result<Self, Error> {
@@ -567,7 +535,7 @@ impl Certificate {
        Ok(Self {
            #[cfg(feature = "native-tls")]
            native_tls: native_tls_cert,
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            rustls: vec![der.into()],
            #[cfg(feature = "boring-tls")]
            boring_tls: boring_tls_cert,
@@ -582,17 +550,20 @@ impl Certificate {
        #[cfg(feature = "boring-tls")]
        let boring_tls_cert = boring::x509::X509::from_pem(pem).map_err(error::tls)?;

-        #[cfg(feature = "rustls")]
+        #[cfg(feature = "rustls-tls")]
        let rustls_cert = {
-            CertificateDer::pem_slice_iter(pem)
-                .collect::<Result<Vec<_>, pki_types::pem::Error>>()
+            use std::io::Cursor;
+
+            let mut pem = Cursor::new(pem);
+            rustls_pemfile::certs(&mut pem)
+                .collect::<io::Result<Vec<_>>>()
                .map_err(|_| error::tls("invalid certificates"))?
        };

        Ok(Self {
            #[cfg(feature = "native-tls")]
            native_tls: native_tls_cert,
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            rustls: rustls_cert,
            #[cfg(feature = "boring-tls")]
            boring_tls: boring_tls_cert,
@@ -611,7 +582,7 @@ impl Debug for Certificate {
 pub struct Identity {
    #[cfg(feature = "native-tls")]
    native_tls: native_tls::Identity,
-    #[cfg(feature = "rustls")]
+    #[cfg(feature = "rustls-tls")]
    rustls_tls: (Vec<CertificateDer<'static>>, PrivateKeyDer<'static>),
    #[cfg(feature = "boring-tls")]
    boring_tls: (boring::x509::X509, PKey<boring::pkey::Private>),
@@ -628,7 +599,7 @@ impl Clone for Identity {
        Identity {
            #[cfg(feature = "native-tls")]
            native_tls: self.native_tls.clone(),
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            rustls_tls: (self.rustls_tls.0.clone(), self.rustls_tls.1.clone_key()),
            #[cfg(feature = "boring-tls")]
            boring_tls: (self.boring_tls.0.clone(), self.boring_tls.1.clone()),
@@ -636,13 +607,13 @@ impl Clone for Identity {
    }
 }

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 impl Identity {
    pub fn from_pem(pem: &[u8], key: &[u8]) -> Result<Self, Error> {
        Ok(Self {
            #[cfg(feature = "native-tls")]
            native_tls: Identity::from_pem_native_tls(pem, key)?,
-            #[cfg(feature = "rustls")]
+            #[cfg(feature = "rustls-tls")]
            rustls_tls: Identity::from_pem_rustls_tls(pem, key)?,
            #[cfg(feature = "boring-tls")]
            boring_tls: Identity::from_pem_boring_tls(pem, key)?,
@@ -654,19 +625,13 @@ impl Identity {
        native_tls::Identity::from_pkcs8(pem, key).map_err(error::tls)
    }

-    #[cfg(feature = "rustls")]
+    #[cfg(feature = "rustls-tls")]
    fn from_pem_rustls_tls(
        pem: &[u8],
        key: &[u8],
    ) -> Result<(Vec<CertificateDer<'static>>, PrivateKeyDer<'static>), Error> {
-        let key = match PrivateKeyDer::from_pem_slice(key) {
-            Ok(key) => key,
-            Err(pki_types::pem::Error::NoItemsFound) => {
-                return Err(error::tls("no private key found"))
-            }
-            Err(err) => return Err(error::tls(err)),
-        };
-
+        let mut key = key;
+        let key = rustls_pemfile::private_key(&mut key).unwrap().unwrap();
        Ok((vec![pem.to_owned().into()], key))
    }

@@ -681,16 +646,16 @@ impl Identity {
    }
 }

-#[cfg(feature = "rustls")]
+#[cfg(feature = "rustls-tls")]
 #[derive(Debug)]
 struct InvalidCertsVerifier {
    ignore_invalid_hostnames: bool,
    ignore_invalid_certs: bool,
    roots: RootCertStore,
-    crypto_provider: Arc<CryptoProvider>,
+    signature_algorithms: WebPkiSupportedAlgorithms,
 }

-#[cfg(feature = "rustls")]
+#[cfg(feature = "rustls-tls")]
 impl ServerCertVerifier for InvalidCertsVerifier {
    fn verify_server_cert(
        &self,
@@ -708,7 +673,7 @@ impl ServerCertVerifier for InvalidCertsVerifier {
                &self.roots,
                intermediates,
                now,
-                self.crypto_provider.signature_verification_algorithms.all,
+                self.signature_algorithms.all,
            )?;
        }

@@ -728,7 +693,7 @@ impl ServerCertVerifier for InvalidCertsVerifier {
            message,
            cert,
            dss,
-            &self.crypto_provider.signature_verification_algorithms,
+            &rustls::crypto::ring::default_provider().signature_verification_algorithms,
        )
    }

@@ -742,12 +707,12 @@ impl ServerCertVerifier for InvalidCertsVerifier {
            message,
            cert,
            dss,
-            &self.crypto_provider.signature_verification_algorithms,
+            &rustls::crypto::ring::default_provider().signature_verification_algorithms,
        )
    }

    fn supported_verify_schemes(&self) -> Vec<SignatureScheme> {
-        self.crypto_provider
+        rustls::crypto::ring::default_provider()
            .signature_verification_algorithms
            .supported_schemes()
    }
--- a/src/transport/smtp/connection_url.rs
+++ b/src/transport/smtp/connection_url.rs
@@ -1,8 +1,6 @@
-use std::borrow::Cow;
-
 use url::Url;

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::client::{Tls, TlsParameters};
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 use super::AsyncSmtpTransportBuilder;
@@ -64,7 +62,7 @@ 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
@@ -82,30 +80,30 @@ pub(crate) fn from_connection_url<B: TransportBuilder>(connection_url: &str) ->
        ("smtp", None) => {
            builder = builder.port(connection_url.port().unwrap_or(SMTP_PORT));
        }
-        #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+        #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
        ("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", feature = "boring-tls"))]
+        #[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", feature = "boring-tls"))]
+        #[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 {
@@ -117,7 +115,7 @@ 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::into_owned)
+                .map(|cow| cow.into_owned())
                .map_err(error::connection)
        };
        let credentials = Credentials::new(
--- a/src/transport/smtp/error.rs
+++ b/src/transport/smtp/error.rs
@@ -68,20 +68,15 @@ impl Error {
    }

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

-    /// Returns true if the error is because the transport was shut down
-    pub fn is_transport_shutdown(&self) -> bool {
-        matches!(self.inner.kind, Kind::TransportShutdown)
-    }
-
    /// Returns the status code, if the error was generated from a response.
    pub fn status(&self) -> Option<Code> {
        match self.inner.kind {
@@ -112,12 +107,10 @@ pub(crate) enum Kind {
    /// TLS error
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    Tls,
-    /// Transport shutdown error
-    TransportShutdown,
 }

 impl fmt::Debug for Error {
@@ -141,16 +134,15 @@ impl fmt::Display for Error {
            Kind::Client => f.write_str("internal client error")?,
            Kind::Network => f.write_str("network error")?,
            Kind::Connection => f.write_str("Connection error")?,
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            Kind::Tls => f.write_str("tls error")?,
-            Kind::TransportShutdown => f.write_str("transport has been shut down")?,
            Kind::Transient(code) => {
                write!(f, "transient error ({code})")?;
            }
            Kind::Permanent(code) => {
                write!(f, "permanent error ({code})")?;
            }
-        }
+        };

        if let Some(e) = &self.inner.source {
            write!(f, ": {e}")?;
@@ -193,11 +185,7 @@ pub(crate) fn connection<E: Into<BoxError>>(e: E) -> Error {
    Error::new(Kind::Connection, Some(e))
 }

-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(crate) fn tls<E: Into<BoxError>>(e: E) -> Error {
    Error::new(Kind::Tls, Some(e))
 }
-
-pub(crate) fn transport_shutdown() -> Error {
-    Error::new::<BoxError>(Kind::TransportShutdown, None)
-}
--- a/src/transport/smtp/extension.rs
+++ b/src/transport/smtp/extension.rs
@@ -129,8 +129,9 @@ impl Display for ServerInfo {
 impl ServerInfo {
    /// Parses a EHLO response to create a `ServerInfo`
    pub fn from_response(response: &Response) -> Result<ServerInfo, Error> {
-        let Some(name) = response.first_word() else {
-            return Err(error::response("Could not read server name"));
+        let name = match response.first_word() {
+            Some(name) => name,
+            None => return Err(error::response("Could not read server name")),
        };

        let mut features: HashSet<Extension> = HashSet::new();
@@ -168,7 +169,7 @@ impl ServerInfo {
                    }
                }
                _ => (),
-            }
+            };
        }

        Ok(ServerInfo {
--- a/src/transport/smtp/mod.rs
+++ b/src/transport/smtp/mod.rs
@@ -26,17 +26,43 @@
 //!
 //! The relay server can be the local email server, a specific host or a third-party service.
 //!
-//! #### Simple example with authentication
+//! #### Simple example
 //!
-//! A good starting point for sending emails via SMTP relay is to
-//! do the following:
+//! This is the most basic example of usage:
 //!
 //! ```rust,no_run
-//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls")))]
+//! # #[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:
+//!
+//! ```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::header::ContentType,
-//!     transport::smtp::authentication::{Credentials, Mechanism},
+//!     transport::smtp::{
+//!         authentication::{Credentials, Mechanism},
+//!         PoolConfig,
+//!     },
 //!     Message, SmtpTransport, Transport,
 //! };
 //!
@@ -45,39 +71,35 @@
 //!     .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 the SMTPS transport
-//! let sender = SmtpTransport::relay("smtp.example.com")?
+//! // Create TLS transport on port 587 with STARTTLS
+//! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
 //!     // Add credentials for authentication
 //!     .credentials(Credentials::new(
 //!         "username".to_owned(),
 //!         "password".to_owned(),
 //!     ))
-//!     // Optionally configure expected authentication mechanism
+//!     // 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
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! # Ok(())
 //! # }
 //! ```
 //!
-//! #### 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`].
+//! You can specify custom TLS settings:
 //!
 //! ```rust,no_run
-//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls")))]
+//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
 //! # fn test() -> Result<(), Box<dyn std::error::Error>> {
 //! use lettre::{
-//!     message::header::ContentType,
-//!     transport::smtp::authentication::{Credentials, Mechanism},
+//!     transport::smtp::client::{Tls, TlsParameters},
 //!     Message, SmtpTransport, Transport,
 //! };
 //!
@@ -86,106 +108,25 @@
 //!     .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 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")))]
-//! # 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)
+//! // Custom TLS configuration
+//! let tls = TlsParameters::builder("smtp.example.com".to_owned())
+//!     .dangerous_accept_invalid_certs(true)
 //!     .build()?;
 //!
-//! // Create the SMTPS transport
+//! // Create TLS transport on port 465
 //! let sender = SmtpTransport::relay("smtp.example.com")?
-//!     .tls(Tls::Wrapper(tls))
+//!     // Custom TLS configuration
+//!     .tls(Tls::Required(tls))
 //!     .build();
 //!
 //! // Send the email via remote relay
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! # 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")))]
-//! # 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;

@@ -199,7 +140,7 @@ pub use self::{
    error::Error,
    transport::{SmtpTransport, SmtpTransportBuilder},
 };
-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::client::TlsParameters;
 use crate::transport::smtp::{
    authentication::{Credentials, Mechanism, DEFAULT_MECHANISMS},
--- a/src/transport/smtp/pool/async_impl.rs
+++ b/src/transport/smtp/pool/async_impl.rs
@@ -1,5 +1,6 @@
 use std::{
    fmt::{self, Debug},
+    mem,
    ops::{Deref, DerefMut},
    sync::{Arc, OnceLock},
    time::{Duration, Instant},
@@ -14,15 +15,11 @@ use super::{
    super::{client::AsyncSmtpConnection, Error},
    PoolConfig,
 };
-use crate::{
-    executor::SpawnHandle,
-    transport::smtp::{async_transport::AsyncSmtpClient, error},
-    Executor,
-};
+use crate::{executor::SpawnHandle, transport::smtp::async_transport::AsyncSmtpClient, Executor};

-pub(crate) struct Pool<E: Executor> {
+pub struct Pool<E: Executor> {
    config: PoolConfig,
-    connections: Mutex<Option<Vec<ParkedConnection>>>,
+    connections: Mutex<Vec<ParkedConnection>>,
    client: AsyncSmtpClient<E>,
    handle: OnceLock<E::Handle>,
 }
@@ -32,16 +29,16 @@ struct ParkedConnection {
    since: Instant,
 }

-pub(crate) struct PooledConnection<E: Executor> {
+pub struct PooledConnection<E: Executor> {
    conn: Option<AsyncSmtpConnection>,
    pool: Arc<Pool<E>>,
 }

 impl<E: Executor> Pool<E> {
-    pub(crate) fn new(config: PoolConfig, client: AsyncSmtpClient<E>) -> Arc<Self> {
+    pub fn new(config: PoolConfig, client: AsyncSmtpClient<E>) -> Arc<Self> {
        let pool = Arc::new(Self {
            config,
-            connections: Mutex::new(Some(Vec::new())),
+            connections: Mutex::new(Vec::new()),
            client,
            handle: OnceLock::new(),
        });
@@ -63,10 +60,6 @@ impl<E: Executor> Pool<E> {
                            #[allow(clippy::needless_collect)]
                            let (count, dropped) = {
                                let mut connections = pool.connections.lock().await;
-                                let Some(connections) = connections.as_mut() else {
-                                    // The transport was shut down
-                                    return;
-                                };

                                let to_drop = connections
                                    .iter()
@@ -85,7 +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) => {
@@ -99,11 +92,6 @@ impl<E: Executor> Pool<E> {
                                };

                                let mut connections = pool.connections.lock().await;
-                                let Some(connections) = connections.as_mut() else {
-                                    // The transport was shut down
-                                    return;
-                                };
-
                                connections.push(ParkedConnection::park(conn));

                                #[cfg(feature = "tracing")]
@@ -121,7 +109,7 @@ impl<E: Executor> Pool<E> {
                                #[cfg(feature = "tracing")]
                                tracing::debug!("dropped {} idle connections", dropped.len());

-                                abort_concurrent(dropped.into_iter().map(ParkedConnection::unpark))
+                                abort_concurrent(dropped.into_iter().map(|conn| conn.unpark()))
                                    .await;
                            }
                        }
@@ -146,29 +134,10 @@ impl<E: Executor> Pool<E> {
        pool
    }

-    pub(crate) async fn shutdown(&self) {
-        let connections = { self.connections.lock().await.take() };
-        if let Some(connections) = connections {
-            stream::iter(connections)
-                .for_each_concurrent(8, |conn| async move {
-                    conn.unpark().abort().await;
-                })
-                .await;
-        }
-
-        if let Some(handle) = self.handle.get() {
-            handle.shutdown().await;
-        }
-    }
-
-    pub(crate) async fn connection(self: &Arc<Self>) -> Result<PooledConnection<E>, Error> {
+    pub async fn connection(self: &Arc<Self>) -> Result<PooledConnection<E>, Error> {
        loop {
            let conn = {
                let mut connections = self.connections.lock().await;
-                let Some(connections) = connections.as_mut() else {
-                    // The transport was shut down
-                    return Err(error::transport_shutdown());
-                };
                connections.pop()
            };

@@ -212,20 +181,13 @@ impl<E: Executor> Pool<E> {
            #[cfg(feature = "tracing")]
            tracing::debug!("recycling connection");

-            let mut connections_guard = self.connections.lock().await;
-
-            if let Some(connections) = connections_guard.as_mut() {
-                if connections.len() >= self.config.max_size as usize {
-                    drop(connections_guard);
-                    conn.abort().await;
-                } else {
-                    let conn = ParkedConnection::park(conn);
-                    connections.push(conn);
-                }
-            } else {
-                // The pool has already been shut down
-                drop(connections_guard);
+            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);
            }
        }
    }
@@ -238,13 +200,7 @@ impl<E: Executor> Debug for Pool<E> {
            .field(
                "connections",
                &match self.connections.try_lock() {
-                    Some(connections) => {
-                        if let Some(connections) = connections.as_ref() {
-                            format!("{} connections", connections.len())
-                        } else {
-                            "SHUT DOWN".to_owned()
-                        }
-                    }
+                    Some(connections) => format!("{} connections", connections.len()),

                    None => "LOCKED".to_owned(),
                },
@@ -266,16 +222,14 @@ impl<E: Executor> Drop for Pool<E> {
        #[cfg(feature = "tracing")]
        tracing::debug!("dropping Pool");

-        let connections = self.connections.get_mut().take();
+        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;
            }

-            if let Some(connections) = connections {
-                abort_concurrent(connections.into_iter().map(ParkedConnection::unpark)).await;
-            }
+            abort_concurrent(connections.into_iter().map(|conn| conn.unpark())).await;
        });
    }
 }
--- a/src/transport/smtp/pool/mod.rs
+++ b/src/transport/smtp/pool/mod.rs
@@ -1,8 +1,8 @@
 use std::time::Duration;

 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
-pub(super) mod async_impl;
-pub(super) mod sync_impl;
+pub mod async_impl;
+pub mod sync_impl;

 /// Configuration for a connection pool
 #[derive(Debug, Clone)]
--- a/src/transport/smtp/pool/sync_impl.rs
+++ b/src/transport/smtp/pool/sync_impl.rs
@@ -1,7 +1,8 @@
 use std::{
    fmt::{self, Debug},
+    mem,
    ops::{Deref, DerefMut},
-    sync::{mpsc, Arc, Mutex, TryLockError},
+    sync::{Arc, Mutex, TryLockError},
    thread,
    time::{Duration, Instant},
 };
@@ -10,12 +11,11 @@ use super::{
    super::{client::SmtpConnection, Error},
    PoolConfig,
 };
-use crate::transport::smtp::{error, transport::SmtpClient};
+use crate::transport::smtp::transport::SmtpClient;

-pub(crate) struct Pool {
+pub struct Pool {
    config: PoolConfig,
-    connections: Mutex<Option<Vec<ParkedConnection>>>,
-    thread_terminator: mpsc::SyncSender<()>,
+    connections: Mutex<Vec<ParkedConnection>>,
    client: SmtpClient,
 }

@@ -24,19 +24,16 @@ struct ParkedConnection {
    since: Instant,
 }

-pub(crate) struct PooledConnection {
+pub struct PooledConnection {
    conn: Option<SmtpConnection>,
    pool: Arc<Pool>,
 }

 impl Pool {
-    pub(crate) fn new(config: PoolConfig, client: SmtpClient) -> Arc<Self> {
-        let (thread_tx, thread_rx) = mpsc::sync_channel(1);
-
+    pub fn new(config: PoolConfig, client: SmtpClient) -> Arc<Self> {
        let pool = Arc::new(Self {
            config,
-            connections: Mutex::new(Some(Vec::new())),
-            thread_terminator: thread_tx,
+            connections: Mutex::new(Vec::new()),
            client,
        });

@@ -57,10 +54,6 @@ impl Pool {
                        #[allow(clippy::needless_collect)]
                        let (count, dropped) = {
                            let mut connections = pool.connections.lock().unwrap();
-                            let Some(connections) = connections.as_mut() else {
-                                // The transport was shut down
-                                return;
-                            };

                            let to_drop = connections
                                .iter()
@@ -79,7 +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) => {
@@ -93,11 +86,6 @@ impl Pool {
                            };

                            let mut connections = pool.connections.lock().unwrap();
-                            let Some(connections) = connections.as_mut() else {
-                                // The transport was shut down
-                                return;
-                            };
-
                            connections.push(ParkedConnection::park(conn));

                            #[cfg(feature = "tracing")]
@@ -121,15 +109,7 @@ impl Pool {
                            }
                        }

-                        drop(pool);
-
-                        match thread_rx.recv_timeout(idle_timeout) {
-                            Ok(()) | Err(mpsc::RecvTimeoutError::Disconnected) => {
-                                // The transport was shut down
-                                return;
-                            }
-                            Err(mpsc::RecvTimeoutError::Timeout) => {}
-                        }
+                        thread::sleep(idle_timeout);
                    }
                })
                .expect("couldn't spawn the Pool thread");
@@ -138,25 +118,10 @@ impl Pool {
        pool
    }

-    pub(crate) fn shutdown(&self) {
-        let connections = { self.connections.lock().unwrap().take() };
-        if let Some(connections) = connections {
-            for conn in connections {
-                conn.unpark().abort();
-            }
-        }
-
-        _ = self.thread_terminator.try_send(());
-    }
-
-    pub(crate) fn connection(self: &Arc<Self>) -> Result<PooledConnection, Error> {
+    pub fn connection(self: &Arc<Self>) -> Result<PooledConnection, Error> {
        loop {
            let conn = {
                let mut connections = self.connections.lock().unwrap();
-                let Some(connections) = connections.as_mut() else {
-                    // The transport was shut down
-                    return Err(error::transport_shutdown());
-                };
                connections.pop()
            };

@@ -200,20 +165,13 @@ impl Pool {
            #[cfg(feature = "tracing")]
            tracing::debug!("recycling connection");

-            let mut connections_guard = self.connections.lock().unwrap();
-
-            if let Some(connections) = connections_guard.as_mut() {
-                if connections.len() >= self.config.max_size as usize {
-                    drop(connections_guard);
-                    conn.abort();
-                } else {
-                    let conn = ParkedConnection::park(conn);
-                    connections.push(conn);
-                }
-            } else {
-                // The pool has already been shut down
-                drop(connections_guard);
+            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);
            }
        }
    }
@@ -226,13 +184,7 @@ impl Debug for Pool {
            .field(
                "connections",
                &match self.connections.try_lock() {
-                    Ok(connections) => {
-                        if let Some(connections) = connections.as_ref() {
-                            format!("{} connections", connections.len())
-                        } else {
-                            "SHUT DOWN".to_owned()
-                        }
-                    }
+                    Ok(connections) => format!("{} connections", connections.len()),

                    Err(TryLockError::WouldBlock) => "LOCKED".to_owned(),
                    Err(TryLockError::Poisoned(_)) => "POISONED".to_owned(),
@@ -248,11 +200,10 @@ impl Drop for Pool {
        #[cfg(feature = "tracing")]
        tracing::debug!("dropping Pool");

-        if let Some(connections) = self.connections.get_mut().unwrap().take() {
-            for conn in connections {
-                let mut conn = conn.unpark();
-                conn.abort();
-            }
+        let connections = mem::take(&mut *self.connections.get_mut().unwrap());
+        for conn in connections {
+            let mut conn = conn.unpark();
+            conn.abort();
        }
    }
 }
--- a/src/transport/smtp/response.rs
+++ b/src/transport/smtp/response.rs
@@ -12,8 +12,8 @@ use nom::{
    bytes::streaming::{tag, take_until},
    combinator::{complete, map},
    multi::many0,
-    sequence::preceded,
-    IResult, Parser,
+    sequence::{preceded, tuple},
+    IResult,
 };

 use crate::transport::smtp::{error, Error};
@@ -221,8 +221,7 @@ fn parse_severity(i: &str) -> IResult<&str, Severity> {
        map(tag("3"), |_| Severity::PositiveIntermediate),
        map(tag("4"), |_| Severity::TransientNegativeCompletion),
        map(tag("5"), |_| Severity::PermanentNegativeCompletion),
-    ))
-    .parse(i)
+    ))(i)
 }

 fn parse_category(i: &str) -> IResult<&str, Category> {
@@ -233,8 +232,7 @@ fn parse_category(i: &str) -> IResult<&str, Category> {
        map(tag("3"), |_| Category::Unspecified3),
        map(tag("4"), |_| Category::Unspecified4),
        map(tag("5"), |_| Category::MailSystem),
-    ))
-    .parse(i)
+    ))(i)
 }

 fn parse_detail(i: &str) -> IResult<&str, Detail> {
@@ -249,20 +247,18 @@ fn parse_detail(i: &str) -> IResult<&str, Detail> {
        map(tag("7"), |_| Detail::Seven),
        map(tag("8"), |_| Detail::Eight),
        map(tag("9"), |_| Detail::Nine),
-    ))
-    .parse(i)
+    ))(i)
 }

 pub(crate) fn parse_response(i: &str) -> IResult<&str, Response> {
-    let (i, lines) = many0((
+    let (i, lines) = many0(tuple((
        parse_code,
        preceded(tag("-"), take_until("\r\n")),
        tag("\r\n"),
-    ))
-    .parse(i)?;
+    )))(i)?;
    let (i, (last_code, last_line)) =
-        (parse_code, preceded(tag(" "), take_until("\r\n"))).parse(i)?;
-    let (i, _) = complete(tag("\r\n")).parse(i)?;
+        tuple((parse_code, preceded(tag(" "), take_until("\r\n"))))(i)?;
+    let (i, _) = complete(tag("\r\n"))(i)?;

    // Check that all codes are equal.
    if !lines.iter().all(|&(code, _, _)| code == last_code) {
--- a/src/transport/smtp/transport.rs
+++ b/src/transport/smtp/transport.rs
@@ -7,35 +7,11 @@ use super::pool::sync_impl::Pool;
 #[cfg(feature = "pool")]
 use super::PoolConfig;
 use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
-#[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{address::Envelope, Transport};

-/// 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`].
+/// Sends emails using the SMTP protocol
 #[cfg_attr(docsrs, doc(cfg(feature = "smtp-transport")))]
 #[derive(Clone)]
 pub struct SmtpTransport {
@@ -56,15 +32,10 @@ impl Transport for SmtpTransport {
        let result = conn.send(envelope, email)?;

        #[cfg(not(feature = "pool"))]
-        conn.abort();
+        conn.quit()?;

        Ok(result)
    }
-
-    fn shutdown(&self) {
-        #[cfg(feature = "pool")]
-        self.inner.shutdown();
-    }
 }

 impl Debug for SmtpTransport {
@@ -82,10 +53,10 @@ impl SmtpTransport {
    ///
    /// Creates an encrypted transport over submissions port, using the provided domain
    /// to validate TLS certificates.
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
        let tls_parameters = TlsParameters::new(relay.into())?;
@@ -106,10 +77,10 @@ impl SmtpTransport {
    ///
    /// An error is returned if the connection can't be upgraded. No credentials
    /// or emails will be sent to the server, protecting from downgrade attacks.
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn starttls_relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
        let tls_parameters = TlsParameters::new(relay.into())?;
@@ -144,72 +115,54 @@ impl SmtpTransport {

    /// Creates a `SmtpTransportBuilder` from a connection URL
    ///
-    /// 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.
+    /// 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 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:
+    /// <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>
    ///
    /// ```rust,no_run
    /// use lettre::{
@@ -217,7 +170,6 @@ 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())
@@ -228,17 +180,20 @@ impl SmtpTransport {
    ///     .unwrap();
    ///
    /// // Open a remote connection to example
-    /// let mailer = SmtpTransport::from_url("smtps://username:password@smtp.example.com")?.build();
+    /// let mailer = SmtpTransport::from_url("smtps://username:password@smtp.example.com:465")
+    ///     .unwrap()
+    ///     .build();
    ///
    /// // Send the email
-    /// mailer.send(&email)?;
-    /// # Ok(())
-    /// # }
+    /// match mailer.send(&email) {
+    ///     Ok(_) => println!("Email sent successfully!"),
+    ///     Err(e) => panic!("Could not send email: {e:?}"),
+    /// }
    /// ```
-    #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn from_url(connection_url: &str) -> Result<SmtpTransportBuilder, Error> {
        super::connection_url::from_connection_url(connection_url)
@@ -291,7 +246,7 @@ impl SmtpTransportBuilder {
        self
    }

-    /// Set the authentication credentials to use
+    /// Set the authentication mechanism to use
    pub fn credentials(mut self, credentials: Credentials) -> Self {
        self.info.credentials = Some(credentials);
        self
@@ -310,38 +265,16 @@ 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", feature = "boring-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
    #[cfg_attr(
        docsrs,
-        doc(cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls")))
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
    )]
    pub fn tls(mut self, tls: Tls) -> Self {
        self.info.tls = tls;
@@ -374,7 +307,7 @@ impl SmtpTransportBuilder {

 /// Build client
 #[derive(Debug, Clone)]
-pub(super) struct SmtpClient {
+pub struct SmtpClient {
    info: SmtpInfo,
 }

@@ -382,10 +315,10 @@ impl SmtpClient {
    /// Creates a new connection directly usable to send emails
    ///
    /// Handles encryption and authentication
-    pub(super) fn connection(&self) -> Result<SmtpConnection, Error> {
+    pub fn connection(&self) -> Result<SmtpConnection, Error> {
        #[allow(clippy::match_single_binding)]
        let tls_parameters = match &self.info.tls {
-            #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
            Tls::Wrapper(tls_parameters) => Some(tls_parameters),
            _ => None,
        };
@@ -399,7 +332,7 @@ impl SmtpClient {
            None,
        )?;

-        #[cfg(any(feature = "native-tls", feature = "rustls", feature = "boring-tls"))]
+        #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
        match &self.info.tls {
            Tls::Opportunistic(tls_parameters) => {
                if conn.can_starttls() {
--- a/src/transport/smtp/util.rs
+++ b/src/transport/smtp/util.rs
@@ -4,9 +4,9 @@ use std::fmt::{Display, Formatter, Result as FmtResult};

 /// Encode a string as xtext
 #[derive(Debug)]
-pub(crate) struct XText<'a>(pub(crate) &'a str);
+pub struct XText<'a>(pub &'a str);

-impl Display for XText<'_> {
+impl<'a> Display for XText<'a> {
    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
        let mut rest = self.0;
        while let Some(idx) = rest.find(|c| c < '!' || c == '+' || c == '=') {
@@ -38,7 +38,9 @@ mod tests {
            ("bjørn", "bjørn"),
            ("Ø+= ❤️‰", "Ø+2B+3D+20❤️‰"),
            ("+", "+2B"),
-        ] {
+        ]
+        .iter()
+        {
            assert_eq!(format!("{}", XText(input)), (*expect).to_owned());
        }
    }
--- a/src/transport/stub/mod.rs
+++ b/src/transport/stub/mod.rs
@@ -11,9 +11,7 @@
 //! ```rust
 //! # #[cfg(feature = "builder")]
 //! # {
-//! use lettre::{
-//!     message::header::ContentType, transport::stub::StubTransport, Message, Transport,
-//! };
+//! use lettre::{transport::stub::StubTransport, Message, Transport};
 //!
 //! # use std::error::Error;
 //! # fn try_main() -> Result<(), Box<dyn Error>> {
@@ -22,11 +20,11 @@
 //!     .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();
-//! sender.send(&email)?;
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
 //! assert_eq!(
 //!     sender.messages(),
 //!     vec![(