Prepare v0.11.8 (#985)

Add a method that, when using a TLS toolkit that supports it, returns
the entire certificate chain. This is useful, for example, when
implementing DANE support which has directives that apply to the issuer
and not just to the leaf certificate.

Not all TLS toolkits support this, so the code will panic if the method
is called when using a TLS toolkit that has no way to return these
certificates.

.github/workflows/test.yml

@@ -13,16 +13,16 @@ env:
 
 jobs:
   rustfmt:
-    name: rustfmt / nightly-2022-02-11
+    name: rustfmt / nightly-2024-09-01
     runs-on: ubuntu-latest
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Install rust
         run: |
-          rustup default nightly-2022-02-11
+          rustup default nightly-2024-09-01
           rustup component add rustfmt
 
       - name: cargo fmt
@@ -34,7 +34,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Install rust
         run: |
@@ -50,19 +50,13 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
-
-      - name: Setup cache
-        uses: actions/cache@v2
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-check
+        uses: actions/checkout@v4
 
       - name: Install rust
         run: rustup update --no-self-update stable
+
+      - name: Setup cache
+        uses: Swatinem/rust-cache@v2
       
       - name: Install cargo hack
         run: cargo install cargo-hack --debug
@@ -81,27 +75,21 @@ jobs:
             rust: stable
           - name: beta
             rust: beta
-          - name: 1.56.0
-            rust: 1.56.0
+          - name: '1.70'
+            rust: '1.70'
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
-
-      - name: Setup cache
-        uses: actions/cache@v2
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-test-${{ matrix.rust }}
+        uses: actions/checkout@v4
 
       - name: Install rust
         run: |
           rustup default ${{ matrix.rust }}
           rustup update --no-self-update ${{ matrix.rust }}
 
+      - name: Setup cache
+        uses: Swatinem/rust-cache@v2
+
       - name: Install postfix  
         run: |
           DEBIAN_FRONTEND=noninteractive sudo apt-get update
@@ -125,7 +113,10 @@ jobs:
         run: sudo apt -y install python3-dkim
 
       - name: Work around early dependencies MSRV bump
-        run: cargo update -p async-global-executor --precise 2.0.4
+        run: |
+          cargo update -p anstyle --precise 1.0.2
+          cargo update -p clap --precise 4.3.24
+          cargo update -p clap_lex --precise 0.5.0
 
       - name: Test with no default features
         run: cargo test --no-default-features
@@ -133,14 +124,17 @@ jobs:
       - name: Test with default features
         run: cargo test
 
-      - name: Test with all features
-        run: cargo test --all-features
+      - name: Test with all features (-native-tls)
+        run: cargo test --no-default-features --features async-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-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
 #    runs-on: ubuntu-latest
 #    steps:
-#      - uses: actions/checkout@v2
+#      - uses: actions/checkout@v4
 #      - uses: actions-rs/toolchain@v1
 #        with:
 #          toolchain: nightly

.gitignore

@@ -4,4 +4,3 @@
 lettre.sublime-*
 lettre.iml
 target/
-/Cargo.lock

CHANGELOG.md

@@ -1,5 +1,271 @@
+<a name="v0.11.8"></a>
+### v0.11.8 (2024-09-03)
+
+#### Features
+
+* Add mTLS support ([#974])
+* Implement `accept_invalid_hostnames` for rustls ([#977])
+* Provide certificate chain for peer certificates when using `rustls` or `boring-tls` ([#976])
+
+#### Changes
+
+* Make `HeaderName` comparisons via `PartialEq` case insensitive ([#980])
+
+#### Misc
+
+* Fix clippy warnings ([#979])
+* Replace manual impl of `#[non_exhaustive]` for `InvalidHeaderName` ([#981])
+
+[#974]: https://github.com/lettre/lettre/pull/974
+[#976]: https://github.com/lettre/lettre/pull/976
+[#977]: https://github.com/lettre/lettre/pull/977
+[#980]: https://github.com/lettre/lettre/pull/980
+[#981]: https://github.com/lettre/lettre/pull/981
+
+<a name="v0.11.7"></a>
+### v0.11.7 (2024-04-23)
+
+#### Misc
+
+* Bump `hostname` to v0.4 ([#956])
+* Fix `tracing` message consistency ([#960])
+* Bump minimum required `rustls` to v0.23.5 ([#958])
+* Dropped use of `ref` syntax in the entire project ([#959])
+
+[#956]: https://github.com/lettre/lettre/pull/956
+[#958]: https://github.com/lettre/lettre/pull/958
+[#959]: https://github.com/lettre/lettre/pull/959
+[#960]: https://github.com/lettre/lettre/pull/960
+
+<a name="v0.11.6"></a>
+### v0.11.6 (2024-03-28)
+
+#### Bug fixes
+
+* Upgraded `email-encoding` to v0.3 - fixing multiple encoding bugs in the process ([#952])
+
+#### Misc
+
+* Updated copyright year in license ([#954])
+
+[#952]: https://github.com/lettre/lettre/pull/952
+[#954]: https://github.com/lettre/lettre/pull/954
+
+<a name="v0.11.5"></a>
+### v0.11.5 (2024-03-25)
+
+#### Features
+
+* Support SMTP SASL draft login challenge ([#911])
+* Add conversion from SMTP response code to integer ([#941])
+
+#### Misc
+
+* Upgrade `rustls` to v0.23 ([#950])
+* Bump `base64` to v0.22 ([#945])
+* Fix typos in documentation ([#943], [#944])
+* Add `Cargo.lock` ([#942])
+
+[#911]: https://github.com/lettre/lettre/pull/911
+[#941]: https://github.com/lettre/lettre/pull/941
+[#942]: https://github.com/lettre/lettre/pull/942
+[#943]: https://github.com/lettre/lettre/pull/943
+[#944]: https://github.com/lettre/lettre/pull/944
+[#945]: https://github.com/lettre/lettre/pull/945
+[#950]: https://github.com/lettre/lettre/pull/950
+
+<a name="v0.11.4"></a>
+### v0.11.4 (2024-01-28)
+
+#### Bug fixes
+
+* Percent decode credentials in SMTP connect URL ([#932], [#934])
+* Fix mimebody DKIM body-hash computation ([#923])
+
+[#923]: https://github.com/lettre/lettre/pull/923
+[#932]: https://github.com/lettre/lettre/pull/932
+[#934]: https://github.com/lettre/lettre/pull/934
+
+<a name="v0.11.3"></a>
+### v0.11.3 (2024-01-02)
+
+#### Features
+
+* Derive `Clone` for `FileTransport` and `AsyncFileTransport` ([#924])
+* Derive `Debug` for `SmtpTransport` ([#925])
+
+#### Misc
+
+* Upgrade `rustls` to v0.22 ([#921])
+* Drop once_cell dependency in favor of OnceLock from std ([#928])
+
+[#921]: https://github.com/lettre/lettre/pull/921
+[#924]: https://github.com/lettre/lettre/pull/924
+[#925]: https://github.com/lettre/lettre/pull/925
+[#928]: https://github.com/lettre/lettre/pull/928
+
+<a name="v0.11.2"></a>
+### v0.11.2 (2023-11-23)
+
+#### Upgrade notes
+
+* MSRV is now 1.70 ([#916])
+
+#### Misc
+
+* Bump `idna` to v0.5 ([#918])
+* Bump `boring` and `tokio-boring` to v4 ([#915])
+
+[#915]: https://github.com/lettre/lettre/pull/915
+[#916]: https://github.com/lettre/lettre/pull/916
+[#918]: https://github.com/lettre/lettre/pull/918
+
+<a name="v0.11.1"></a>
+### v0.11.1 (2023-10-24)
+
+#### Bug fixes
+
+* Fix `webpki-roots` certificate store setup ([#909])
+
+[#909]: https://github.com/lettre/lettre/pull/909
+
+<a name="v0.11.0"></a>
+### v0.11.0 (2023-10-15)
+
+While this release technically contains breaking changes, we expect most projects
+to be able to upgrade by only bumping the version in `Cargo.toml`.
+
+#### Upgrade notes
+
+* MSRV is now 1.65 ([#869] and [#881])
+* `AddressError` is now marked as `#[non_exhaustive]` ([#839])
+
+#### Features
+
+* Improve mailbox parsing ([#839])
+* Add construction of SMTP transport from URL ([#901])
+* Add `From<Address>` implementation for `Mailbox` ([#879])
+
+#### Misc
+
+* Bump `socket2` to v0.5 ([#868])
+* Bump `idna` to v0.4, `fastrand` to v2, `quoted_printable` to v0.5, `rsa` to v0.9 ([#882])
+* Bump `webpki-roots` to v0.25 ([#884] and [#890])
+* Bump `ed25519-dalek` to v2 fixing RUSTSEC-2022-0093 ([#896])
+* Bump `boring`ssl crates to v3 ([#897])
+
+[#839]: https://github.com/lettre/lettre/pull/839
+[#868]: https://github.com/lettre/lettre/pull/868
+[#869]: https://github.com/lettre/lettre/pull/869
+[#879]: https://github.com/lettre/lettre/pull/879
+[#881]: https://github.com/lettre/lettre/pull/881
+[#882]: https://github.com/lettre/lettre/pull/882
+[#884]: https://github.com/lettre/lettre/pull/884
+[#890]: https://github.com/lettre/lettre/pull/890
+[#896]: https://github.com/lettre/lettre/pull/896
+[#897]: https://github.com/lettre/lettre/pull/897
+[#901]: https://github.com/lettre/lettre/pull/901
+
+<a name="v0.10.4"></a>
+### v0.10.4 (2023-04-02)
+
+#### Misc
+
+* Bumped rustls to 0.21 and all related dependencies ([#867])
+
+[#867]: https://github.com/lettre/lettre/pull/867
+
+<a name="v0.10.3"></a>
+### v0.10.3 (2023-02-20)
+
+#### Announcements
+
+It was found that what had been used until now as a basic lettre 0.10
+`MessageBuilder::body` example failed to mention that for maximum
+compatibility with various email clients a `Content-Type` header
+should always be present in the message.
+
+##### Before
+
+```rust
+Message::builder()
+  // [...] some headers skipped for brevity
+  .body(String::from("A plaintext or html body"))?
+```
+
+##### Patch
+
+```diff
+ Message::builder()
+   // [...] some headers skipped for brevity
++  .header(ContentType::TEXT_PLAIN) // or `TEXT_HTML` if the body is html
+   .body(String::from("A plaintext or html body"))?
+```
+
+#### Features
+
+* Add support for rustls-native-certs when using rustls ([#843])
+
+[#843]: https://github.com/lettre/lettre/pull/843
+
+<a name="v0.10.2"></a>
+### v0.10.2 (2023-01-29)
+
+#### Upgrade notes
+
+* MSRV is now 1.60 ([#828])
+
+#### Features
+
+* Allow providing a custom `tokio` stream for `AsyncSmtpTransport` ([#805])
+* Return whole SMTP error message ([#821])
+
+#### Bug fixes
+
+* Mailbox displays wrongly when containing a comma and a non-ascii char in its name ([#827])
+* Require `quoted_printable` ^0.4.6 in order to fix encoding of tabs and spaces at the end of line ([#837])
+
+#### Misc
+
+* Increase tracing ([#848])
+* Bump `idna` to 0.3 ([#816])
+* Update `base64` to 0.21 ([#840] and [#851])
+* Update `rsa` to 0.8 ([#829] and [#852])
+
+[#805]: https://github.com/lettre/lettre/pull/805
+[#816]: https://github.com/lettre/lettre/pull/816
+[#821]: https://github.com/lettre/lettre/pull/821
+[#827]: https://github.com/lettre/lettre/pull/827
+[#828]: https://github.com/lettre/lettre/pull/828
+[#829]: https://github.com/lettre/lettre/pull/829
+[#837]: https://github.com/lettre/lettre/pull/837
+[#840]: https://github.com/lettre/lettre/pull/840
+[#848]: https://github.com/lettre/lettre/pull/848
+[#851]: https://github.com/lettre/lettre/pull/851
+[#852]: https://github.com/lettre/lettre/pull/852
+
+<a name="v0.10.1"></a>
+### v0.10.1 (2022-07-20)
+
+#### Features
+
+* Add `boring-tls` support for `SmtpTransport` and `AsyncSmtpTransport`. The latter is only supported with the tokio runtime. ([#797]) ([#798])
+* Make the minimum TLS version configurable. ([#799]) ([#800])
+
+#### Bug Fixes
+
+* Ensure connections are closed on abort. ([#801])
+* Fix SMTP dot stuffing. ([#803])
+
+[#797]: https://github.com/lettre/lettre/pull/797
+[#798]: https://github.com/lettre/lettre/pull/798
+[#799]: https://github.com/lettre/lettre/pull/799
+[#800]: https://github.com/lettre/lettre/pull/800
+[#801]: https://github.com/lettre/lettre/pull/801
+[#803]: https://github.com/lettre/lettre/pull/803
+
 <a name="v0.10.0"></a>
-### v0.10.0 (unreleased)
+### v0.10.0 (2022-06-29)
 
 #### Upgrade notes
 
@@ -54,7 +320,7 @@ Several breaking changes were made between 0.9 and 0.10, but changes should be s
 * Update `hostname` to 0.3
 * Update to `nom` 6
 * Replace `log` with `tracing`
-* Move CI to Github Actions
+* Move CI to GitHub Actions
 * Use criterion for benchmarks
 
 <a name="v0.9.2"></a>

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 ## Contributing to Lettre
 
-The following guidelines are inspired from the [hyper project](https://github.com/hyperium/hyper/blob/master/CONTRIBUTING.md).
+The following guidelines are inspired by the [hyper project](https://github.com/hyperium/hyper/blob/master/CONTRIBUTING.md).
 
 ### Code formatting
 

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "lettre"
 # remember to update html_root_url and README.md (Cargo.toml example and deps.rs badge)
-version = "0.10.0"
+version = "0.11.8"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -11,7 +11,7 @@ authors = ["Alexis Mousset <contact@amousset.me>", "Paolo Barbolini <paolo@paolo
 categories = ["email", "network-programming"]
 keywords = ["email", "smtp", "mailer", "message", "sendmail"]
 edition = "2021"
-rust-version = "1.56"
+rust-version = "1.70"
 
 [badges]
 is-it-maintained-issue-resolution = { repository = "lettre/lettre" }
@@ -19,17 +19,17 @@ is-it-maintained-open-issues = { repository = "lettre/lettre" }
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-idna = "0.2"
-once_cell = { version = "1", optional = true }
+chumsky = "0.9"
+idna = "1"
 tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # feature
 
 # builder
 httpdate = { version = "1", optional = true }
 mime = { version = "0.3.4", optional = true }
-fastrand = { version = "1.4", optional = true }
-quoted_printable = { version = "0.4", optional = true }
-base64 = { version = "0.13", optional = true }
-email-encoding = { version = "0.1.1", optional = true }
+fastrand = { version = "2.0", optional = true }
+quoted_printable = { version = "0.5", optional = true }
+base64 = { version = "0.22", optional = true }
+email-encoding = { version = "0.3", optional = true }
 
 # file transport
 uuid = { version = "1", features = ["v4"], optional = true }
@@ -38,14 +38,19 @@ serde_json = { version = "1", optional = true }
 
 # smtp-transport
 nom = { version = "7", optional = true }
-hostname = { version = "0.3", optional = true } # feature
-socket2 = { version = "0.4.4", 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", optional = true } # feature
-rustls = { version = "0.20", features = ["dangerous_configuration"], optional = true }
-rustls-pemfile = { version = "1", optional = true }
-webpki-roots = { version = "0.22", 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 = "0.7", optional = true }
+rustls-pki-types = { version = "1.7", optional = true }
+webpki-roots = { version = "0.26", optional = true }
+boring = { version = "4", optional = true }
 
 # async
 futures-io = { version = "0.3.7", optional = true }
@@ -53,27 +58,26 @@ futures-util = { version = "0.3.7", default-features = false, features = ["io"],
 async-trait = { version = "0.1", optional = true }
 
 ## async-std
-async-std = { version = "1.8", optional = true, features = ["unstable"] }
-#async-native-tls = { version = "0.3.3", optional = true }
-futures-rustls = { version = "0.22", optional = true }
+async-std = { version = "1.8", optional = true }
+futures-rustls = { version = "0.26", default-features = false, features = ["logging", "tls12", "ring"], optional = true }
 
 ## tokio
-tokio1_crate = { package = "tokio", version = "1", features = ["fs", "rt", "process", "time", "net", "io-util"], optional = true }
+tokio1_crate = { package = "tokio", version = "1", optional = true }
 tokio1_native_tls_crate = { package = "tokio-native-tls", version = "0.3", optional = true }
-tokio1_rustls = { package = "tokio-rustls", version = "0.23", optional = true }
+tokio1_rustls = { package = "tokio-rustls", version = "0.26", default-features = false, features = ["logging", "tls12", "ring"], optional = true }
+tokio1_boring = { package = "tokio-boring", version = "4", optional = true }
 
 ## dkim
-sha2 = { version = "0.10", optional = true }
-rsa = { version = "0.6.0", optional = true }
-ed25519-dalek = { version = "1.0.1", optional = true }
-regex = { version = "1", default-features = false, features = ["std"], optional = true }
+sha2 = { version = "0.10", optional = true, features = ["oid"] }
+rsa = { version = "0.9", optional = true }
+ed25519-dalek = { version = "2", optional = true }
 
 # email formats
 email_address = { version = "0.2.1", default-features = false }
 
 [dev-dependencies]
 pretty_assertions = "1"
-criterion = "0.3"
+criterion = "0.5"
 tracing = { version = "0.1.16", default-features = false, features = ["std"] }
 tracing-subscriber = "0.3"
 glob = "0.3"
@@ -81,36 +85,45 @@ walkdir = "2"
 tokio1_crate = { package = "tokio", version = "1", features = ["macros", "rt-multi-thread"] }
 async-std = { version = "1.8", features = ["attributes"] }
 serde_json = "1"
-maud = "0.23"
+maud = "0.26"
 
 [[bench]]
 harness = false
 name = "transport_smtp"
 
+[[bench]]
+harness = false
+name = "mailbox_parsing"
+
 [features]
 default = ["smtp-transport", "pool", "native-tls", "hostname", "builder"]
-builder = ["httpdate", "mime", "fastrand", "quoted_printable", "email-encoding"]
-mime03 = ["mime"]
+builder = ["dep:httpdate", "dep:mime", "dep:fastrand", "dep:quoted_printable", "dep:email-encoding"]
+mime03 = ["dep:mime"]
 
 # transports
-file-transport = ["uuid"]
-file-transport-envelope = ["serde", "serde_json", "file-transport"]
-sendmail-transport = []
-smtp-transport = ["base64", "nom", "socket2", "once_cell"]
+file-transport = ["dep:uuid", "tokio1_crate?/fs", "tokio1_crate?/io-util"]
+file-transport-envelope = ["serde", "dep:serde_json", "file-transport"]
+sendmail-transport = ["tokio1_crate?/process", "tokio1_crate?/io-util", "async-std?/unstable"]
+smtp-transport = ["dep:base64", "dep:nom", "dep:socket2", "dep:url", "dep:percent-encoding", "tokio1_crate?/rt", "tokio1_crate?/time", "tokio1_crate?/net"]
 
-pool = ["futures-util"]
+pool = ["dep:futures-util"]
 
-rustls-tls = ["webpki-roots", "rustls", "rustls-pemfile"]
+rustls-tls = ["dep:webpki-roots", "dep:rustls", "dep:rustls-pemfile", "dep:rustls-pki-types"]
+
+boring-tls = ["dep:boring"]
 
 # async
-async-std1 = ["async-std", "async-trait", "futures-io", "futures-util"]
-#async-std1-native-tls = ["async-std1", "native-tls", "async-native-tls"]
-async-std1-rustls-tls = ["async-std1", "rustls-tls", "futures-rustls"]
-tokio1 = ["tokio1_crate", "async-trait", "futures-io", "futures-util"]
-tokio1-native-tls = ["tokio1", "native-tls", "tokio1_native_tls_crate"]
-tokio1-rustls-tls = ["tokio1", "rustls-tls", "tokio1_rustls"]
+async-std1 = ["dep:async-std", "dep:async-trait", "dep:futures-io", "dep:futures-util"]
+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-tls = ["tokio1", "rustls-tls", "dep:tokio1_rustls"]
+tokio1-boring-tls = ["tokio1", "boring-tls", "dep:tokio1_boring"]
 
-dkim = ["base64", "sha2", "rsa", "ed25519-dalek", "regex", "once_cell"]
+dkim = ["dep:base64", "dep:sha2", "dep:rsa", "dep:ed25519-dalek"]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(lettre_ignore_tls_mismatch)'] }
 
 [package.metadata.docs.rs]
 all-features = true

LICENSE

@@ -1,5 +1,5 @@
-Copyright (c) 2014-2022 Alexis Mousset <contact@amousset.me>
-Copyright (c) 2019-2022 Paolo Barbolini <paolo@paolo565.org>
+Copyright (c) 2014-2024 Alexis Mousset <contact@amousset.me>
+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

README.md

@@ -28,8 +28,8 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0">
-    <img src="https://deps.rs/crate/lettre/0.10.0/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.11.8">
+    <img src="https://deps.rs/crate/lettre/0.11.8/status.svg"
       alt="dependency status" />
   </a>
 </div>
@@ -53,20 +53,21 @@ 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 1.56, 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 1.56.0 or newer.
+This library requires Rust 1.70 or newer.
 To use this library, add the following to your `Cargo.toml`:
 
 ```toml
 [dependencies]
-lettre = "0.10"
+lettre = "0.11"
 ```
 
 ```rust,no_run
+use lettre::message::header::ContentType;
 use lettre::transport::smtp::authentication::Credentials;
 use lettre::{Message, SmtpTransport, Transport};
 
@@ -75,10 +76,11 @@ let email = Message::builder()
     .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
     .to("Hei <hei@domain.tld>".parse().unwrap())
     .subject("Happy new year")
+    .header(ContentType::TEXT_PLAIN)
     .body(String::from("Be happy!"))
     .unwrap();
 
-let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
 // Open a remote connection to gmail
 let mailer = SmtpTransport::relay("smtp.gmail.com")
@@ -89,7 +91,7 @@ let mailer = SmtpTransport::relay("smtp.gmail.com")
 // Send the email
 match mailer.send(&email) {
     Ok(_) => println!("Email sent successfully!"),
-    Err(e) => panic!("Could not send email: {:?}", e),
+    Err(e) => panic!("Could not send email: {e:?}"),
 }
 ```
 

SECURITY.md

@@ -2,7 +2,7 @@
 
 The lettre project team welcomes security reports and is committed to providing prompt attention to security issues.
 Security issues should be reported privately via [security@lettre.rs](mailto:security@lettre.rs). Security issues
-should not be reported via the public Github Issue tracker.
+should not be reported via the public GitHub Issue tracker.
 
 ## Security advisories
 

benches/mailbox_parsing.rs

@@ -0,0 +1,27 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use lettre::message::{Mailbox, Mailboxes};
+
+fn bench_parse_single(mailbox: &str) {
+    assert!(mailbox.parse::<Mailbox>().is_ok());
+}
+
+fn bench_parse_multiple(mailboxes: &str) {
+    assert!(mailboxes.parse::<Mailboxes>().is_ok());
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    c.bench_function("parse single mailbox", |b| {
+        b.iter(|| bench_parse_single(black_box("\"Benchmark test\" <test@mail.local>")))
+    });
+
+    c.bench_function("parse multiple mailboxes", |b| {
+        b.iter(|| {
+            bench_parse_multiple(black_box(
+                "\"Benchmark test\" <test@mail.local>, Test <test@mail.local>, <test@mail.local>, test@mail.local",
+            ))
+        })
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);

examples/asyncstd1_smtp_starttls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
-    AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncStd1Executor, AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -12,10 +12,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
@@ -27,6 +28,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/asyncstd1_smtp_tls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
-    AsyncTransport, Message,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncStd1Executor, AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -12,10 +12,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
@@ -27,6 +28,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/autoconfigure.rs

@@ -41,7 +41,7 @@ fn main() {
 
     // Plaintext connection which MUST then successfully upgrade to TLS via STARTTLS
     {
-        tracing::info!("Trying to establish a plaintext connection to {} and then updating it via the SMTP STARTTLS extension", smtp_host);
+        tracing::info!("Trying to establish a plaintext connection to {} and then upgrading it via the SMTP STARTTLS extension", smtp_host);
 
         let transport = SmtpTransport::starttls_relay(&smtp_host)
             .expect("build SmtpTransport::starttls_relay")

examples/smtp.rs

@@ -1,4 +1,4 @@
-use lettre::{Message, SmtpTransport, Transport};
+use lettre::{message::header::ContentType, Message, SmtpTransport, Transport};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,6 +8,7 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
@@ -17,6 +18,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_selfsigned.rs

@@ -1,6 +1,7 @@
 use std::fs;
 
 use lettre::{
+    message::header::ContentType,
     transport::smtp::{
         authentication::Credentials,
         client::{Certificate, Tls, TlsParameters},
@@ -16,18 +17,19 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
     // Use a custom certificate stored on disk to securely verify the server's certificate
     let pem_cert = fs::read("certificate.pem").unwrap();
     let cert = Certificate::from_pem(&pem_cert).unwrap();
-    let tls = TlsParameters::builder("smtp.server.com".to_string())
+    let tls = TlsParameters::builder("smtp.server.com".to_owned())
         .add_root_certificate(cert)
         .build()
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to the smtp server
     let mailer = SmtpTransport::builder_dangerous("smtp.server.com")
@@ -39,6 +41,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_starttls.rs

@@ -1,4 +1,7 @@
-use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
+use lettre::{
+    message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+    SmtpTransport, Transport,
+};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,10 +11,11 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer = SmtpTransport::starttls_relay("smtp.gmail.com")
@@ -22,6 +26,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/smtp_tls.rs

@@ -1,4 +1,7 @@
-use lettre::{transport::smtp::authentication::Credentials, Message, SmtpTransport, Transport};
+use lettre::{
+    message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+    SmtpTransport, Transport,
+};
 
 fn main() {
     tracing_subscriber::fmt::init();
@@ -8,10 +11,11 @@ fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer = SmtpTransport::relay("smtp.gmail.com")
@@ -22,6 +26,6 @@ fn main() {
     // Send the email
     match mailer.send(&email) {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/tokio1_smtp_starttls.rs

@@ -2,8 +2,8 @@
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
-    Tokio1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncTransport, Message, Tokio1Executor,
 };
 use tokio1_crate as tokio;
 
@@ -16,10 +16,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail using STARTTLS
     let mailer: AsyncSmtpTransport<Tokio1Executor> =
@@ -31,6 +32,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

examples/tokio1_smtp_tls.rs

@@ -2,8 +2,8 @@
 // since it uses Rust 2018 crate renaming to import tokio.
 // Won't be needed in user's code.
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
-    Tokio1Executor,
+    message::header::ContentType, transport::smtp::authentication::Credentials, AsyncSmtpTransport,
+    AsyncTransport, Message, Tokio1Executor,
 };
 use tokio1_crate as tokio;
 
@@ -16,10 +16,11 @@ async fn main() {
         .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
         .to("Hei <hei@domain.tld>".parse().unwrap())
         .subject("Happy new async year")
+        .header(ContentType::TEXT_PLAIN)
         .body(String::from("Be happy with async!"))
         .unwrap();
 
-    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
     // Open a remote connection to gmail
     let mailer: AsyncSmtpTransport<Tokio1Executor> =
@@ -31,6 +32,6 @@ async fn main() {
     // Send the email
     match mailer.send(email).await {
         Ok(_) => println!("Email sent successfully!"),
-        Err(e) => panic!("Could not send email: {:?}", e),
+        Err(e) => panic!("Could not send email: {e:?}"),
     }
 }

src/address/envelope.rs

@@ -14,11 +14,71 @@ pub struct Envelope {
     /// The envelope recipient's addresses
     ///
     /// This can not be empty.
+    #[cfg_attr(
+        feature = "serde",
+        serde(deserialize_with = "serde_forward_path::deserialize")
+    )]
     forward_path: Vec<Address>,
     /// The envelope sender address
     reverse_path: Option<Address>,
 }
 
+/// just like the default implementation to deserialize `Vec<Address>` but it
+/// forbids **de**serializing empty lists
+#[cfg(feature = "serde")]
+mod serde_forward_path {
+    use super::Address;
+    /// dummy type required for serde
+    /// see example: https://serde.rs/deserialize-map.html
+    struct CustomVisitor;
+    impl<'de> serde::de::Visitor<'de> for CustomVisitor {
+        type Value = Vec<Address>;
+
+        fn expecting(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            formatter.write_str("a non-empty list of recipient addresses")
+        }
+
+        fn visit_seq<S>(self, mut access: S) -> Result<Self::Value, S::Error>
+        where
+            S: serde::de::SeqAccess<'de>,
+        {
+            let mut seq: Vec<Address> = Vec::with_capacity(access.size_hint().unwrap_or(0));
+            while let Some(key) = access.next_element()? {
+                seq.push(key);
+            }
+            if seq.is_empty() {
+                Err(serde::de::Error::invalid_length(seq.len(), &self))
+            } else {
+                Ok(seq)
+            }
+        }
+    }
+    pub fn deserialize<'de, D>(deserializer: D) -> Result<Vec<Address>, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        deserializer.deserialize_seq(CustomVisitor {})
+    }
+
+    #[cfg(test)]
+    mod tests {
+        #[test]
+        fn deserializing_empty_recipient_list_returns_error() {
+            assert!(
+                serde_json::from_str::<crate::address::Envelope>(r#"{"forward_path": []}"#)
+                    .is_err()
+            );
+        }
+        #[test]
+        fn deserializing_non_empty_recipient_list_is_ok() {
+            serde_json::from_str::<crate::address::Envelope>(
+                r#"{ "forward_path": [ {"user":"foo", "domain":"example.com"} ] }"#,
+            )
+            .unwrap();
+        }
+    }
+}
+
 impl Envelope {
     /// Creates a new envelope, which may fail if `to` is empty.
     ///

src/address/types.rs

@@ -15,7 +15,7 @@ use idna::domain_to_ascii;
 ///
 /// This type contains email in canonical form (_user@domain.tld_).
 ///
-/// **NOTE**: Enable feature "serde" to be able serialize/deserialize it using [serde](https://serde.rs/).
+/// **NOTE**: Enable feature "serde" to be able to serialize/deserialize it using [serde](https://serde.rs/).
 ///
 /// # Examples
 ///
@@ -184,7 +184,7 @@ where
         let domain = domain.as_ref();
         Address::check_domain(domain)?;
 
-        let serialized = format!("{}@{}", user, domain);
+        let serialized = format!("{user}@{domain}");
         Ok(Address {
             serialized,
             at_start: user.len(),
@@ -227,6 +227,7 @@ fn check_address(val: &str) -> Result<usize, AddressError> {
 }
 
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[non_exhaustive]
 /// Errors in email addresses parsing
 pub enum AddressError {
     /// Missing domain or user
@@ -237,6 +238,8 @@ pub enum AddressError {
     InvalidUser,
     /// Invalid email domain
     InvalidDomain,
+    /// Invalid input found
+    InvalidInput,
 }
 
 impl Error for AddressError {}
@@ -248,6 +251,7 @@ impl Display for AddressError {
             AddressError::Unbalanced => f.write_str("Unbalanced angle bracket"),
             AddressError::InvalidUser => f.write_str("Invalid email user"),
             AddressError::InvalidDomain => f.write_str("Invalid email domain"),
+            AddressError::InvalidInput => f.write_str("Invalid input"),
         }
     }
 }

src/base64.rs

@@ -0,0 +1,12 @@
+use ::base64::{
+    engine::{general_purpose::STANDARD, Engine},
+    DecodeError,
+};
+
+pub(crate) fn encode<T: AsRef<[u8]>>(input: T) -> String {
+    STANDARD.encode(input)
+}
+
+pub(crate) fn decode<T: AsRef<[u8]>>(input: T) -> Result<Vec<u8>, DecodeError> {
+    STANDARD.decode(input)
+}

src/executor.rs

@@ -134,7 +134,7 @@ impl Executor for Tokio1Executor {
         #[allow(clippy::match_single_binding)]
         let tls_parameters = match tls {
             #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
-            Tls::Wrapper(ref tls_parameters) => Some(tls_parameters.clone()),
+            Tls::Wrapper(tls_parameters) => Some(tls_parameters.clone()),
             _ => None,
         };
         #[allow(unused_mut)]
@@ -149,12 +149,12 @@ impl Executor for Tokio1Executor {
 
         #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
         match tls {
-            Tls::Opportunistic(ref tls_parameters) => {
+            Tls::Opportunistic(tls_parameters) => {
                 if conn.can_starttls() {
                     conn.starttls(tls_parameters.clone(), hello_name).await?;
                 }
             }
-            Tls::Required(ref tls_parameters) => {
+            Tls::Required(tls_parameters) => {
                 conn.starttls(tls_parameters.clone(), hello_name).await?;
             }
             _ => (),
@@ -216,7 +216,7 @@ impl Executor for AsyncStd1Executor {
 
     #[cfg(feature = "smtp-transport")]
     fn sleep(duration: Duration) -> Self::Sleep {
-        let fut = async move { async_std::task::sleep(duration).await };
+        let fut = async_std::task::sleep(duration);
         Box::pin(fut)
     }
 
@@ -230,8 +230,8 @@ impl Executor for AsyncStd1Executor {
     ) -> Result<AsyncSmtpConnection, Error> {
         #[allow(clippy::match_single_binding)]
         let tls_parameters = match tls {
-            #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
-            Tls::Wrapper(ref tls_parameters) => Some(tls_parameters.clone()),
+            #[cfg(feature = "async-std1-rustls-tls")]
+            Tls::Wrapper(tls_parameters) => Some(tls_parameters.clone()),
             _ => None,
         };
         #[allow(unused_mut)]
@@ -243,14 +243,14 @@ impl Executor for AsyncStd1Executor {
         )
         .await?;
 
-        #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
+        #[cfg(feature = "async-std1-rustls-tls")]
         match tls {
-            Tls::Opportunistic(ref tls_parameters) => {
+            Tls::Opportunistic(tls_parameters) => {
                 if conn.can_starttls() {
                     conn.starttls(tls_parameters.clone(), hello_name).await?;
                 }
             }
-            Tls::Required(ref tls_parameters) => {
+            Tls::Required(tls_parameters) => {
                 conn.starttls(tls_parameters.clone(), hello_name).await?;
             }
             _ => (),

src/lib.rs

@@ -6,7 +6,7 @@
 //! * Secure defaults
 //! * Async support
 //!
-//! Lettre requires Rust 1.56.0 or newer.
+//! Lettre requires Rust 1.70 or newer.
 //!
 //! ## Features
 //!
@@ -41,6 +41,15 @@
 //!
 //! NOTE: native-tls isn't supported with `async-std`
 //!
+//! #### SMTP over TLS via the boring crate (Boring TLS)
+//!
+//! _Secure SMTP connections using TLS from the `boring-tls` crate_
+//!
+//! * **boring-tls**: TLS support for the synchronous version of the API
+//! * **tokio1-boring-tls**: TLS support for the `tokio1` async version of the API
+//!
+//! NOTE: boring-tls isn't supported with `async-std`
+//!
 //! #### SMTP over TLS via the rustls crate
 //!
 //! _Secure SMTP connections using TLS from the `rustls-tls` crate_
@@ -100,7 +109,7 @@
 //! [mime 0.3]: https://docs.rs/mime/0.3
 //! [DKIM]: https://datatracker.ietf.org/doc/html/rfc6376
 
-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.11.8")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -126,12 +135,18 @@
     clippy::manual_assert,
     clippy::unnecessary_join,
     clippy::wildcard_imports,
+    clippy::str_to_string,
+    clippy::empty_structs_with_brackets,
     clippy::zero_sized_map_values
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 #[cfg(not(lettre_ignore_tls_mismatch))]
 mod compiletime_checks {
+    #[cfg(all(feature = "native-tls", feature = "boring-tls"))]
+    compile_error!("feature \"native-tls\" and feature \"boring-tls\" cannot be enabled at the same time, otherwise
+    the executable will fail to link.");
+
     #[cfg(all(
         feature = "tokio1",
         feature = "native-tls",
@@ -150,21 +165,16 @@ mod compiletime_checks {
     If you'd like to use `native-tls` make sure that the `rustls-tls` feature hasn't been enabled by mistake.
     Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
 
-    /*
     #[cfg(all(
-        feature = "async-std1",
-        feature = "native-tls",
-        not(feature = "async-std1-native-tls")
-    ))]
-    compile_error!("Lettre is being built with the `async-std1` and the `native-tls` features, but the `async-std1-native-tls` feature hasn't been turned on.
-    If you'd like to use rustls make sure that the `native-tls` hasn't been enabled by mistake (you may need to import lettre without default features)
-    If you're building a library which depends on lettre import it without default features and enable just the features you need.");
-    */
-    #[cfg(all(
-        feature = "async-std1",
-        feature = "native-tls",
-        not(feature = "async-std1-native-tls")
+        feature = "tokio1",
+        feature = "boring-tls",
+        not(feature = "tokio1-boring-tls")
     ))]
+    compile_error!("Lettre is being built with the `tokio1` and the `boring-tls` features, but the `tokio1-boring-tls` feature hasn't been turned on.
+    If you'd like to use `boring-tls` make sure that the `rustls-tls` feature hasn't been enabled by mistake.
+    Make sure to apply the same to any of your crate dependencies that use the `lettre` crate.");
+
+    #[cfg(all(feature = "async-std1", 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-tls` and did not activate `native-tls`, disable the default-features of lettre in `Cargo.toml` and manually add the required features.
@@ -181,6 +191,8 @@ Make sure to apply the same to any of your crate dependencies that use the `lett
 }
 
 pub mod address;
+#[cfg(any(feature = "smtp-transport", feature = "dkim"))]
+mod base64;
 pub mod error;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 mod executor;
@@ -193,11 +205,11 @@ use std::error::Error as StdError;
 
 #[cfg(feature = "async-std1")]
 pub use self::executor::AsyncStd1Executor;
-#[cfg(all(any(feature = "tokio1", feature = "async-std1")))]
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
 pub use self::executor::Executor;
 #[cfg(feature = "tokio1")]
 pub use self::executor::Tokio1Executor;
-#[cfg(all(any(feature = "tokio1", feature = "async-std1")))]
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
 #[doc(inline)]
 pub use self::transport::AsyncTransport;
 pub use crate::address::Address;

src/message/attachment.rs

@@ -13,9 +13,9 @@ pub struct Attachment {
 
 #[derive(Clone)]
 enum Disposition {
-    /// file name
+    /// File name
     Attached(String),
-    /// content id
+    /// Content id
     Inline(String),
 }
 
@@ -96,7 +96,7 @@ impl Attachment {
                 builder.header(header::ContentDisposition::attachment(&filename))
             }
             Disposition::Inline(content_id) => builder
-                .header(header::ContentId::from(format!("<{}>", content_id)))
+                .header(header::ContentId::from(format!("<{content_id}>")))
                 .header(header::ContentDisposition::inline()),
         };
         builder = builder.header(content_type);

src/message/body.rs

@@ -496,7 +496,7 @@ mod test {
     #[test]
     fn base64_encode_bytes_wrapping() {
         let encoded = Body::new_with_encoding(
-            vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9].repeat(20),
+            [0, 1, 2, 3, 4, 5, 6, 7, 8, 9].repeat(20),
             ContentTransferEncoding::Base64,
         )
         .unwrap();

src/message/dkim.rs

@@ -2,14 +2,11 @@ use std::{
     borrow::Cow,
     error::Error as StdError,
     fmt::{self, Display},
-    iter::IntoIterator,
     time::SystemTime,
 };
 
 use ed25519_dalek::Signer;
-use once_cell::sync::Lazy;
-use regex::bytes::Regex;
-use rsa::{pkcs1::DecodeRsaPrivateKey, Hash, PaddingScheme, RsaPrivateKey};
+use rsa::{pkcs1::DecodeRsaPrivateKey, pkcs1v15::Pkcs1v15Sign, RsaPrivateKey};
 use sha2::{Digest, Sha256};
 
 use crate::message::{
@@ -110,7 +107,7 @@ pub struct DkimSigningKey(InnerDkimSigningKey);
 #[derive(Debug)]
 enum InnerDkimSigningKey {
     Rsa(RsaPrivateKey),
-    Ed25519(ed25519_dalek::Keypair),
+    Ed25519(ed25519_dalek::SigningKey),
 }
 
 impl DkimSigningKey {
@@ -124,12 +121,16 @@ impl DkimSigningKey {
                     .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Rsa(err)))?,
             ),
             DkimSigningAlgorithm::Ed25519 => {
-                InnerDkimSigningKey::Ed25519(
-                    ed25519_dalek::Keypair::from_bytes(&base64::decode(private_key).map_err(
-                        |err| DkimSigningKeyError(InnerDkimSigningKeyError::Base64(err)),
-                    )?)
-                    .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Ed25519(err)))?,
-                )
+                InnerDkimSigningKey::Ed25519(ed25519_dalek::SigningKey::from_bytes(
+                    &crate::base64::decode(private_key)
+                        .map_err(|err| DkimSigningKeyError(InnerDkimSigningKeyError::Base64(err)))?
+                        .try_into()
+                        .map_err(|_| {
+                            DkimSigningKeyError(InnerDkimSigningKeyError::Ed25519(
+                                ed25519_dalek::ed25519::Error::new(),
+                            ))
+                        })?,
+                ))
             }
         }))
     }
@@ -142,19 +143,18 @@ impl DkimSigningKey {
 }
 
 /// A struct to describe Dkim configuration applied when signing a message
-/// selector: the name of the key publied in DNS
-/// domain: the domain for which we sign the message
-/// private_key: private key in PKCS1 string format
-/// headers: a list of headers name to be included in the signature. Signing of more than one
-/// header with same name is not supported
-/// canonicalization: the canonicalization to be applied on the message
-/// pub signing_algorithm: the signing algorithm to be used when signing
 #[derive(Debug)]
 pub struct DkimConfig {
+    /// The name of the key published in DNS
     selector: String,
+    /// The domain for which we sign the message
     domain: String,
+    /// The private key in PKCS1 string format
     private_key: DkimSigningKey,
+    /// A list of header names to be included in the signature. Signing of more than one
+    /// header with the same name is not supported
     headers: Vec<HeaderName>,
+    /// The signing algorithm to be used when signing
     canonicalization: DkimCanonicalization,
 }
 
@@ -219,24 +219,34 @@ fn dkim_header_format(
 
 /// Canonicalize the body of an email
 fn dkim_canonicalize_body(
-    body: &[u8],
+    mut body: &[u8],
     canonicalization: DkimCanonicalizationType,
 ) -> Cow<'_, [u8]> {
-    static RE: Lazy<Regex> = Lazy::new(|| Regex::new("(\r\n)+$").unwrap());
-    static RE_DOUBLE_SPACE: Lazy<Regex> = Lazy::new(|| Regex::new("[\\t ]+").unwrap());
-    static RE_SPACE_EOL: Lazy<Regex> = Lazy::new(|| Regex::new("[\t ]\r\n").unwrap());
     match canonicalization {
-        DkimCanonicalizationType::Simple => RE.replace(body, &b"\r\n"[..]),
-        DkimCanonicalizationType::Relaxed => {
-            let body = RE_DOUBLE_SPACE.replace_all(body, &b" "[..]);
-            let body = match RE_SPACE_EOL.replace_all(&body, &b"\r\n"[..]) {
-                Cow::Borrowed(_body) => body,
-                Cow::Owned(body) => Cow::Owned(body),
-            };
-            match RE.replace(&body, &b"\r\n"[..]) {
-                Cow::Borrowed(_body) => body,
-                Cow::Owned(body) => Cow::Owned(body),
+        DkimCanonicalizationType::Simple => {
+            // Remove empty lines at end
+            while body.ends_with(b"\r\n\r\n") {
+                body = &body[..body.len() - 2];
             }
+            Cow::Borrowed(body)
+        }
+        DkimCanonicalizationType::Relaxed => {
+            let mut out = Vec::with_capacity(body.len());
+            loop {
+                match body {
+                    [b' ' | b'\t', b'\r', b'\n', ..] => {}
+                    [b' ' | b'\t', b' ' | b'\t', ..] => {}
+                    [b' ' | b'\t', ..] => out.push(b' '),
+                    [c, ..] => out.push(*c),
+                    [] => break,
+                }
+                body = &body[1..];
+            }
+            // Remove empty lines at end
+            while out.ends_with(b"\r\n\r\n") {
+                out.truncate(out.len() - 2);
+            }
+            Cow::Owned(out)
         }
     }
 }
@@ -333,7 +343,7 @@ fn dkim_canonicalize_headers<'a>(
     }
 }
 
-/// Sign with Dkim a message by adding Dkim-Signture header created with configuration expressed by
+/// Sign with Dkim a message by adding Dkim-Signature header created with configuration expressed by
 /// dkim_config
 
 pub fn dkim_sign(message: &mut Message, dkim_config: &DkimConfig) {
@@ -346,11 +356,11 @@ fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timesta
         .unwrap()
         .as_secs();
     let headers = message.headers();
-    let body_hash = Sha256::digest(&dkim_canonicalize_body(
+    let body_hash = Sha256::digest(dkim_canonicalize_body(
         &message.body_raw(),
         dkim_config.canonicalization.body,
     ));
-    let bh = base64::encode(body_hash);
+    let bh = crate::base64::encode(body_hash);
     let mut signed_headers_list =
         dkim_config
             .headers
@@ -382,16 +392,13 @@ fn dkim_sign_fixed_time(message: &mut Message, dkim_config: &DkimConfig, timesta
     hashed_headers.update(canonicalized_dkim_header.trim_end().as_bytes());
     let hashed_headers = hashed_headers.finalize();
     let signature = match &dkim_config.private_key.0 {
-        InnerDkimSigningKey::Rsa(private_key) => base64::encode(
+        InnerDkimSigningKey::Rsa(private_key) => crate::base64::encode(
             private_key
-                .sign(
-                    PaddingScheme::new_pkcs1v15_sign(Some(Hash::SHA2_256)),
-                    &hashed_headers,
-                )
+                .sign(Pkcs1v15Sign::new::<Sha256>(), &hashed_headers)
                 .unwrap(),
         ),
         InnerDkimSigningKey::Ed25519(private_key) => {
-            base64::encode(private_key.sign(&hashed_headers).to_bytes())
+            crate::base64::encode(private_key.sign(&hashed_headers).to_bytes())
         }
     };
     let dkim_header = dkim_header_format(
@@ -416,8 +423,9 @@ mod test {
             header::{HeaderName, HeaderValue},
             Header, Message,
         },
-        dkim_canonicalize_headers, dkim_sign_fixed_time, DkimCanonicalization,
-        DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm, DkimSigningKey,
+        dkim_canonicalize_body, dkim_canonicalize_headers, dkim_sign_fixed_time,
+        DkimCanonicalization, DkimCanonicalizationType, DkimConfig, DkimSigningAlgorithm,
+        DkimSigningKey,
     };
     use crate::StdError;
 
@@ -471,9 +479,9 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
             .from("Test O'Leary <test+ezrz@example.net>".parse().unwrap())
             .to("Test2 <test2@example.org>".parse().unwrap())
             .date(std::time::UNIX_EPOCH)
-            .header(TestHeader("test  test very very long with spaces and extra spaces   \twill be folded to several lines ".to_string()))
+            .header(TestHeader("test  test very very long with spaces and extra spaces   \twill be folded to several lines ".to_owned()))
             .subject("Test with utf-8 ë")
-            .body("test\r\n\r\ntest   \ttest\r\n\r\n\r\n".to_string()).unwrap()
+            .body("test\r\n\r\ntest   \ttest\r\n\r\n\r\n".to_owned()).unwrap()
     }
 
     #[test]
@@ -490,6 +498,24 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         assert_eq!(dkim_canonicalize_headers(["From", "Test"], &message.headers, DkimCanonicalizationType::Relaxed),"from:=?utf-8?b?VGVzdCBPJ0xlYXJ5?= <test+ezrz@example.net>\r\ntest:test test very very long with spaces and extra spaces will be folded to several lines\r\n")
     }
 
+    #[test]
+    fn test_body_simple_canonicalize() {
+        let body = b" C \r\nD \t E\r\n\r\n\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Simple).into_owned(),
+            b" C \r\nD \t E\r\n"
+        );
+    }
+
+    #[test]
+    fn test_body_relaxed_canonicalize() {
+        let body = b" C \r\nD \t E\r\n\tF\r\n\t\r\n\r\n\r\n";
+        assert_eq!(
+            dkim_canonicalize_body(body, DkimCanonicalizationType::Relaxed).into_owned(),
+            b" C\r\nD E\r\n F\r\n"
+        );
+    }
+
     #[test]
     fn test_signature_rsa_simple() {
         let mut message = test_message();
@@ -497,8 +523,8 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         dkim_sign_fixed_time(
             &mut message,
             &DkimConfig::new(
-                "dkimtest".to_string(),
-                "example.org".to_string(),
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
                 signing_key,
                 vec![
                     HeaderName::new_from_ascii_str("Date"),
@@ -546,8 +572,8 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         dkim_sign_fixed_time(
             &mut message,
             &DkimConfig::new(
-                "dkimtest".to_string(),
-                "example.org".to_string(),
+                "dkimtest".to_owned(),
+                "example.org".to_owned(),
                 signing_key,
                 vec![
                     HeaderName::new_from_ascii_str("Date"),
@@ -564,7 +590,7 @@ cJ5Ku0OTwRtSMaseRPX+T4EfG1Caa/eunPPN4rh+CSup2BVVarOT
         );
         let signed = message.formatted();
         let signed = std::str::from_utf8(&signed).unwrap();
-        println!("{}", signed);
+        println!("{signed}");
         assert_eq!(
             signed,
             std::concat!(

src/message/header/content.rs

@@ -13,12 +13,14 @@ use crate::BoxError;
 /// use-caches this header shouldn't be set manually.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Default)]
 pub enum ContentTransferEncoding {
     /// ASCII
     SevenBit,
     /// Quoted-Printable encoding
     QuotedPrintable,
     /// base64 encoding
+    #[default]
     Base64,
     /// Requires `8BITMIME`
     EightBit,
@@ -67,12 +69,6 @@ impl FromStr for ContentTransferEncoding {
     }
 }
 
-impl Default for ContentTransferEncoding {
-    fn default() -> Self {
-        ContentTransferEncoding::Base64
-    }
-}
-
 #[cfg(test)]
 mod test {
     use pretty_assertions::assert_eq;
@@ -99,7 +95,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "7bit".to_string(),
+            "7bit".to_owned(),
         ));
 
         assert_eq!(
@@ -109,7 +105,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "base64".to_string(),
+            "base64".to_owned(),
         ));
 
         assert_eq!(

src/message/header/content_disposition.rs

@@ -1,6 +1,6 @@
 use std::fmt::Write;
 
-use email_encoding::headers::EmailWriter;
+use email_encoding::headers::writer::EmailWriter;
 
 use super::{Header, HeaderName, HeaderValue};
 use crate::BoxError;
@@ -16,13 +16,13 @@ impl ContentDisposition {
     pub fn inline() -> Self {
         Self(HeaderValue::dangerous_new_pre_encoded(
             Self::name(),
-            "inline".to_string(),
-            "inline".to_string(),
+            "inline".to_owned(),
+            "inline".to_owned(),
         ))
     }
 
     /// An attachment which should be displayed inline into the message, but that also
-    /// species the filename in case it were to be downloaded
+    /// species the filename in case it is downloaded
     pub fn inline_with_name(file_name: &str) -> Self {
         Self::with_name("inline", file_name)
     }
@@ -33,17 +33,19 @@ impl ContentDisposition {
     }
 
     fn with_name(kind: &str, file_name: &str) -> Self {
-        let raw_value = format!("{}; filename=\"{}\"", kind, file_name);
+        let raw_value = format!("{kind}; filename=\"{file_name}\"");
 
         let mut encoded_value = String::new();
         let line_len = "Content-Disposition: ".len();
-        let mut w = EmailWriter::new(&mut encoded_value, line_len, false);
-        w.write_str(kind).expect("writing `kind` returned an error");
-        w.write_char(';').expect("writing `;` returned an error");
-        w.space();
+        {
+            let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false);
+            w.write_str(kind).expect("writing `kind` returned an error");
+            w.write_char(';').expect("writing `;` returned an error");
+            w.space();
 
-        email_encoding::headers::rfc2231::encode("filename", file_name, &mut w)
-            .expect("some Write implementation returned an error");
+            email_encoding::headers::rfc2231::encode("filename", file_name, &mut w)
+                .expect("some Write implementation returned an error");
+        }
 
         Self(HeaderValue::dangerous_new_pre_encoded(
             Self::name(),
@@ -88,12 +90,12 @@ mod test {
 
         headers.set(ContentDisposition::inline());
 
-        assert_eq!(format!("{}", headers), "Content-Disposition: inline\r\n");
+        assert_eq!(format!("{headers}"), "Content-Disposition: inline\r\n");
 
         headers.set(ContentDisposition::attachment("something.txt"));
 
         assert_eq!(
-            format!("{}", headers),
+            format!("{headers}"),
             "Content-Disposition: attachment; filename=\"something.txt\"\r\n"
         );
     }
@@ -104,7 +106,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
-            "inline".to_string(),
+            "inline".to_owned(),
         ));
 
         assert_eq!(
@@ -114,7 +116,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Disposition"),
-            "attachment; filename=\"something.txt\"".to_string(),
+            "attachment; filename=\"something.txt\"".to_owned(),
         ));
 
         assert_eq!(

src/message/header/content_type.rs

@@ -11,12 +11,12 @@ use crate::BoxError;
 
 /// `Content-Type` of the body
 ///
-/// This struct can represent any valid [mime type], which can be parsed via
+/// This struct can represent any valid [MIME type], which can be parsed via
 /// [`ContentType::parse`]. Constants are provided for the most-used mime-types.
 ///
 /// Defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-5)
 ///
-/// [mime type]: https://www.iana.org/assignments/media-types/media-types.xhtml
+/// [MIME type]: https://www.iana.org/assignments/media-types/media-types.xhtml
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ContentType(Mime);
 
@@ -135,8 +135,7 @@ mod serde {
                     match ContentType::parse(mime) {
                         Ok(content_type) => Ok(content_type),
                         Err(_) => Err(E::custom(format!(
-                            "Couldn't parse the following MIME-Type: {}",
-                            mime
+                            "Couldn't parse the following MIME-Type: {mime}"
                         ))),
                     }
                 }
@@ -179,14 +178,14 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
-            "text/plain; charset=utf-8".to_string(),
+            "text/plain; charset=utf-8".to_owned(),
         ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_PLAIN));
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Type"),
-            "text/html; charset=utf-8".to_string(),
+            "text/html; charset=utf-8".to_owned(),
         ));
 
         assert_eq!(headers.get::<ContentType>(), Some(ContentType::TEXT_HTML));

src/message/header/date.rs

@@ -90,7 +90,7 @@ mod test {
 
         assert_eq!(
             headers.to_string(),
-            "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n".to_string()
+            "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n".to_owned()
         );
 
         // Tue, 15 Nov 1994 08:12:32 GMT
@@ -110,7 +110,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
-            "Tue, 15 Nov 1994 08:12:31 +0000".to_string(),
+            "Tue, 15 Nov 1994 08:12:31 +0000".to_owned(),
         ));
 
         assert_eq!(
@@ -122,7 +122,7 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Date"),
-            "Tue, 15 Nov 1994 08:12:32 +0000".to_string(),
+            "Tue, 15 Nov 1994 08:12:32 +0000".to_owned(),
         ));
 
         assert_eq!(

src/message/header/mailbox.rs

@@ -1,4 +1,4 @@
-use email_encoding::headers::EmailWriter;
+use email_encoding::headers::writer::EmailWriter;
 
 use super::{Header, HeaderName, HeaderValue};
 use crate::{
@@ -30,8 +30,10 @@ macro_rules! mailbox_header {
             fn display(&self) -> HeaderValue {
                 let mut encoded_value = String::new();
                 let line_len = $header_name.len() + ": ".len();
-                let mut w = EmailWriter::new(&mut encoded_value, line_len, false);
-                self.0.encode(&mut w).expect("writing `Mailbox` returned an error");
+                {
+                    let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false);
+                    self.0.encode(&mut w).expect("writing `Mailbox` returned an error");
+                }
 
                 HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
@@ -78,8 +80,10 @@ macro_rules! mailboxes_header {
             fn display(&self) -> HeaderValue {
                 let mut encoded_value = String::new();
                 let line_len = $header_name.len() + ": ".len();
-                let mut w = EmailWriter::new(&mut encoded_value, line_len, false);
-                self.0.encode(&mut w).expect("writing `Mailboxes` returned an error");
+                {
+                    let mut w = EmailWriter::new(&mut encoded_value, line_len, 0, false);
+                    self.0.encode(&mut w).expect("writing `Mailboxes` returned an error");
+                }
 
                 HeaderValue::dangerous_new_pre_encoded(Self::name(), self.0.to_string(), encoded_value)
             }
@@ -248,7 +252,7 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "kayo@example.com".to_string(),
+            "kayo@example.com".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
@@ -261,7 +265,7 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "K. <kayo@example.com>".to_string(),
+            "K. <kayo@example.com>".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from)));
@@ -277,7 +281,7 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "kayo@example.com, pony@domain.tld".to_string(),
+            "kayo@example.com, pony@domain.tld".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
@@ -293,7 +297,7 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "K. <kayo@example.com>, Pony P. <pony@domain.tld>".to_string(),
+            "K. <kayo@example.com>, Pony P. <pony@domain.tld>".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
@@ -302,14 +306,30 @@ mod test {
     #[test]
     fn parse_multi_with_name_containing_comma() {
         let from: Vec<Mailbox> = vec![
-            "Test, test <1@example.com>".parse().unwrap(),
-            "Test2, test2 <2@example.com>".parse().unwrap(),
+            "\"Test, test\" <1@example.com>".parse().unwrap(),
+            "\"Test2, test2\" <2@example.com>".parse().unwrap(),
         ];
 
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Test, test <1@example.com>, Test2, test2 <2@example.com>".to_string(),
+            "\"Test, test\" <1@example.com>, \"Test2, test2\" <2@example.com>".to_owned(),
+        ));
+
+        assert_eq!(headers.get::<From>(), Some(From(from.into())));
+    }
+
+    #[test]
+    fn parse_multi_with_name_containing_double_quotes() {
+        let from: Vec<Mailbox> = vec![
+            "\"Test, test\" <1@example.com>".parse().unwrap(),
+            "\"Test2, \"test2\"\" <2@example.com>".parse().unwrap(),
+        ];
+
+        let mut headers = Headers::new();
+        headers.insert_raw(HeaderValue::new(
+            HeaderName::new_from_ascii_str("From"),
+            "\"Test, test\" <1@example.com>, \"Test2, \"test2\"\" <2@example.com>".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), Some(From(from.into())));
@@ -320,9 +340,20 @@ mod test {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Test, test <1@example.com>, Test2, test2".to_string(),
+            "\"Test, test\" <1@example.com>, \"Test2, test2\"".to_owned(),
         ));
 
         assert_eq!(headers.get::<From>(), None);
     }
+
+    #[test]
+    fn mailbox_format_address_with_angle_bracket() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(Some("<3".into()), "i@love.example".parse().unwrap())
+            ),
+            r#""<3" <i@love.example>"#
+        );
+    }
 }

src/message/header/mod.rs

@@ -7,7 +7,7 @@ use std::{
     ops::Deref,
 };
 
-use email_encoding::headers::EmailWriter;
+use email_encoding::headers::writer::EmailWriter;
 
 pub use self::{
     content::*,
@@ -66,7 +66,7 @@ impl Headers {
         }
     }
 
-    /// Returns a copy of an `Header` present in `Headers`
+    /// Returns a copy of a `Header` present in `Headers`
     ///
     /// Returns `None` if `Header` isn't present in `Headers`.
     pub fn get<H: Header>(&self) -> Option<H> {
@@ -124,22 +124,18 @@ impl Headers {
     }
 
     pub(crate) fn find_header(&self, name: &str) -> Option<&HeaderValue> {
-        self.headers
-            .iter()
-            .find(|value| name.eq_ignore_ascii_case(&value.name))
+        self.headers.iter().find(|value| name == value.name)
     }
 
     fn find_header_mut(&mut self, name: &str) -> Option<&mut HeaderValue> {
-        self.headers
-            .iter_mut()
-            .find(|value| name.eq_ignore_ascii_case(&value.name))
+        self.headers.iter_mut().find(|value| name == value.name)
     }
 
     fn find_header_index(&self, name: &str) -> Option<usize> {
         self.headers
             .iter()
             .enumerate()
-            .find(|(_i, value)| name.eq_ignore_ascii_case(&value.name))
+            .find(|(_i, value)| name == value.name)
             .map(|(i, _)| i)
     }
 }
@@ -161,18 +157,9 @@ impl Display for Headers {
 /// A possible error when converting a `HeaderName` from another type.
 // comes from `http` crate
 #[allow(missing_copy_implementations)]
-#[derive(Clone)]
-pub struct InvalidHeaderName {
-    _priv: (),
-}
-
-impl fmt::Debug for InvalidHeaderName {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("InvalidHeaderName")
-            // skip _priv noise
-            .finish()
-    }
-}
+#[derive(Debug, Clone)]
+#[non_exhaustive]
+pub struct InvalidHeaderName;
 
 impl fmt::Display for InvalidHeaderName {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -189,14 +176,11 @@ pub struct HeaderName(Cow<'static, str>);
 impl HeaderName {
     /// Creates a new header name
     pub fn new_from_ascii(ascii: String) -> Result<Self, InvalidHeaderName> {
-        if !ascii.is_empty()
-            && ascii.len() <= 76
-            && ascii.is_ascii()
-            && !ascii.contains(|c| c == ':' || c == ' ')
+        if !ascii.is_empty() && ascii.len() <= 76 && ascii.is_ascii() && !ascii.contains([':', ' '])
         {
             Ok(Self(Cow::Owned(ascii)))
         } else {
-            Err(InvalidHeaderName { _priv: () })
+            Err(InvalidHeaderName)
         }
     }
 
@@ -257,23 +241,19 @@ impl AsRef<str> for HeaderName {
 
 impl PartialEq<HeaderName> for HeaderName {
     fn eq(&self, other: &HeaderName) -> bool {
-        let s1: &str = self.as_ref();
-        let s2: &str = other.as_ref();
-        s1 == s2
+        self.eq_ignore_ascii_case(other)
     }
 }
 
 impl PartialEq<&str> for HeaderName {
     fn eq(&self, other: &&str) -> bool {
-        let s: &str = self.as_ref();
-        s == *other
+        self.eq_ignore_ascii_case(other)
     }
 }
 
 impl PartialEq<HeaderName> for &str {
     fn eq(&self, other: &HeaderName) -> bool {
-        let s: &str = other.as_ref();
-        *self == s
+        self.eq_ignore_ascii_case(other)
     }
 }
 
@@ -310,7 +290,7 @@ impl HeaderValue {
     /// acceptable for use if `encoded_value` contains only ascii
     /// printable characters and is already line folded.
     ///
-    /// When in doubt use [`HeaderValue::new`].
+    /// When in doubt, use [`HeaderValue::new`].
     pub fn dangerous_new_pre_encoded(
         name: HeaderName,
         raw_value: String,
@@ -323,10 +303,12 @@ impl HeaderValue {
         }
     }
 
+    #[cfg(feature = "dkim")]
     pub(crate) fn get_raw(&self) -> &str {
         &self.raw_value
     }
 
+    #[cfg(feature = "dkim")]
     pub(crate) fn get_encoded(&self) -> &str {
         &self.encoded_value
     }
@@ -340,28 +322,21 @@ struct HeaderValueEncoder<'a> {
 
 impl<'a> HeaderValueEncoder<'a> {
     fn encode(name: &str, value: &'a str, f: &'a mut impl fmt::Write) -> fmt::Result {
-        let (words_iter, encoder) = Self::new(name, value, f);
-        encoder.format(words_iter)
+        let encoder = Self::new(name, f);
+        encoder.format(value.split_inclusive(' '))
     }
 
-    fn new(
-        name: &str,
-        value: &'a str,
-        writer: &'a mut dyn Write,
-    ) -> (WordsPlusFillIterator<'a>, Self) {
+    fn new(name: &str, writer: &'a mut dyn Write) -> Self {
         let line_len = name.len() + ": ".len();
-        let writer = EmailWriter::new(writer, line_len, false);
+        let writer = EmailWriter::new(writer, line_len, 0, false);
 
-        (
-            WordsPlusFillIterator { s: value },
-            Self {
-                writer,
-                encode_buf: String::new(),
-            },
-        )
+        Self {
+            writer,
+            encode_buf: String::new(),
+        }
     }
 
-    fn format(mut self, words_iter: WordsPlusFillIterator<'_>) -> fmt::Result {
+    fn format(mut self, words_iter: impl Iterator<Item = &'a str>) -> fmt::Result {
         for next_word in words_iter {
             let allowed = allowed_str(next_word);
 
@@ -389,71 +364,20 @@ impl<'a> HeaderValueEncoder<'a> {
             return Ok(());
         }
 
-        // It is important that we don't encode leading whitespace otherwise it breaks wrapping.
-        let first_not_allowed = self
-            .encode_buf
-            .bytes()
-            .enumerate()
-            .find(|(_i, c)| !allowed_char(*c))
-            .map(|(i, _)| i);
-        // May as well also write the tail in plain text.
-        let last_not_allowed = self
-            .encode_buf
-            .bytes()
-            .enumerate()
-            .rev()
-            .find(|(_i, c)| !allowed_char(*c))
-            .map(|(i, _)| i + 1);
+        let prefix = self.encode_buf.trim_end_matches(' ');
+        email_encoding::headers::rfc2047::encode(prefix, &mut self.writer)?;
 
-        let (prefix, to_encode, suffix) = match first_not_allowed {
-            Some(first_not_allowed) => {
-                let last_not_allowed = last_not_allowed.unwrap();
-
-                let (remaining, suffix) = self.encode_buf.split_at(last_not_allowed);
-                let (prefix, to_encode) = remaining.split_at(first_not_allowed);
-
-                (prefix, to_encode, suffix)
-            }
-            None => ("", self.encode_buf.as_str(), ""),
-        };
-
-        self.writer.folding().write_str(prefix)?;
-        email_encoding::headers::rfc2047::encode(to_encode, &mut self.writer)?;
-        self.writer.folding().write_str(suffix)?;
+        // TODO: add a better API for doing this in email-encoding
+        let spaces = self.encode_buf.len() - prefix.len();
+        for _ in 0..spaces {
+            self.writer.space();
+        }
 
         self.encode_buf.clear();
         Ok(())
     }
 }
 
-/// Iterator yielding a string split by space, but spaces are included before the next word.
-struct WordsPlusFillIterator<'a> {
-    s: &'a str,
-}
-
-impl<'a> Iterator for WordsPlusFillIterator<'a> {
-    type Item = &'a str;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if self.s.is_empty() {
-            return None;
-        }
-
-        let next_word = self
-            .s
-            .bytes()
-            .enumerate()
-            .skip(1)
-            .find(|&(_i, c)| c == b' ')
-            .map(|(i, _)| i)
-            .unwrap_or(self.s.len());
-
-        let word = &self.s[..next_word];
-        self.s = &self.s[word.len()..];
-        Some(word)
-    }
-}
-
 fn allowed_str(s: &str) -> bool {
     s.bytes().all(allowed_char)
 }
@@ -466,7 +390,8 @@ const fn allowed_char(c: u8) -> bool {
 mod tests {
     use pretty_assertions::assert_eq;
 
-    use super::{HeaderName, HeaderValue, Headers};
+    use super::{HeaderName, HeaderValue, Headers, To};
+    use crate::message::Mailboxes;
 
     #[test]
     fn valid_headername() {
@@ -522,6 +447,60 @@ mod tests {
         let _ = HeaderName::new_from_ascii_str("");
     }
 
+    #[test]
+    fn headername_headername_eq() {
+        assert_eq!(
+            HeaderName::new_from_ascii_str("From"),
+            HeaderName::new_from_ascii_str("From")
+        );
+    }
+
+    #[test]
+    fn headername_str_eq() {
+        assert_eq!(HeaderName::new_from_ascii_str("From"), "From");
+    }
+
+    #[test]
+    fn str_headername_eq() {
+        assert_eq!("From", HeaderName::new_from_ascii_str("From"));
+    }
+
+    #[test]
+    fn headername_headername_eq_case_insensitive() {
+        assert_eq!(
+            HeaderName::new_from_ascii_str("From"),
+            HeaderName::new_from_ascii_str("from")
+        );
+    }
+
+    #[test]
+    fn headername_str_eq_case_insensitive() {
+        assert_eq!(HeaderName::new_from_ascii_str("From"), "from");
+    }
+
+    #[test]
+    fn str_headername_eq_case_insensitive() {
+        assert_eq!("from", HeaderName::new_from_ascii_str("From"));
+    }
+
+    #[test]
+    fn headername_headername_ne() {
+        assert_ne!(
+            HeaderName::new_from_ascii_str("From"),
+            HeaderName::new_from_ascii_str("To")
+        );
+    }
+
+    #[test]
+    fn headername_str_ne() {
+        assert_ne!(HeaderName::new_from_ascii_str("From"), "To");
+    }
+
+    #[test]
+    fn str_headername_ne() {
+        assert_ne!("From", HeaderName::new_from_ascii_str("To"));
+    }
+
     // names taken randomly from https://it.wikipedia.org/wiki/Pinco_Pallino
 
     #[test]
@@ -529,7 +508,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "John Doe <example@example.com>, Jean Dupont <jean@example.com>".to_string(),
+            "John Doe <example@example.com>, Jean Dupont <jean@example.com>".to_owned(),
         ));
 
         assert_eq!(
@@ -543,7 +522,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand <jemand@example.com>, Jean Dupont <jean@example.com>".to_string(),
+            "Ascii <example@example.com>, John Doe <johndoe@example.com, John Smith <johnsmith@example.com>, Pinco Pallino <pincopallino@example.com>, Jemand <jemand@example.com>, Jean Dupont <jean@example.com>".to_owned(),
         ));
 
         assert_eq!(
@@ -561,7 +540,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
+            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_owned()
         ));
 
         assert_eq!(
@@ -580,7 +559,7 @@ mod tests {
         headers.insert_raw(
             HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know".to_string()
+            "Hello! IGuessTheLastLineWasntLongEnoughSoLetsTryAgainShallWeWhatDoYouThinkItsGoingToHappenIGuessWereAboutToFindOut! I don't know".to_owned()
         ));
 
         assert_eq!(
@@ -598,7 +577,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_string()
+            "1abcdefghijklmnopqrstuvwxyz2abcdefghijklmnopqrstuvwxyz3abcdefghijklmnopqrstuvwxyz4abcdefghijklmnopqrstuvwxyz5abcdefghijklmnopqrstuvwxyz6abcdefghijklmnopqrstuvwxyz".to_owned()
         ));
 
         assert_eq!(
@@ -612,12 +591,12 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "Seán <sean@example.com>".to_string(),
+            "Seán <sean@example.com>".to_owned(),
         ));
 
         assert_eq!(
             headers.to_string(),
-            "To: Se=?utf-8?b?w6E=?=n <sean@example.com>\r\n"
+            "To: =?utf-8?b?U2XDoW4=?= <sean@example.com>\r\n"
         );
     }
 
@@ -626,7 +605,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "🌎 <world@example.com>".to_string(),
+            "🌎 <world@example.com>".to_owned(),
         ));
 
         assert_eq!(
@@ -638,19 +617,43 @@ mod tests {
     #[test]
     fn format_special_with_folding() {
         let mut headers = Headers::new();
-        headers.insert_raw(HeaderValue::new(
-            HeaderName::new_from_ascii_str("To"),
-            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_string(),
-         ) );
+        let to = To::from(Mailboxes::from_iter([
+            "🌍 <world@example.com>".parse().unwrap(),
+            "🦆 Everywhere <ducks@example.com>".parse().unwrap(),
+            "Иванов Иван Иванович <ivanov@example.com>".parse().unwrap(),
+            "Jānis Bērziņš <janis@example.com>".parse().unwrap(),
+            "Seán Ó Rudaí <sean@example.com>".parse().unwrap(),
+        ]));
+        headers.set(to);
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?= Everywhere\r\n",
-                " <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9INCY0LLQsNC9?=\r\n",
-                " =?utf-8?b?0L7QstC40Yc=?= <ivanov@example.com>, J=?utf-8?b?xIFuaXMgQsST?=\r\n",
-                " =?utf-8?b?cnppxYbFoQ==?= <janis@example.com>, Se=?utf-8?b?w6FuIMOTIFJ1?=\r\n",
-                " =?utf-8?b?ZGHDrQ==?= <sean@example.com>\r\n",
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhiBFdmVyeXdo?=\r\n",
+                " =?utf-8?b?ZXJl?= <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LI=?=\r\n",
+                " =?utf-8?b?0LDQvSDQmNCy0LDQvdC+0LLQuNGH?= <ivanov@example.com>,\r\n",
+                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, =?utf-8?b?U2U=?=\r\n",
+                " =?utf-8?b?w6FuIMOTIFJ1ZGHDrQ==?= <sean@example.com>\r\n",
+            )
+        );
+    }
+
+    #[test]
+    fn format_special_with_folding_raw() {
+        let mut headers = Headers::new();
+        headers.insert_raw(HeaderValue::new(
+            HeaderName::new_from_ascii_str("To"),
+            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_owned(),
+        ));
+
+        assert_eq!(
+            headers.to_string(),
+            concat!(
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?=\r\n",
+                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9?=\r\n",
+                " =?utf-8?b?INCY0LLQsNC90L7QstC40Yc=?= <ivanov@example.com>,\r\n",
+                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, =?utf-8?b?U2U=?=\r\n",
+                " =?utf-8?b?w6FuIMOTIFJ1ZGHDrQ==?= <sean@example.com>\r\n",
             )
         );
     }
@@ -661,7 +664,7 @@ mod tests {
         headers.insert_raw(
             HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_string(),)
+            "🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳🥳".to_owned(),)
         );
 
         assert_eq!(
@@ -682,7 +685,7 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! \r\n This is \" bad \0. 👋".to_string(),
+            "Hello! \r\n This is \" bad \0. 👋".to_owned(),
         ));
 
         assert_eq!(
@@ -697,22 +700,22 @@ mod tests {
         headers.insert_raw(
             HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_string()
+            "Hello! This is lettre, and this IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I guess that's it!".to_owned()
             )
         );
         headers.insert_raw(
             HeaderValue::new(
             HeaderName::new_from_ascii_str("To"),
-            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_string(),
+            "🌍 <world@example.com>, 🦆 Everywhere <ducks@example.com>, Иванов Иван Иванович <ivanov@example.com>, Jānis Bērziņš <janis@example.com>, Seán Ó Rudaí <sean@example.com>".to_owned(),
             )
         );
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("From"),
-            "Someone <somewhere@example.com>".to_string(),
+            "Someone <somewhere@example.com>".to_owned(),
         ));
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Content-Transfer-Encoding"),
-            "quoted-printable".to_string(),
+            "quoted-printable".to_owned(),
         ));
 
         assert_eq!(
@@ -721,11 +724,11 @@ mod tests {
                 "Subject: Hello! This is lettre, and this\r\n",
                 " IsAVeryLongLineDoYouKnowWhatsGoingToHappenIGuessWeAreGoingToFindOut. Ok I\r\n",
                 " guess that's it!\r\n",
-                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?= Everywhere\r\n",
-                " <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9INCY0LLQsNC9?=\r\n",
-                " =?utf-8?b?0L7QstC40Yc=?= <ivanov@example.com>, J=?utf-8?b?xIFuaXMgQsST?=\r\n",
-                " =?utf-8?b?cnppxYbFoQ==?= <janis@example.com>, Se=?utf-8?b?w6FuIMOTIFJ1?=\r\n",
-                " =?utf-8?b?ZGHDrQ==?= <sean@example.com>\r\n",
+                "To: =?utf-8?b?8J+MjQ==?= <world@example.com>, =?utf-8?b?8J+mhg==?=\r\n",
+                " Everywhere <ducks@example.com>, =?utf-8?b?0JjQstCw0L3QvtCyINCY0LLQsNC9?=\r\n",
+                " =?utf-8?b?INCY0LLQsNC90L7QstC40Yc=?= <ivanov@example.com>,\r\n",
+                " =?utf-8?b?SsSBbmlzIELEk3J6acWGxaE=?= <janis@example.com>, =?utf-8?b?U2U=?=\r\n",
+                " =?utf-8?b?w6FuIMOTIFJ1ZGHDrQ==?= <sean@example.com>\r\n",
                 "From: Someone <somewhere@example.com>\r\n",
                 "Content-Transfer-Encoding: quoted-printable\r\n",
             )
@@ -737,14 +740,14 @@ mod tests {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "＋仮名 :a;go; ;;;;;s;;;;;;;;;;;;;;;;fffeinmjgggggggggｆっ".to_string(),
+            "＋仮名 :a;go; ;;;;;s;;;;;;;;;;;;;;;;fffeinmjgggggggggｆっ".to_owned(),
         ));
 
         assert_eq!(
             headers.to_string(),
             concat!(
-                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go;\r\n",
-                " ;;;;;s;;;;;;;;;;;;;;;;fffeinmjggggggggg=?utf-8?b?772G44Gj?=\r\n"
+                "Subject: =?utf-8?b?77yL5Luu5ZCN?= :a;go; =?utf-8?b?Ozs7OztzOzs7Ozs7Ozs7?=\r\n",
+                " =?utf-8?b?Ozs7Ozs7O2ZmZmVpbm1qZ2dnZ2dnZ2dn772G44Gj?=\r\n",
             )
         );
     }

src/message/header/special.rs

@@ -91,14 +91,14 @@ mod test {
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
-            "1.0".to_string(),
+            "1.0".to_owned(),
         ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MIME_VERSION_1_0));
 
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("MIME-Version"),
-            "0.1".to_string(),
+            "0.1".to_owned(),
         ));
 
         assert_eq!(headers.get::<MimeVersion>(), Some(MimeVersion::new(0, 1)));

src/message/header/textual.rs

@@ -109,12 +109,23 @@ mod test {
         );
     }
 
+    #[test]
+    fn format_utf8_word() {
+        let mut headers = Headers::new();
+        headers.set(Subject("Administratör".into()));
+
+        assert_eq!(
+            headers.to_string(),
+            "Subject: =?utf-8?b?QWRtaW5pc3RyYXTDtnI=?=\r\n"
+        );
+    }
+
     #[test]
     fn parse_ascii() {
         let mut headers = Headers::new();
         headers.insert_raw(HeaderValue::new(
             HeaderName::new_from_ascii_str("Subject"),
-            "Sample subject".to_string(),
+            "Sample subject".to_owned(),
         ));
 
         assert_eq!(

src/message/mailbox/mod.rs

@@ -1,3 +1,4 @@
+mod parsers;
 #[cfg(feature = "serde")]
 mod serde;
 mod types;

src/message/mailbox/parsers/mod.rs

@@ -0,0 +1,5 @@
+mod rfc2234;
+mod rfc2822;
+mod rfc5336;
+
+pub(crate) use rfc2822::{mailbox, mailbox_list};

src/message/mailbox/parsers/rfc2234.rs

@@ -0,0 +1,32 @@
+//! Partial parsers implementation of [RFC2234]: Augmented BNF for
+//! Syntax Specifications: ABNF.
+//!
+//! [RFC2234]: https://datatracker.ietf.org/doc/html/rfc2234
+
+use chumsky::{error::Cheap, prelude::*};
+
+// 6.1  Core Rules
+// https://datatracker.ietf.org/doc/html/rfc2234#section-6.1
+
+// ALPHA          =  %x41-5A / %x61-7A   ; A-Z / a-z
+pub(super) fn alpha() -> impl Parser<char, char, Error = Cheap<char>> {
+    filter(|c: &char| c.is_ascii_alphabetic())
+}
+
+// DIGIT          =  %x30-39
+//                        ; 0-9
+pub(super) fn digit() -> impl Parser<char, char, Error = Cheap<char>> {
+    filter(|c: &char| c.is_ascii_digit())
+}
+
+// DQUOTE         =  %x22
+//                        ; " (Double Quote)
+pub(super) fn dquote() -> impl Parser<char, char, Error = Cheap<char>> {
+    just('"')
+}
+
+// WSP            =  SP / HTAB
+//                        ; white space
+pub(super) fn wsp() -> impl Parser<char, char, Error = Cheap<char>> {
+    choice((just(' '), just('\t')))
+}

src/message/mailbox/parsers/rfc2822.rs

@@ -0,0 +1,248 @@
+//! Partial parsers implementation of [RFC2822]: Internet Message
+//! Format.
+//!
+//! [RFC2822]: https://datatracker.ietf.org/doc/html/rfc2822
+
+use chumsky::{error::Cheap, prelude::*};
+
+use super::{rfc2234, rfc5336};
+
+// 3.2.1. Primitive Tokens
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
+
+// NO-WS-CTL       =       %d1-8 /         ; US-ASCII control characters
+//                         %d11 /          ;  that do not include the
+//                         %d12 /          ;  carriage return, line feed,
+//                         %d14-31 /       ;  and white space characters
+//                         %d127
+fn no_ws_ctl() -> impl Parser<char, char, Error = Cheap<char>> {
+    filter(|c| matches!(u32::from(*c), 1..=8 | 11 | 12 | 14..=31 | 127))
+}
+
+// text            =       %d1-9 /         ; Characters excluding CR and LF
+//                         %d11 /
+//                         %d12 /
+//                         %d14-127 /
+//                         obs-text
+fn text() -> impl Parser<char, char, Error = Cheap<char>> {
+    filter(|c| matches!(u32::from(*c), 1..=9 | 11 | 12 | 14..=127))
+}
+
+// 3.2.2. Quoted characters
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
+
+// quoted-pair     =       ("\" text) / obs-qp
+fn quoted_pair() -> impl Parser<char, char, Error = Cheap<char>> {
+    just('\\').ignore_then(text())
+}
+
+// 3.2.3. Folding white space and comments
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.3
+
+// FWS             =       ([*WSP CRLF] 1*WSP) /   ; Folding white space
+//                         obs-FWS
+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 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()
+}
+
+// 3.2.4. Atom
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.4
+
+// atext           =       ALPHA / DIGIT / ; Any character except controls,
+//                         "!" / "#" /     ;  SP, and specials.
+//                         "$" / "%" /     ;  Used for atoms
+//                         "&" / "'" /
+//                         "*" / "+" /
+//                         "-" / "/" /
+//                         "=" / "?" /
+//                         "^" / "_" /
+//                         "`" / "{" /
+//                         "|" / "}" /
+//                         "~"
+pub(super) fn atext() -> impl Parser<char, char, Error = Cheap<char>> {
+    choice((
+        rfc2234::alpha(),
+        rfc2234::digit(),
+        filter(|c| {
+            matches!(
+                *c,
+                '!' | '#'
+                    | '$'
+                    | '%'
+                    | '&'
+                    | '\''
+                    | '*'
+                    | '+'
+                    | '-'
+                    | '/'
+                    | '='
+                    | '?'
+                    | '^'
+                    | '_'
+                    | '`'
+                    | '{'
+                    | '|'
+                    | '}'
+                    | '~'
+            )
+        }),
+        // also allow non ASCII UTF8 chars
+        rfc5336::utf8_non_ascii(),
+    ))
+}
+
+// atom            =       [CFWS] 1*atext [CFWS]
+pub(super) fn atom() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    cfws().chain(atext().repeated().at_least(1))
+}
+
+// dot-atom        =       [CFWS] dot-atom-text [CFWS]
+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 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))
+            .repeated()
+            .at_least(1)
+            .flatten(),
+    )
+}
+
+// 3.2.5. Quoted strings
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
+
+// qtext           =       NO-WS-CTL /     ; Non white space controls
+//
+//                         %d33 /          ; The rest of the US-ASCII
+//                         %d35-91 /       ;  characters not including "\"
+//                         %d93-126        ;  or the quote character
+fn qtext() -> impl Parser<char, char, Error = Cheap<char>> {
+    choice((
+        filter(|c| matches!(u32::from(*c), 33 | 35..=91 | 93..=126)),
+        no_ws_ctl(),
+    ))
+}
+
+// qcontent        =       qtext / quoted-pair
+pub(super) fn qcontent() -> impl Parser<char, char, Error = Cheap<char>> {
+    choice((qtext(), quoted_pair(), rfc5336::utf8_non_ascii()))
+}
+
+// quoted-string   =       [CFWS]
+//                         DQUOTE *([FWS] qcontent) [FWS] DQUOTE
+//                         [CFWS]
+fn quoted_string() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    rfc2234::dquote()
+        .ignore_then(fws().chain(qcontent()).repeated().flatten())
+        .then_ignore(text::whitespace())
+        .then_ignore(rfc2234::dquote())
+}
+
+// 3.2.6. Miscellaneous tokens
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.6
+
+// word            =       atom / quoted-string
+fn word() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    choice((quoted_string(), atom()))
+}
+
+// phrase          =       1*word / obs-phrase
+fn phrase() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    choice((obs_phrase(), word().repeated().at_least(1).flatten()))
+}
+
+// 3.4. Address Specification
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.4
+
+// mailbox         =       name-addr / addr-spec
+pub(crate) fn mailbox() -> impl Parser<char, (Option<String>, (String, String)), Error = Cheap<char>>
+{
+    choice((name_addr(), addr_spec().map(|addr| (None, addr)))).then_ignore(end())
+}
+
+// name-addr       =       [display-name] angle-addr
+fn name_addr() -> impl Parser<char, (Option<String>, (String, String)), Error = Cheap<char>> {
+    display_name().collect().or_not().then(angle_addr())
+}
+
+// angle-addr      =       [CFWS] "<" addr-spec ">" [CFWS] / obs-angle-addr
+fn angle_addr() -> impl Parser<char, (String, String), Error = Cheap<char>> {
+    addr_spec()
+        .delimited_by(just('<').ignored(), just('>').ignored())
+        .padded()
+}
+
+// display-name    =       phrase
+fn display_name() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    phrase()
+}
+
+// mailbox-list    =       (mailbox *("," mailbox)) / obs-mbox-list
+pub(crate) fn mailbox_list(
+) -> impl Parser<char, Vec<(Option<String>, (String, String))>, Error = Cheap<char>> {
+    choice((name_addr(), addr_spec().map(|addr| (None, addr))))
+        .separated_by(just(',').padded())
+        .then_ignore(end())
+}
+
+// 3.4.1. Addr-spec specification
+// https://datatracker.ietf.org/doc/html/rfc2822#section-3.4.1
+
+// addr-spec       =       local-part "@" domain
+pub fn addr_spec() -> impl Parser<char, (String, String), Error = Cheap<char>> {
+    local_part()
+        .collect()
+        .then_ignore(just('@'))
+        .then(domain().collect())
+}
+
+// local-part      =       dot-atom / quoted-string / obs-local-part
+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 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()))
+}
+
+// 4.1. Miscellaneous obsolete tokens
+// https://datatracker.ietf.org/doc/html/rfc2822#section-4.1
+
+// obs-phrase      =       word *(word / "." / CFWS)
+fn obs_phrase() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    // NOTE: the CFWS is already captured by the word, no need to add
+    // it there.
+    word().chain(
+        choice((word(), just('.').repeated().exactly(1)))
+            .repeated()
+            .flatten(),
+    )
+}
+
+// 4.4. Obsolete Addressing
+// https://datatracker.ietf.org/doc/html/rfc2822#section-4.4
+
+// obs-local-part  =       word *("." word)
+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 fn obs_domain() -> impl Parser<char, Vec<char>, Error = Cheap<char>> {
+    atom().chain(just('.').chain(atom()).repeated().flatten())
+}

src/message/mailbox/parsers/rfc5336.rs

@@ -0,0 +1,17 @@
+//! Partial parsers implementation of [RFC5336]: SMTP Extension for
+//! Internationalized Email Addresses.
+//!
+//! [RFC5336]: https://datatracker.ietf.org/doc/html/rfc5336
+
+use chumsky::{error::Cheap, prelude::*};
+
+// 3.3.  Extended Mailbox Address Syntax
+// https://datatracker.ietf.org/doc/html/rfc5336#section-3.3
+
+// UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4
+// UTF8-2 =  <See Section 4 of RFC 3629>
+// UTF8-3 =  <See Section 4 of RFC 3629>
+// UTF8-4 =  <See Section 4 of RFC 3629>
+pub(super) fn utf8_non_ascii() -> impl Parser<char, char, Error = Cheap<char>> {
+    filter(|c: &char| c.len_utf8() > 1)
+}

src/message/mailbox/serde.rs

@@ -13,7 +13,7 @@ impl Serialize for Mailbox {
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 
@@ -111,7 +111,7 @@ impl Serialize for Mailboxes {
     where
         S: Serializer,
     {
-        serializer.serialize_str(&self.to_string())
+        serializer.collect_str(self)
     }
 }
 
@@ -179,7 +179,7 @@ mod test {
     }
 
     #[test]
-    fn parse_mailbox_object_address_stirng() {
+    fn parse_mailbox_object_address_string() {
         let m: Mailbox = from_str(r#"{ "name": "Kai", "email": "kayo@example.com" }"#).unwrap();
         assert_eq!(m, "Kai <kayo@example.com>".parse().unwrap());
     }
@@ -198,7 +198,7 @@ mod test {
             from_str(r#""yin@dtb.com, Hei <hei@dtb.com>, Kai <kayo@example.com>""#).unwrap();
         assert_eq!(
             m,
-            "<yin@dtb.com>, Hei <hei@dtb.com>, Kai <kayo@example.com>"
+            "yin@dtb.com, Hei <hei@dtb.com>, Kai <kayo@example.com>"
                 .parse()
                 .unwrap()
         );
@@ -211,7 +211,7 @@ mod test {
                 .unwrap();
         assert_eq!(
             m,
-            "<yin@dtb.com>, Hei <hei@dtb.com>, Kai <kayo@example.com>"
+            "yin@dtb.com, Hei <hei@dtb.com>, Kai <kayo@example.com>"
                 .parse()
                 .unwrap()
         );

src/message/mailbox/types.rs

@@ -5,15 +5,17 @@ use std::{
     str::FromStr,
 };
 
-use email_encoding::headers::EmailWriter;
+use chumsky::prelude::*;
+use email_encoding::headers::writer::EmailWriter;
 
+use super::parsers;
 use crate::address::{Address, AddressError};
 
 /// Represents an email address with an optional name for the sender/recipient.
 ///
 /// This type contains email address and the sender/recipient name (_Some Name \<user@domain.tld\>_ or _withoutname@domain.tld_).
 ///
-/// **NOTE**: Enable feature "serde" to be able serialize/deserialize it using [serde](https://serde.rs/).
+/// **NOTE**: Enable feature "serde" to be able to serialize/deserialize it using [serde](https://serde.rs/).
 ///
 /// # Examples
 ///
@@ -86,7 +88,7 @@ impl Mailbox {
 
 impl Display for Mailbox {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
-        if let Some(ref name) = self.name {
+        if let Some(name) = &self.name {
             let name = name.trim();
             if !name.is_empty() {
                 write_word(f, name)?;
@@ -108,40 +110,24 @@ impl<S: Into<String>, T: Into<String>> TryFrom<(S, T)> for Mailbox {
     }
 }
 
-/*
-impl<S: AsRef<&str>, T: AsRef<&str>> TryFrom<(S, T)> for Mailbox {
-    type Error = AddressError;
-
-    fn try_from(header: (S, T)) -> Result<Self, Self::Error> {
-        let (name, address) = header;
-        Ok(Mailbox::new(Some(name.as_ref()), address.as_ref().parse()?))
-    }
-}*/
-
 impl FromStr for Mailbox {
     type Err = AddressError;
 
     fn from_str(src: &str) -> Result<Mailbox, Self::Err> {
-        match (src.find('<'), src.find('>')) {
-            (Some(addr_open), Some(addr_close)) if addr_open < addr_close => {
-                let name = src.split_at(addr_open).0;
-                let addr_open = addr_open + 1;
-                let addr = src.split_at(addr_open).1.split_at(addr_close - addr_open).0;
-                let addr = addr.parse()?;
-                let name = name.trim();
-                let name = if name.is_empty() {
-                    None
-                } else {
-                    Some(name.into())
-                };
-                Ok(Mailbox::new(name, addr))
-            }
-            (Some(_), _) => Err(AddressError::Unbalanced),
-            _ => {
-                let addr = src.parse()?;
-                Ok(Mailbox::new(None, addr))
-            }
-        }
+        let (name, (user, domain)) = parsers::mailbox().parse(src).map_err(|_errs| {
+            // TODO: improve error management
+            AddressError::InvalidInput
+        })?;
+
+        let mailbox = Mailbox::new(name, Address::new(user, domain)?);
+
+        Ok(mailbox)
+    }
+}
+
+impl From<Address> for Mailbox {
+    fn from(value: Address) -> Self {
+        Self::new(None, value)
     }
 }
 
@@ -149,7 +135,7 @@ impl FromStr for Mailbox {
 ///
 /// This type contains a sequence of mailboxes (_Some Name \<user@domain.tld\>, Another Name \<other@domain.tld\>, withoutname@domain.tld, ..._).
 ///
-/// **NOTE**: Enable feature "serde" to be able serialize/deserialize it using [serde](https://serde.rs/).
+/// **NOTE**: Enable feature "serde" to be able to serialize/deserialize it using [serde](https://serde.rs/).
 #[derive(Debug, Clone, PartialOrd, Ord, PartialEq, Eq, Hash)]
 pub struct Mailboxes(Vec<Mailbox>);
 
@@ -356,34 +342,16 @@ impl Display for Mailboxes {
 impl FromStr for Mailboxes {
     type Err = AddressError;
 
-    fn from_str(mut src: &str) -> Result<Self, Self::Err> {
+    fn from_str(src: &str) -> Result<Self, Self::Err> {
         let mut mailboxes = Vec::new();
 
-        if !src.is_empty() {
-            // n-1 elements
-            let mut skip = 0;
-            while let Some(i) = src[skip..].find(',') {
-                let left = &src[..skip + i];
+        let parsed_mailboxes = parsers::mailbox_list().parse(src).map_err(|_errs| {
+            // TODO: improve error management
+            AddressError::InvalidInput
+        })?;
 
-                match left.trim().parse() {
-                    Ok(mailbox) => {
-                        mailboxes.push(mailbox);
-
-                        src = &src[left.len() + ",".len()..];
-                        skip = 0;
-                    }
-                    Err(AddressError::MissingParts) => {
-                        skip = left.len() + ",".len();
-                    }
-                    Err(err) => {
-                        return Err(err);
-                    }
-                }
-            }
-
-            // last element
-            let mailbox = src.trim().parse()?;
-            mailboxes.push(mailbox);
+        for (name, (user, domain)) in parsed_mailboxes {
+            mailboxes.push(Mailbox::new(name, Address::new(user, domain)?))
         }
 
         Ok(Mailboxes(mailboxes))
@@ -397,7 +365,7 @@ fn write_word(f: &mut Formatter<'_>, s: &str) -> FmtResult {
     } else {
         // Quoted string: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
         f.write_char('"')?;
-        for &c in s.as_bytes() {
+        for c in s.chars() {
             write_quoted_string_char(f, c)?;
         }
         f.write_char('"')?;
@@ -436,45 +404,46 @@ fn is_valid_atom_char(c: u8) -> bool {
 		b'}' |
 		b'~' |
 
-		// Not techically allowed but will be escaped into allowed characters.
+		// Not technically allowed but will be escaped into allowed characters.
 		128..=255)
 }
 
 // https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.5
-fn write_quoted_string_char(f: &mut Formatter<'_>, c: u8) -> FmtResult {
+fn write_quoted_string_char(f: &mut Formatter<'_>, c: char) -> FmtResult {
     match c {
-		// NO-WS-CTL: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
-		1..=8 | 11 | 12 | 14..=31 | 127 |
+        // Can not be encoded.
+        '\n' | '\r' => Err(std::fmt::Error),
 
-		// Note, not qcontent but can be put before or after any qcontent.
-		b'\t' |
-		b' ' |
+        // Note, not qcontent but can be put before or after any qcontent.
+        '\t' | ' ' => f.write_char(c),
 
-		// The rest of the US-ASCII except \ and "
-		33 |
-		35..=91 |
-		93..=126 |
+        c if match c as u32 {
+            // NO-WS-CTL: https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.1
+            1..=8 | 11 | 12 | 14..=31 | 127 |
 
-		// Non-ascii characters will be escaped separately later.
-		128..=255
+            // The rest of the US-ASCII except \ and "
+            33 |
+            35..=91 |
+            93..=126 |
 
-		=> f.write_char(c.into()),
+            // Non-ascii characters will be escaped separately later.
+            128.. => true,
+            _ => false,
+        } =>
+        {
+            f.write_char(c)
+        }
 
-		// Can not be encoded.
-		b'\n' | b'\r' => Err(std::fmt::Error),
-
-		c => {
-			// quoted-pair https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
-			f.write_char('\\')?;
-			f.write_char(c.into())
-		}
-	}
+        _ => {
+            // quoted-pair https://datatracker.ietf.org/doc/html/rfc2822#section-3.2.2
+            f.write_char('\\')?;
+            f.write_char(c)
+        }
+    }
 }
 
 #[cfg(test)]
 mod test {
-    use std::convert::TryInto;
-
     use pretty_assertions::assert_eq;
 
     use super::Mailbox;
@@ -515,6 +484,34 @@ mod test {
         );
     }
 
+    #[test]
+    fn mailbox_format_address_with_comma_and_non_ascii() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some("Laşt, First".into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Laşt, First" <kayo@example.com>"#
+        );
+    }
+
+    #[test]
+    fn mailbox_format_address_with_comma_and_quoted_non_ascii() {
+        assert_eq!(
+            format!(
+                "{}",
+                Mailbox::new(
+                    Some(r#"Laşt, "First""#.into()),
+                    "kayo@example.com".parse().unwrap()
+                )
+            ),
+            r#""Laşt, \"First\"" <kayo@example.com>"#
+        );
+    }
+
     #[test]
     fn mailbox_format_address_with_color() {
         assert_eq!(
@@ -589,7 +586,7 @@ mod test {
     #[test]
     fn parse_address_from_tuple() {
         assert_eq!(
-            ("K.".to_string(), "kayo@example.com".to_string()).try_into(),
+            ("K.".to_owned(), "kayo@example.com".to_owned()).try_into(),
             Ok(Mailbox::new(
                 Some("K.".into()),
                 "kayo@example.com".parse().unwrap()

src/message/mimebody.rs

@@ -17,6 +17,16 @@ pub(super) enum Part {
     Multi(MultiPart),
 }
 
+impl Part {
+    #[cfg(feature = "dkim")]
+    pub(super) fn format_body(&self, out: &mut Vec<u8>) {
+        match self {
+            Part::Single(part) => part.format_body(out),
+            Part::Multi(part) => part.format_body(out),
+        }
+    }
+}
+
 impl EmailFormat for Part {
     fn format(&self, out: &mut Vec<u8>) {
         match self {
@@ -100,14 +110,14 @@ impl SinglePart {
         SinglePartBuilder::new()
     }
 
-    /// Directly create a `SinglePart` from an plain UTF-8 content
+    /// Directly create a `SinglePart` from a plain UTF-8 content
     pub fn plain<T: IntoBody>(body: T) -> Self {
         Self::builder()
             .header(header::ContentType::TEXT_PLAIN)
             .body(body)
     }
 
-    /// Directly create a `SinglePart` from an UTF-8 HTML content
+    /// Directly create a `SinglePart` from a UTF-8 HTML content
     pub fn html<T: IntoBody>(body: T) -> Self {
         Self::builder()
             .header(header::ContentType::TEXT_HTML)
@@ -132,6 +142,12 @@ impl SinglePart {
         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");
+    }
 }
 
 impl EmailFormat for SinglePart {
@@ -139,8 +155,7 @@ impl EmailFormat for SinglePart {
         write!(out, "{}", self.headers)
             .expect("A Write implementation panicked while formatting headers");
         out.extend_from_slice(b"\r\n");
-        out.extend_from_slice(&self.body);
-        out.extend_from_slice(b"\r\n");
+        self.format_body(out);
     }
 }
 
@@ -149,17 +164,17 @@ impl EmailFormat for SinglePart {
 pub enum MultiPartKind {
     /// Mixed kind to combine unrelated content parts
     ///
-    /// For example this kind can be used to mix email message and attachments.
+    /// For example, this kind can be used to mix an email message and attachments.
     Mixed,
 
     /// Alternative kind to join several variants of same email contents.
     ///
-    /// That kind is recommended to use for joining plain (text) and rich (HTML) messages into single email message.
+    /// That kind is recommended to use for joining plain (text) and rich (HTML) messages into a single email message.
     Alternative,
 
     /// Related kind to mix content and related resources.
     ///
-    /// For example, you can include images into HTML content using that.
+    /// For example, you can include images in HTML content using that.
     Related,
 
     /// Encrypted kind for encrypted messages
@@ -190,9 +205,9 @@ impl MultiPartKind {
             },
             boundary,
             match self {
-                Self::Encrypted { protocol } => format!("; protocol=\"{}\"", protocol),
+                Self::Encrypted { protocol } => format!("; protocol=\"{protocol}\""),
                 Self::Signed { protocol, micalg } =>
-                    format!("; protocol=\"{}\"; micalg=\"{}\"", protocol, micalg),
+                    format!("; protocol=\"{protocol}\"; micalg=\"{micalg}\""),
                 _ => String::new(),
             }
         )
@@ -373,14 +388,9 @@ impl MultiPart {
         self.format(&mut out);
         out
     }
-}
-
-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");
 
+    /// Format only the multipart body
+    fn format_body(&self, out: &mut Vec<u8>) {
         let boundary = self.boundary();
 
         for part in &self.parts {
@@ -396,12 +406,20 @@ impl EmailFormat for MultiPart {
     }
 }
 
+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");
+        self.format_body(out);
+    }
+}
+
 #[cfg(test)]
 mod test {
     use pretty_assertions::assert_eq;
 
     use super::*;
-    use crate::message::header;
 
     #[test]
     fn single_part_binary() {

src/message/mod.rs

@@ -14,7 +14,7 @@
 //! The easiest way of creating a message, which uses a plain text body.
 //!
 //! ```rust
-//! use lettre::message::Message;
+//! use lettre::message::{header::ContentType, Message};
 //!
 //! # use std::error::Error;
 //! # fn main() -> Result<(), Box<dyn Error>> {
@@ -23,6 +23,7 @@
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
 //!     .to("Hei <hei@domain.tld>".parse()?)
 //!     .subject("Happy new year")
+//!     .header(ContentType::TEXT_PLAIN)
 //!     .body(String::from("Be happy!"))?;
 //! # Ok(())
 //! # }
@@ -38,6 +39,7 @@
 //! To: Hei <hei@domain.tld>
 //! Subject: Happy new year
 //! Date: Sat, 12 Dec 2020 16:33:19 GMT
+//! Content-Type: text/plain; charset=utf-8
 //! Content-Transfer-Encoding: 7bit
 //!
 //! Be happy!
@@ -343,9 +345,9 @@ impl MessageBuilder {
                 let hostname = hostname::get()
                     .map_err(|_| ())
                     .and_then(|s| s.into_string().map_err(|_| ()))
-                    .unwrap_or_else(|_| DEFAULT_MESSAGE_ID_DOMAIN.to_string());
+                    .unwrap_or_else(|_| DEFAULT_MESSAGE_ID_DOMAIN.to_owned());
                 #[cfg(not(feature = "hostname"))]
-                let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_string();
+                let hostname = DEFAULT_MESSAGE_ID_DOMAIN.to_owned();
 
                 self.header(header::MessageId::from(
                     // https://tools.ietf.org/html/rfc5322#section-3.6.4
@@ -356,7 +358,7 @@ impl MessageBuilder {
     }
 
     /// Set [User-Agent
-    /// header](https://tools.ietf.org/html/draft-melnikov-email-user-agent-004)
+    /// header](https://tools.ietf.org/html/draft-melnikov-email-user-agent-00)
     pub fn user_agent(self, id: String) -> Self {
         self.header(header::UserAgent::from(id))
     }
@@ -386,13 +388,13 @@ impl MessageBuilder {
 
     /// Keep the `Bcc` header
     ///
-    /// By default the `Bcc` header is removed from the email after
+    /// By default, the `Bcc` header is removed from the email after
     /// using it to generate the message envelope. In some cases though,
     /// like when saving the email as an `.eml`, or sending through
     /// some transports (like the Gmail API) that don't take a separate
     /// envelope value, it becomes necessary to keep the `Bcc` header.
     ///
-    /// Calling this method overrides the default behaviour.
+    /// Calling this method overrides the default behavior.
     pub fn keep_bcc(mut self) -> Self {
         self.drop_bcc = false;
         self
@@ -501,6 +503,11 @@ impl Message {
         &self.headers
     }
 
+    /// Get a mutable reference to the headers
+    pub fn headers_mut(&mut self) -> &mut Headers {
+        &mut self.headers
+    }
+
     /// Get `Message` envelope
     pub fn envelope(&self) -> &Envelope {
         &self.envelope
@@ -518,7 +525,7 @@ impl Message {
     pub(crate) fn body_raw(&self) -> Vec<u8> {
         let mut out = Vec::new();
         match &self.body {
-            MessageBody::Mime(p) => p.format(&mut out),
+            MessageBody::Mime(p) => p.format_body(&mut out),
             MessageBody::Raw(r) => out.extend_from_slice(r),
         };
         out.extend_from_slice(b"\r\n");
@@ -539,7 +546,7 @@ impl Message {
     ///     .reply_to("Bob <bob@example.org>".parse().unwrap())
     ///     .to("Carla <carla@example.net>".parse().unwrap())
     ///     .subject("Hello")
-    ///     .body("Hi there, it's a test email, with utf-8 chars ë!\n\n\n".to_string())
+    ///     .body("Hi there, it's a test email, with utf-8 chars ë!\n\n\n".to_owned())
     ///     .unwrap();
     /// let key = "-----BEGIN RSA PRIVATE KEY-----
     /// MIIEowIBAAKCAQEAt2gawjoybf0mAz0mSX0cq1ah5F9cPazZdCwLnFBhRufxaZB8
@@ -570,8 +577,8 @@ impl Message {
     /// -----END RSA PRIVATE KEY-----";
     /// let signing_key = DkimSigningKey::new(key, DkimSigningAlgorithm::Rsa).unwrap();
     /// message.sign(&DkimConfig::default_config(
-    ///     "dkimtest".to_string(),
-    ///     "example.org".to_string(),
+    ///     "dkimtest".to_owned(),
+    ///     "example.org".to_owned(),
     ///     signing_key,
     /// ));
     /// println!(
@@ -628,7 +635,7 @@ mod test {
     }
 
     #[test]
-    fn email_miminal_message() {
+    fn email_minimal_message() {
         assert!(Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
             .to("NoBody <nobody@domain.tld>".parse().unwrap())
@@ -673,7 +680,7 @@ mod test {
                 "Date: Tue, 15 Nov 1994 08:12:31 +0000\r\n",
                 "From: =?utf-8?b?0JrQsNC4?= <kayo@example.com>\r\n",
                 "To: \"Pony O.P.\" <pony@domain.tld>\r\n",
-                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvQ==?=!\r\n",
+                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvSE=?=\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Happy new year!"
@@ -711,7 +718,7 @@ mod test {
                 "Bcc: hidden@example.com\r\n",
                 "From: =?utf-8?b?0JrQsNC4?= <kayo@example.com>\r\n",
                 "To: \"Pony O.P.\" <pony@domain.tld>\r\n",
-                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvQ==?=!\r\n",
+                "Subject: =?utf-8?b?0Y/So9CwINC10Lsg0LHQtdC705nQvSE=?=\r\n",
                 "Content-Transfer-Encoding: 7bit\r\n",
                 "\r\n",
                 "Happy new year!"

src/transport/file/error.rs

@@ -54,7 +54,7 @@ impl fmt::Debug for Error {
 
         builder.field("kind", &self.inner.kind);
 
-        if let Some(ref source) = self.inner.source {
+        if let Some(source) = &self.inner.source {
             builder.field("source", source);
         }
 
@@ -70,8 +70,8 @@ impl fmt::Display for Error {
             Kind::Envelope => f.write_str("internal client error")?,
         };
 
-        if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+        if let Some(e) = &self.inner.source {
+            write!(f, ": {e}")?;
         }
 
         Ok(())

src/transport/file/mod.rs

@@ -157,7 +157,7 @@ mod error;
 type Id = String;
 
 /// Writes the content and the envelope information to a file
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(docsrs, doc(cfg(feature = "file-transport")))]
 pub struct FileTransport {
@@ -167,7 +167,7 @@ pub struct FileTransport {
 }
 
 /// Asynchronously writes the content and the envelope information to a file
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(docsrs, doc(cfg(any(feature = "tokio1", feature = "async-std1"))))]
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
@@ -208,18 +208,18 @@ impl FileTransport {
     pub fn read(&self, email_id: &str) -> Result<(Envelope, Vec<u8>), Error> {
         use std::fs;
 
-        let eml_file = self.path.join(format!("{}.eml", email_id));
+        let eml_file = self.path.join(format!("{email_id}.eml"));
         let eml = fs::read(eml_file).map_err(error::io)?;
 
-        let json_file = self.path.join(format!("{}.json", email_id));
-        let json = fs::read(&json_file).map_err(error::io)?;
+        let json_file = self.path.join(format!("{email_id}.json"));
+        let json = fs::read(json_file).map_err(error::io)?;
         let envelope = serde_json::from_slice(&json).map_err(error::envelope)?;
 
         Ok((envelope, eml))
     }
 
     fn path(&self, email_id: &Uuid, extension: &str) -> PathBuf {
-        self.path.join(format!("{}.{}", email_id, extension))
+        self.path.join(format!("{email_id}.{extension}"))
     }
 }
 
@@ -255,10 +255,10 @@ where
     /// Reads the envelope and the raw message content.
     #[cfg(feature = "file-transport-envelope")]
     pub async fn read(&self, email_id: &str) -> Result<(Envelope, Vec<u8>), Error> {
-        let eml_file = self.inner.path.join(format!("{}.eml", email_id));
+        let eml_file = self.inner.path.join(format!("{email_id}.eml"));
         let eml = E::fs_read(&eml_file).await.map_err(error::io)?;
 
-        let json_file = self.inner.path.join(format!("{}.json", email_id));
+        let json_file = self.inner.path.join(format!("{email_id}.json"));
         let json = E::fs_read(&json_file).await.map_err(error::io)?;
         let envelope = serde_json::from_slice(&json).map_err(error::envelope)?;
 
@@ -276,6 +276,8 @@ impl Transport for FileTransport {
         let email_id = Uuid::new_v4();
 
         let file = self.path(&email_id, "eml");
+        #[cfg(feature = "tracing")]
+        tracing::debug!(?file, "writing email to");
         fs::write(file, email).map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]
@@ -306,6 +308,8 @@ where
         let email_id = Uuid::new_v4();
 
         let file = self.inner.path(&email_id, "eml");
+        #[cfg(feature = "tracing")]
+        tracing::debug!(?file, "writing email to");
         E::fs_write(&file, email).await.map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]

src/transport/mod.rs

@@ -12,7 +12,7 @@
 //!
 //! * a service from your Cloud or hosting provider
 //! * an email server ([MTA] for Mail Transfer Agent, like Postfix or Exchange), running either
-//! locally on your servers or accessible over the network
+//!   locally on your servers or accessible over the network
 //! * a dedicated external service, like Mailchimp, Mailgun, etc.
 //!
 //! In most cases, the best option is to:
@@ -32,7 +32,7 @@
 //! | [`smtp`]     | SMTP     | [`SmtpTransport`]     | [`AsyncSmtpTransport`]     | Uses the SMTP protocol to send emails to a relay server |
 //! | [`sendmail`] | Sendmail | [`SendmailTransport`] | [`AsyncSendmailTransport`] | Uses the `sendmail` command to send emails              |
 //! | [`file`]     | File     | [`FileTransport`]     | [`AsyncFileTransport`]     | Saves the email as an `.eml` file                       |
-//! | [`stub`]     | Debug    | [`StubTransport`]     | [`StubTransport`]          | Drops the email - Useful for debugging                  |
+//! | [`stub`]     | Debug    | [`StubTransport`]     | [`AsyncStubTransport`]     | Drops the email - Useful for debugging                  |
 //!
 //! ## Building an email
 //!
@@ -65,7 +65,7 @@
 //!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+//! let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 //!
 //! // Open a remote connection to the SMTP relay server
 //! let mailer = SmtpTransport::relay("smtp.gmail.com")?
@@ -75,7 +75,7 @@
 //! // Send the email
 //! match mailer.send(&email) {
 //!     Ok(_) => println!("Email sent successfully!"),
-//!     Err(e) => panic!("Could not send email: {:?}", e),
+//!     Err(e) => panic!("Could not send email: {e:?}"),
 //! }
 //! # Ok(())
 //! # }
@@ -97,6 +97,7 @@
 //! [`FileTransport`]: crate::FileTransport
 //! [`AsyncFileTransport`]: crate::AsyncFileTransport
 //! [`StubTransport`]: crate::transport::stub::StubTransport
+//! [`AsyncStubTransport`]: crate::transport::stub::AsyncStubTransport
 
 #[cfg(any(feature = "async-std1", feature = "tokio1"))]
 use async_trait::async_trait;
@@ -127,6 +128,9 @@ pub trait Transport {
     #[cfg(feature = "builder")]
     #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
     fn send(&self, message: &Message) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::trace!("starting to send an email");
+
         let raw = message.formatted();
         self.send_raw(message.envelope(), &raw)
     }
@@ -149,6 +153,9 @@ pub trait AsyncTransport {
     #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
     // TODO take &Message
     async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::trace!("starting to send an email");
+
         let raw = message.formatted();
         let envelope = message.envelope();
         self.send_raw(envelope, &raw).await

src/transport/sendmail/error.rs

@@ -52,7 +52,7 @@ impl fmt::Debug for Error {
 
         builder.field("kind", &self.inner.kind);
 
-        if let Some(ref source) = self.inner.source {
+        if let Some(source) = &self.inner.source {
             builder.field("source", source);
         }
 
@@ -67,8 +67,8 @@ impl fmt::Display for Error {
             Kind::Client => f.write_str("internal client error")?,
         };
 
-        if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+        if let Some(e) = &self.inner.source {
+            write!(f, ": {e}")?;
         }
 
         Ok(())

src/transport/sendmail/mod.rs

@@ -232,6 +232,9 @@ impl Transport for SendmailTransport {
     type Error = Error;
 
     fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.command, "sending email with");
+
         // Spawn the sendmail command
         let mut process = self.command(envelope).spawn().map_err(error::client)?;
 
@@ -261,6 +264,9 @@ impl AsyncTransport for AsyncSendmailTransport<AsyncStd1Executor> {
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         use async_std::io::prelude::WriteExt;
 
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.inner.command, "sending email with");
+
         let mut command = self.async_std_command(envelope);
 
         // Spawn the sendmail command
@@ -293,6 +299,9 @@ impl AsyncTransport for AsyncSendmailTransport<Tokio1Executor> {
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         use tokio1_crate::io::AsyncWriteExt;
 
+        #[cfg(feature = "tracing")]
+        tracing::debug!(command = ?self.inner.command, "sending email with");
+
         let mut command = self.tokio1_command(envelope);
 
         // Spawn the sendmail command

src/transport/smtp/async_transport.rs

@@ -1,7 +1,8 @@
+#[cfg(feature = "pool")]
+use std::sync::Arc;
 use std::{
     fmt::{self, Debug},
     marker::PhantomData,
-    sync::Arc,
     time::Duration,
 };
 
@@ -81,7 +82,6 @@ where
     #[cfg(any(
         feature = "tokio1-native-tls",
         feature = "tokio1-rustls-tls",
-        feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
     #[cfg_attr(
@@ -102,7 +102,7 @@ where
             .tls(Tls::Wrapper(tls_parameters)))
     }
 
-    /// Simple an secure transport, using STARTTLS to obtain encrypted connections
+    /// Simple and secure transport, using STARTTLS to obtain encrypted connections
     ///
     /// Alternative to [`AsyncSmtpTransport::relay`](#method.relay), for SMTP servers
     /// that don't take SMTPS connections.
@@ -116,7 +116,6 @@ where
     #[cfg(any(
         feature = "tokio1-native-tls",
         feature = "tokio1-rustls-tls",
-        feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
     #[cfg_attr(
@@ -150,28 +149,114 @@ where
     ///
     /// * No authentication
     /// * No TLS
-    /// * A 60 seconds timeout for smtp commands
+    /// * A 60-seconds timeout for smtp commands
     /// * Port 25
     ///
     /// Consider using [`AsyncSmtpTransport::relay`](#method.relay) or
     /// [`AsyncSmtpTransport::starttls_relay`](#method.starttls_relay) instead,
     /// if possible.
     pub fn builder_dangerous<T: Into<String>>(server: T) -> AsyncSmtpTransportBuilder {
-        let info = SmtpInfo {
-            server: server.into(),
-            ..Default::default()
-        };
-        AsyncSmtpTransportBuilder {
-            info,
-            #[cfg(feature = "pool")]
-            pool_config: PoolConfig::default(),
-        }
+        AsyncSmtpTransportBuilder::new(server)
+    }
+
+    /// Creates a `AsyncSmtpTransportBuilder` from a connection URL
+    ///
+    /// The protocol, credentials, host and port can be provided in a single URL.
+    /// Use the scheme `smtp` for an unencrypted relay (optionally in combination with the
+    /// `tls` parameter to allow/require STARTTLS) or `smtps` for SMTP over TLS.
+    /// The path section of the url can be used to set an alternative name for
+    /// the HELO / EHLO command.
+    /// For example `smtps://username:password@smtp.example.com/client.example.com:465`
+    /// will set the HELO / EHLO name `client.example.com`.
+    ///
+    /// <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::{
+    ///     message::header::ContentType, transport::smtp::authentication::Credentials,
+    ///     AsyncSmtpTransport, AsyncTransport, Message, Tokio1Executor,
+    /// };
+    /// # use tokio1_crate as tokio;
+    ///
+    /// # #[tokio::main]
+    /// # async 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())
+    ///     .to("Hei <hei@domain.tld>".parse().unwrap())
+    ///     .subject("Happy new year")
+    ///     .header(ContentType::TEXT_PLAIN)
+    ///     .body(String::from("Be happy!"))
+    ///     .unwrap();
+    ///
+    /// // Open a remote connection to gmail
+    /// let mailer: AsyncSmtpTransport<Tokio1Executor> =
+    ///     AsyncSmtpTransport::<Tokio1Executor>::from_url(
+    ///         "smtps://username:password@smtp.example.com:465",
+    ///     )
+    ///     .unwrap()
+    ///     .build();
+    ///
+    /// // Send the email
+    /// match mailer.send(email).await {
+    ///     Ok(_) => println!("Email sent successfully!"),
+    ///     Err(e) => panic!("Could not send email: {e:?}"),
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
+    pub fn from_url(connection_url: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
+        super::connection_url::from_connection_url(connection_url)
     }
 
     /// Tests the SMTP connection
     ///
     /// `test_connection()` tests the connection by using the SMTP NOOP command.
-    /// The connection is closed afterwards if a connection pool is not used.
+    /// The connection is closed afterward if a connection pool is not used.
     pub async fn test_connection(&self) -> Result<bool, Error> {
         let mut conn = self.inner.connection().await?;
 
@@ -218,6 +303,20 @@ pub struct AsyncSmtpTransportBuilder {
 
 /// Builder for the SMTP `AsyncSmtpTransport`
 impl AsyncSmtpTransportBuilder {
+    // Create new builder with default parameters
+    pub(crate) fn new<T: Into<String>>(server: T) -> Self {
+        let info = SmtpInfo {
+            server: server.into(),
+            ..Default::default()
+        };
+
+        AsyncSmtpTransportBuilder {
+            info,
+            #[cfg(feature = "pool")]
+            pool_config: PoolConfig::default(),
+        }
+    }
+
     /// Set the name used during EHLO
     pub fn hello_name(mut self, name: ClientId) -> Self {
         self.info.hello_name = name;
@@ -252,7 +351,6 @@ impl AsyncSmtpTransportBuilder {
     #[cfg(any(
         feature = "tokio1-native-tls",
         feature = "tokio1-rustls-tls",
-        feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
     #[cfg_attr(

src/transport/smtp/authentication.rs

@@ -51,7 +51,7 @@ pub enum Mechanism {
     /// [RFC 4616](https://tools.ietf.org/html/rfc4616)
     Plain,
     /// LOGIN authentication mechanism
-    /// Obsolete but needed for some providers (like office365)
+    /// Obsolete but needed for some providers (like Office 365)
     ///
     /// Defined in [draft-murchison-sasl-login-00](https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt).
     Login,
@@ -71,7 +71,7 @@ impl Display for Mechanism {
 }
 
 impl Mechanism {
-    /// Does the mechanism supports initial response
+    /// Does the mechanism support initial response?
     pub fn supports_initial_response(self) -> bool {
         match self {
             Mechanism::Plain | Mechanism::Xoauth2 => true,
@@ -98,11 +98,13 @@ impl Mechanism {
                 let decoded_challenge = challenge
                     .ok_or_else(|| error::client("This mechanism does expect a challenge"))?;
 
-                if ["User Name", "Username:", "Username"].contains(&decoded_challenge) {
+                if ["User Name", "Username:", "Username", "User Name\0"]
+                    .contains(&decoded_challenge)
+                {
                     return Ok(credentials.authentication_identity.clone());
                 }
 
-                if ["Password", "Password:"].contains(&decoded_challenge) {
+                if ["Password", "Password:", "Password\0"].contains(&decoded_challenge) {
                     return Ok(credentials.secret.clone());
                 }
 
@@ -127,7 +129,7 @@ mod test {
     fn test_plain() {
         let mechanism = Mechanism::Plain;
 
-        let credentials = Credentials::new("username".to_string(), "password".to_string());
+        let credentials = Credentials::new("username".to_owned(), "password".to_owned());
 
         assert_eq!(
             mechanism.response(&credentials, None).unwrap(),
@@ -140,7 +142,7 @@ mod test {
     fn test_login() {
         let mechanism = Mechanism::Login;
 
-        let credentials = Credentials::new("alice".to_string(), "wonderland".to_string());
+        let credentials = Credentials::new("alice".to_owned(), "wonderland".to_owned());
 
         assert_eq!(
             mechanism.response(&credentials, Some("Username")).unwrap(),
@@ -158,8 +160,8 @@ mod test {
         let mechanism = Mechanism::Xoauth2;
 
         let credentials = Credentials::new(
-            "username".to_string(),
-            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_string(),
+            "username".to_owned(),
+            "vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==".to_owned(),
         );
 
         assert_eq!(
@@ -172,7 +174,7 @@ mod test {
     #[test]
     fn test_from_user_pass_for_credentials() {
         assert_eq!(
-            Credentials::new("alice".to_string(), "wonderland".to_string()),
+            Credentials::new("alice".to_owned(), "wonderland".to_owned()),
             Credentials::from(("alice", "wonderland"))
         );
     }

src/transport/smtp/client/async_connection.rs

@@ -2,6 +2,8 @@ use std::{fmt::Display, net::IpAddr, time::Duration};
 
 use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 
+#[cfg(feature = "tokio1")]
+use super::async_net::AsyncTokioStream;
 #[cfg(feature = "tracing")]
 use super::escape_crlf;
 use super::{AsyncNetworkStream, ClientCodec, TlsParameters};
@@ -46,10 +48,50 @@ impl AsyncSmtpConnection {
         &self.server_info
     }
 
-    /// Connects to the configured server
+    /// Connects with existing async stream
     ///
     /// Sends EHLO and parses server information
     #[cfg(feature = "tokio1")]
+    pub async fn connect_with_transport(
+        stream: Box<dyn AsyncTokioStream>,
+        hello_name: &ClientId,
+    ) -> Result<AsyncSmtpConnection, Error> {
+        let stream = AsyncNetworkStream::use_existing_tokio1(stream);
+        Self::connect_impl(stream, hello_name).await
+    }
+
+    /// Connects to the configured server
+    ///
+    /// If `tls_parameters` is `Some`, then the connection will use Implicit TLS (sometimes
+    /// referred to as `SMTPS`). See also [`AsyncSmtpConnection::starttls`].
+    ///
+    /// If `local_address` is `Some`, then the address provided shall be used to bind the
+    /// connection to a specific local address using [`tokio1_crate::net::TcpSocket::bind`].
+    ///
+    /// Sends EHLO and parses server information
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// # use std::time::Duration;
+    /// # use lettre::transport::smtp::{client::{AsyncSmtpConnection, TlsParameters}, extension::ClientId};
+    /// # use tokio1_crate::{self as tokio, net::ToSocketAddrs as _};
+    /// #
+    /// # #[tokio::main]
+    /// # async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    /// let connection = AsyncSmtpConnection::connect_tokio1(
+    ///     ("example.com", 465),
+    ///     Some(Duration::from_secs(60)),
+    ///     &ClientId::default(),
+    ///     Some(TlsParameters::new("example.com".to_owned())?),
+    ///     None,
+    /// )
+    /// .await
+    /// .unwrap();
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(feature = "tokio1")]
     pub async fn connect_tokio1<T: tokio1_crate::net::ToSocketAddrs>(
         server: T,
         timeout: Option<Duration>,
@@ -118,7 +160,7 @@ impl AsyncSmtpConnection {
             mail_options.push(MailParameter::SmtpUtfEight);
         }
 
-        // Check for non-ascii content in message
+        // Check for non-ascii content in the message
         if !email.is_ascii() {
             if !self.server_info().supports_feature(Extension::EightBitMime) {
                 return Err(error::client(
@@ -158,6 +200,12 @@ impl AsyncSmtpConnection {
         !self.is_encrypted() && self.server_info.supports_feature(Extension::StartTls)
     }
 
+    /// Upgrade the connection using `STARTTLS`.
+    ///
+    /// As described in [rfc3207]. Note that this mechanism has been deprecated in [rfc8314].
+    ///
+    /// [rfc3207]: https://www.rfc-editor.org/rfc/rfc3207
+    /// [rfc8314]: https://www.rfc-editor.org/rfc/rfc8314
     #[allow(unused_variables)]
     pub async fn starttls(
         &mut self,
@@ -194,6 +242,7 @@ impl AsyncSmtpConnection {
             self.panic = true;
             let _ = self.command(Quit).await;
         }
+        let _ = self.stream.close().await;
     }
 
     /// Sets the underlying stream
@@ -211,7 +260,7 @@ impl AsyncSmtpConnection {
         self.command(Noop).await.is_ok()
     }
 
-    /// Sends an AUTH command with the given mechanism, and handles challenge if needed
+    /// Sends an AUTH command with the given mechanism, and handles the challenge if needed
     pub async fn auth(
         &mut self,
         mechanisms: &[Mechanism],
@@ -302,7 +351,7 @@ impl AsyncSmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     }
                 }
@@ -320,8 +369,14 @@ impl AsyncSmtpConnection {
     }
 
     /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
         self.stream.get_ref().peer_certificate()
     }
+
+    /// All the X509 certificates of the chain (DER encoded)
+    #[cfg(any(feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
+        self.stream.get_ref().certificate_chain()
+    }
 }

src/transport/smtp/client/async_net.rs

@@ -1,13 +1,11 @@
 use std::{
-    io, mem,
+    fmt, io, mem,
     net::{IpAddr, SocketAddr},
     pin::Pin,
     task::{Context, Poll},
     time::Duration,
 };
 
-#[cfg(feature = "async-std1-native-tls")]
-use async_native_tls::TlsStream as AsyncStd1TlsStream;
 #[cfg(feature = "async-std1")]
 use async_std::net::{TcpStream as AsyncStd1TcpStream, ToSocketAddrs as AsyncStd1ToSocketAddrs};
 use futures_io::{
@@ -16,8 +14,12 @@ use futures_io::{
 };
 #[cfg(feature = "async-std1-rustls-tls")]
 use futures_rustls::client::TlsStream as AsyncStd1RustlsTlsStream;
+#[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;
 #[cfg(feature = "tokio1")]
-use tokio1_crate::io::{AsyncRead as _, AsyncWrite as _, ReadBuf as Tokio1ReadBuf};
+use tokio1_crate::io::{AsyncRead, AsyncWrite, ReadBuf as Tokio1ReadBuf};
 #[cfg(feature = "tokio1")]
 use tokio1_crate::net::{
     TcpSocket as Tokio1TcpSocket, TcpStream as Tokio1TcpStream,
@@ -31,7 +33,7 @@ use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;
 #[cfg(any(
     feature = "tokio1-native-tls",
     feature = "tokio1-rustls-tls",
-    feature = "async-std1-native-tls",
+    feature = "tokio1-boring-tls",
     feature = "async-std1-rustls-tls"
 ))]
 use super::InnerTlsParameters;
@@ -41,32 +43,46 @@ use crate::transport::smtp::client::net::resolved_address_filter;
 use crate::transport::smtp::{error, Error};
 
 /// A network stream
+#[derive(Debug)]
 pub struct AsyncNetworkStream {
     inner: InnerAsyncNetworkStream,
 }
 
+#[cfg(feature = "tokio1")]
+pub trait AsyncTokioStream: AsyncRead + AsyncWrite + Send + Sync + Unpin + fmt::Debug {
+    fn peer_addr(&self) -> io::Result<SocketAddr>;
+}
+
+#[cfg(feature = "tokio1")]
+impl AsyncTokioStream for Tokio1TcpStream {
+    fn peer_addr(&self) -> io::Result<SocketAddr> {
+        self.peer_addr()
+    }
+}
+
 /// Represents the different types of underlying network streams
 // usually only one TLS backend at a time is going to be enabled,
 // so clippy::large_enum_variant doesn't make sense here
 #[allow(clippy::large_enum_variant)]
 #[allow(dead_code)]
+#[derive(Debug)]
 enum InnerAsyncNetworkStream {
     /// Plain Tokio 1.x TCP stream
     #[cfg(feature = "tokio1")]
-    Tokio1Tcp(Tokio1TcpStream),
+    Tokio1Tcp(Box<dyn AsyncTokioStream>),
     /// Encrypted Tokio 1.x TCP stream
     #[cfg(feature = "tokio1-native-tls")]
-    Tokio1NativeTls(Tokio1TlsStream<Tokio1TcpStream>),
+    Tokio1NativeTls(Tokio1TlsStream<Box<dyn AsyncTokioStream>>),
     /// Encrypted Tokio 1.x TCP stream
     #[cfg(feature = "tokio1-rustls-tls")]
-    Tokio1RustlsTls(Tokio1RustlsTlsStream<Tokio1TcpStream>),
+    Tokio1RustlsTls(Tokio1RustlsTlsStream<Box<dyn AsyncTokioStream>>),
+    /// Encrypted Tokio 1.x TCP stream
+    #[cfg(feature = "tokio1-boring-tls")]
+    Tokio1BoringTls(Tokio1SslStream<Box<dyn AsyncTokioStream>>),
     /// Plain Tokio 1.x TCP stream
     #[cfg(feature = "async-std1")]
     AsyncStd1Tcp(AsyncStd1TcpStream),
     /// Encrypted Tokio 1.x TCP stream
-    #[cfg(feature = "async-std1-native-tls")]
-    AsyncStd1NativeTls(AsyncStd1TlsStream<AsyncStd1TcpStream>),
-    /// Encrypted Tokio 1.x TCP stream
     #[cfg(feature = "async-std1-rustls-tls")]
     AsyncStd1RustlsTls(AsyncStd1RustlsTlsStream<AsyncStd1TcpStream>),
     /// Can't be built
@@ -84,21 +100,21 @@ impl AsyncNetworkStream {
 
     /// Returns peer's address
     pub fn peer_addr(&self) -> IoResult<SocketAddr> {
-        match self.inner {
+        match &self.inner {
             #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(ref s) => s.peer_addr(),
+            InnerAsyncNetworkStream::Tokio1Tcp(s) => s.peer_addr(),
             #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(ref s) => {
+            InnerAsyncNetworkStream::Tokio1NativeTls(s) => {
                 s.get_ref().get_ref().get_ref().peer_addr()
             }
             #[cfg(feature = "tokio1-rustls-tls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(ref s) => s.get_ref().0.peer_addr(),
+            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(ref s) => s.peer_addr(),
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(ref s) => s.get_ref().peer_addr(),
+            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => s.peer_addr(),
             #[cfg(feature = "async-std1-rustls-tls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(ref s) => s.get_ref().0.peer_addr(),
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => s.get_ref().0.peer_addr(),
             InnerAsyncNetworkStream::None => {
                 debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
                 Err(IoError::new(
@@ -109,6 +125,11 @@ impl AsyncNetworkStream {
         }
     }
 
+    #[cfg(feature = "tokio1")]
+    pub fn use_existing_tokio1(stream: Box<dyn AsyncTokioStream>) -> AsyncNetworkStream {
+        AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(stream))
+    }
+
     #[cfg(feature = "tokio1")]
     pub async fn connect_tokio1<T: Tokio1ToSocketAddrs>(
         server: T,
@@ -167,7 +188,8 @@ impl AsyncNetworkStream {
         }
 
         let tcp_stream = try_connect(server, timeout, local_addr).await?;
-        let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream));
+        let mut stream =
+            AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(Box::new(tcp_stream)));
         if let Some(tls_parameters) = tls_parameters {
             stream.upgrade_tls(tls_parameters).await?;
         }
@@ -180,9 +202,9 @@ impl AsyncNetworkStream {
         timeout: Option<Duration>,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        // Unfortunately there doesn't currently seem to be a way to set the local address
+        // Unfortunately, there doesn't currently seem to be a way to set the local address.
         // Whilst we can create a AsyncStd1TcpStream from an existing socket, it needs to first have
-        // connected which is a blocking operation.
+        // been connected, which is a blocking operation.
         async fn try_connect_timeout<T: AsyncStd1ToSocketAddrs>(
             server: T,
             timeout: Duration,
@@ -229,14 +251,22 @@ impl AsyncNetworkStream {
         match &self.inner {
             #[cfg(all(
                 feature = "tokio1",
-                not(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))
+                not(any(
+                    feature = "tokio1-native-tls",
+                    feature = "tokio1-rustls-tls",
+                    feature = "tokio1-boring-tls"
+                ))
             ))]
             InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                 let _ = tls_parameters;
                 panic!("Trying to upgrade an AsyncNetworkStream without having enabled either the tokio1-native-tls or the tokio1-rustls-tls feature");
             }
 
-            #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
+            #[cfg(any(
+                feature = "tokio1-native-tls",
+                feature = "tokio1-rustls-tls",
+                feature = "tokio1-boring-tls"
+            ))]
             InnerAsyncNetworkStream::Tokio1Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
@@ -250,16 +280,13 @@ impl AsyncNetworkStream {
                     .map_err(error::connection)?;
                 Ok(())
             }
-            #[cfg(all(
-                feature = "async-std1",
-                not(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))
-            ))]
+            #[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 either the async-std1-native-tls or the async-std1-rustls-tls feature");
+                panic!("Trying to upgrade an AsyncNetworkStream without having enabled the async-std1-rustls-tls feature");
             }
 
-            #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
+            #[cfg(feature = "async-std1-rustls-tls")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerAsyncNetworkStream::None);
@@ -278,12 +305,16 @@ impl AsyncNetworkStream {
     }
 
     #[allow(unused_variables)]
-    #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
+    #[cfg(any(
+        feature = "tokio1-native-tls",
+        feature = "tokio1-rustls-tls",
+        feature = "tokio1-boring-tls"
+    ))]
     async fn upgrade_tokio1_tls(
-        tcp_stream: Tokio1TcpStream,
+        tcp_stream: Box<dyn AsyncTokioStream>,
         tls_parameters: TlsParameters,
     ) -> Result<InnerAsyncNetworkStream, Error> {
-        let domain = tls_parameters.domain().to_string();
+        let domain = tls_parameters.domain().to_owned();
 
         match tls_parameters.connector {
             #[cfg(feature = "native-tls")]
@@ -310,7 +341,6 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "tokio1-rustls-tls")]
                 return {
-                    use rustls::ServerName;
                     use tokio1_rustls::TlsConnector;
 
                     let domain = ServerName::try_from(domain.as_str())
@@ -318,17 +348,33 @@ impl AsyncNetworkStream {
 
                     let connector = TlsConnector::from(config);
                     let stream = connector
-                        .connect(domain, tcp_stream)
+                        .connect(domain.to_owned(), tcp_stream)
                         .await
                         .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::Tokio1RustlsTls(stream))
                 };
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                #[cfg(not(feature = "tokio1-boring-tls"))]
+                panic!("built without the tokio1-boring-tls feature");
+
+                #[cfg(feature = "tokio1-boring-tls")]
+                return {
+                    let mut config = connector.configure().map_err(error::connection)?;
+                    config.set_verify_hostname(tls_parameters.accept_invalid_hostnames);
+
+                    let stream = tokio1_boring::connect(config, &domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
+                    Ok(InnerAsyncNetworkStream::Tokio1BoringTls(stream))
+                };
+            }
         }
     }
 
     #[allow(unused_variables)]
-    #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
+    #[cfg(feature = "async-std1-rustls-tls")]
     async fn upgrade_asyncstd1_tls(
         tcp_stream: AsyncStd1TcpStream,
         mut tls_parameters: TlsParameters,
@@ -339,22 +385,6 @@ impl AsyncNetworkStream {
             #[cfg(feature = "native-tls")]
             InnerTlsParameters::NativeTls(connector) => {
                 panic!("native-tls isn't supported with async-std yet. See https://github.com/lettre/lettre/pull/531#issuecomment-757893531");
-
-                /*
-                #[cfg(not(feature = "async-std1-native-tls"))]
-                panic!("built without the async-std1-native-tls feature");
-
-                #[cfg(feature = "async-std1-native-tls")]
-                return {
-                    use async_native_tls::TlsConnector;
-
-                    // TODO: fix
-                    let connector: TlsConnector = todo!();
-                    // let connector = TlsConnector::from(connector);
-                    let stream = connector.connect(&domain, tcp_stream).await?;
-                    Ok(InnerAsyncNetworkStream::AsyncStd1NativeTls(stream))
-                };
-                */
             }
             #[cfg(feature = "rustls-tls")]
             InnerTlsParameters::RustlsTls(config) => {
@@ -364,40 +394,85 @@ impl AsyncNetworkStream {
                 #[cfg(feature = "async-std1-rustls-tls")]
                 return {
                     use futures_rustls::TlsConnector;
-                    use rustls::ServerName;
 
                     let domain = ServerName::try_from(domain.as_str())
                         .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
 
                     let connector = TlsConnector::from(config);
                     let stream = connector
-                        .connect(domain, tcp_stream)
+                        .connect(domain.to_owned(), tcp_stream)
                         .await
                         .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream))
                 };
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                panic!("boring-tls isn't supported with async-std yet.");
+            }
         }
     }
 
     pub fn is_encrypted(&self) -> bool {
-        match self.inner {
+        match &self.inner {
             #[cfg(feature = "tokio1")]
             InnerAsyncNetworkStream::Tokio1Tcp(_) => false,
             #[cfg(feature = "tokio1-native-tls")]
             InnerAsyncNetworkStream::Tokio1NativeTls(_) => true,
             #[cfg(feature = "tokio1-rustls-tls")]
             InnerAsyncNetworkStream::Tokio1RustlsTls(_) => true,
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(_) => true,
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(_) => false,
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(_) => true,
             #[cfg(feature = "async-std1-rustls-tls")]
             InnerAsyncNetworkStream::AsyncStd1RustlsTls(_) => true,
             InnerAsyncNetworkStream::None => false,
         }
     }
 
+    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
+        match &self.inner {
+            #[cfg(feature = "tokio1")]
+            InnerAsyncNetworkStream::Tokio1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "tokio1-native-tls")]
+            InnerAsyncNetworkStream::Tokio1NativeTls(_) => panic!("Unsupported"),
+            #[cfg(feature = "tokio1-rustls-tls")]
+            InnerAsyncNetworkStream::Tokio1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .iter()
+                .map(|c| c.to_vec())
+                .collect()),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_cert_chain()
+                .unwrap()
+                .iter()
+                .map(|c| c.to_der().map_err(error::tls))
+                .collect::<Result<Vec<_>, _>>()?),
+            #[cfg(feature = "async-std1")]
+            InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
+                Err(error::client("Connection is not encrypted"))
+            }
+            #[cfg(feature = "async-std1-rustls-tls")]
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
+                .get_ref()
+                .1
+                .peer_certificates()
+                .unwrap()
+                .iter()
+                .map(|c| c.to_vec())
+                .collect()),
+            InnerAsyncNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
+
     pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
         match &self.inner {
             #[cfg(feature = "tokio1")]
@@ -420,14 +495,18 @@ impl AsyncNetworkStream {
                 .unwrap()
                 .first()
                 .unwrap()
-                .clone()
-                .0),
+                .to_vec()),
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_certificate()
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
             #[cfg(feature = "async-std1")]
             InnerAsyncNetworkStream::AsyncStd1Tcp(_) => {
                 Err(error::client("Connection is not encrypted"))
             }
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(t) => panic!("Unsupported"),
             #[cfg(feature = "async-std1-rustls-tls")]
             InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream) => Ok(stream
                 .get_ref()
@@ -436,8 +515,7 @@ impl AsyncNetworkStream {
                 .unwrap()
                 .first()
                 .unwrap()
-                .clone()
-                .0),
+                .to_vec()),
             InnerAsyncNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
         }
     }
@@ -449,9 +527,9 @@ impl FuturesAsyncRead for AsyncNetworkStream {
         cx: &mut Context<'_>,
         buf: &mut [u8],
     ) -> Poll<IoResult<usize>> {
-        match self.inner {
+        match &mut self.inner {
             #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(ref mut s) => {
+            InnerAsyncNetworkStream::Tokio1Tcp(s) => {
                 let mut b = Tokio1ReadBuf::new(buf);
                 match Pin::new(s).poll_read(cx, &mut b) {
                     Poll::Ready(Ok(())) => Poll::Ready(Ok(b.filled().len())),
@@ -460,7 +538,7 @@ impl FuturesAsyncRead for AsyncNetworkStream {
                 }
             }
             #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => {
+            InnerAsyncNetworkStream::Tokio1NativeTls(s) => {
                 let mut b = Tokio1ReadBuf::new(buf);
                 match Pin::new(s).poll_read(cx, &mut b) {
                     Poll::Ready(Ok(())) => Poll::Ready(Ok(b.filled().len())),
@@ -469,7 +547,16 @@ impl FuturesAsyncRead for AsyncNetworkStream {
                 }
             }
             #[cfg(feature = "tokio1-rustls-tls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => {
+            InnerAsyncNetworkStream::Tokio1RustlsTls(s) => {
+                let mut b = Tokio1ReadBuf::new(buf);
+                match Pin::new(s).poll_read(cx, &mut b) {
+                    Poll::Ready(Ok(())) => Poll::Ready(Ok(b.filled().len())),
+                    Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
+                    Poll::Pending => Poll::Pending,
+                }
+            }
+            #[cfg(feature = "tokio1-boring-tls")]
+            InnerAsyncNetworkStream::Tokio1BoringTls(s) => {
                 let mut b = Tokio1ReadBuf::new(buf);
                 match Pin::new(s).poll_read(cx, &mut b) {
                     Poll::Ready(Ok(())) => Poll::Ready(Ok(b.filled().len())),
@@ -478,15 +565,9 @@ impl FuturesAsyncRead for AsyncNetworkStream {
                 }
             }
             #[cfg(feature = "async-std1")]
-            InnerAsyncNetworkStream::AsyncStd1Tcp(ref mut s) => Pin::new(s).poll_read(cx, buf),
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(ref mut s) => {
-                Pin::new(s).poll_read(cx, buf)
-            }
+            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_read(cx, buf),
             #[cfg(feature = "async-std1-rustls-tls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(ref mut s) => {
-                Pin::new(s).poll_read(cx, buf)
-            }
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_read(cx, buf),
             InnerAsyncNetworkStream::None => {
                 debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
                 Poll::Ready(Ok(0))
@@ -501,23 +582,19 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
         cx: &mut Context<'_>,
         buf: &[u8],
     ) -> Poll<IoResult<usize>> {
-        match self.inner {
+        match &mut self.inner {
             #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(ref mut s) => Pin::new(s).poll_write(cx, buf),
+            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_write(cx, buf),
+            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "tokio1-rustls-tls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_write(cx, buf),
+            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(ref mut s) => Pin::new(s).poll_write(cx, buf),
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(ref mut s) => {
-                Pin::new(s).poll_write(cx, buf)
-            }
+            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_write(cx, buf),
             #[cfg(feature = "async-std1-rustls-tls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(ref mut s) => {
-                Pin::new(s).poll_write(cx, buf)
-            }
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_write(cx, buf),
             InnerAsyncNetworkStream::None => {
                 debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
                 Poll::Ready(Ok(0))
@@ -526,19 +603,19 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
     }
 
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<IoResult<()>> {
-        match self.inner {
+        match &mut self.inner {
             #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(ref mut s) => Pin::new(s).poll_flush(cx),
+            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_flush(cx),
+            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "tokio1-rustls-tls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_flush(cx),
+            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(ref mut s) => Pin::new(s).poll_flush(cx),
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(ref mut s) => Pin::new(s).poll_flush(cx),
+            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_flush(cx),
             #[cfg(feature = "async-std1-rustls-tls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(ref mut s) => Pin::new(s).poll_flush(cx),
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_flush(cx),
             InnerAsyncNetworkStream::None => {
                 debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
                 Poll::Ready(Ok(()))
@@ -547,19 +624,19 @@ impl FuturesAsyncWrite for AsyncNetworkStream {
     }
 
     fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<IoResult<()>> {
-        match self.inner {
+        match &mut self.inner {
             #[cfg(feature = "tokio1")]
-            InnerAsyncNetworkStream::Tokio1Tcp(ref mut s) => Pin::new(s).poll_shutdown(cx),
+            InnerAsyncNetworkStream::Tokio1Tcp(s) => Pin::new(s).poll_shutdown(cx),
             #[cfg(feature = "tokio1-native-tls")]
-            InnerAsyncNetworkStream::Tokio1NativeTls(ref mut s) => Pin::new(s).poll_shutdown(cx),
+            InnerAsyncNetworkStream::Tokio1NativeTls(s) => Pin::new(s).poll_shutdown(cx),
             #[cfg(feature = "tokio1-rustls-tls")]
-            InnerAsyncNetworkStream::Tokio1RustlsTls(ref mut s) => Pin::new(s).poll_shutdown(cx),
+            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(ref mut s) => Pin::new(s).poll_close(cx),
-            #[cfg(feature = "async-std1-native-tls")]
-            InnerAsyncNetworkStream::AsyncStd1NativeTls(ref mut s) => Pin::new(s).poll_close(cx),
+            InnerAsyncNetworkStream::AsyncStd1Tcp(s) => Pin::new(s).poll_close(cx),
             #[cfg(feature = "async-std1-rustls-tls")]
-            InnerAsyncNetworkStream::AsyncStd1RustlsTls(ref mut s) => Pin::new(s).poll_close(cx),
+            InnerAsyncNetworkStream::AsyncStd1RustlsTls(s) => Pin::new(s).poll_close(cx),
             InnerAsyncNetworkStream::None => {
                 debug_assert!(false, "InnerAsyncNetworkStream::None must never be built");
                 Poll::Ready(Ok(()))

src/transport/smtp/client/connection.rs

@@ -100,7 +100,7 @@ impl SmtpConnection {
             mail_options.push(MailParameter::SmtpUtfEight);
         }
 
-        // Check for non-ascii content in message
+        // Check for non-ascii content in the message
         if !email.is_ascii() {
             if !self.server_info().supports_feature(Extension::EightBitMime) {
                 return Err(error::client(
@@ -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-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-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");
@@ -179,6 +183,7 @@ impl SmtpConnection {
             self.panic = true;
             let _ = self.command(Quit);
         }
+        let _ = self.stream.get_mut().shutdown(std::net::Shutdown::Both);
     }
 
     /// Sets the underlying stream
@@ -202,7 +207,7 @@ impl SmtpConnection {
         self.command(Noop).is_ok()
     }
 
-    /// Sends an AUTH command with the given mechanism, and handles challenge if needed
+    /// Sends an AUTH command with the given mechanism, and handles the challenge if needed
     pub fn auth(
         &mut self,
         mechanisms: &[Mechanism],
@@ -238,11 +243,12 @@ impl SmtpConnection {
 
     /// Sends the message content
     pub fn message(&mut self, message: &[u8]) -> Result<Response, Error> {
-        let mut out_buf: Vec<u8> = vec![];
         let mut codec = ClientCodec::new();
+        let mut out_buf = Vec::with_capacity(message.len());
         codec.encode(message, &mut out_buf);
         self.write(out_buf.as_slice())?;
         self.write(b"\r\n.\r\n")?;
+
         self.read_response()
     }
 
@@ -279,7 +285,7 @@ impl SmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     };
                 }
@@ -297,8 +303,14 @@ impl SmtpConnection {
     }
 
     /// The X509 certificate of the server (DER encoded)
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     pub fn peer_certificate(&self) -> Result<Vec<u8>, Error> {
         self.stream.get_ref().peer_certificate()
     }
+
+    /// All the X509 certificates of the chain (DER encoded)
+    #[cfg(any(feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn certificate_chain(&self) -> Result<Vec<Vec<u8>>, Error> {
+        self.stream.get_ref().certificate_chain()
+    }
 }

src/transport/smtp/client/mod.rs

@@ -11,7 +11,7 @@
 //!     client::SmtpConnection, commands::*, extension::ClientId, SMTP_PORT,
 //! };
 //!
-//! let hello = ClientId::Domain("my_hostname".to_string());
+//! let hello = ClientId::Domain("my_hostname".to_owned());
 //! let mut client = SmtpConnection::connect(&("localhost", SMTP_PORT), None, &hello, None, None)?;
 //! client.command(Mail::new(Some("user@example.com".parse()?), vec![]))?;
 //! client.command(Rcpt::new("user@example.org".parse()?, vec![]))?;
@@ -29,12 +29,16 @@ use std::fmt::Debug;
 pub use self::async_connection::AsyncSmtpConnection;
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
 pub use self::async_net::AsyncNetworkStream;
+#[cfg(feature = "tokio1")]
+pub use self::async_net::AsyncTokioStream;
 use self::net::NetworkStream;
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(super) use self::tls::InnerTlsParameters;
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+pub use self::tls::TlsVersion;
 pub use self::{
     connection::SmtpConnection,
-    tls::{Certificate, Tls, TlsParameters, TlsParametersBuilder},
+    tls::{Certificate, CertificateStore, Identity, Tls, TlsParameters, TlsParametersBuilder},
 };
 
 #[cfg(any(feature = "tokio1", feature = "async-std1"))]
@@ -46,60 +50,57 @@ mod net;
 mod tls;
 
 /// The codec used for transparency
-#[derive(Default, Clone, Copy, Debug)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug)]
 struct ClientCodec {
-    escape_count: u8,
+    status: CodecStatus,
 }
 
 impl ClientCodec {
     /// Creates a new client codec
     pub fn new() -> Self {
-        ClientCodec::default()
+        Self {
+            status: CodecStatus::StartOfNewLine,
+        }
     }
 
     /// Adds transparency
     fn encode(&mut self, frame: &[u8], buf: &mut Vec<u8>) {
-        match frame.len() {
-            0 => {
-                match self.escape_count {
-                    0 => buf.extend_from_slice(b"\r\n.\r\n"),
-                    1 => buf.extend_from_slice(b"\n.\r\n"),
-                    2 => buf.extend_from_slice(b".\r\n"),
-                    _ => unreachable!(),
+        for &b in frame {
+            buf.push(b);
+            match (b, self.status) {
+                (b'\r', _) => {
+                    self.status = CodecStatus::StartingNewLine;
                 }
-                self.escape_count = 0;
-            }
-            _ => {
-                let mut start = 0;
-                for (idx, byte) in frame.iter().enumerate() {
-                    match self.escape_count {
-                        0 => self.escape_count = if *byte == b'\r' { 1 } else { 0 },
-                        1 => self.escape_count = if *byte == b'\n' { 2 } else { 0 },
-                        2 => {
-                            self.escape_count = if *byte == b'.' {
-                                3
-                            } else if *byte == b'\r' {
-                                1
-                            } else {
-                                0
-                            }
-                        }
-                        _ => unreachable!(),
-                    }
-                    if self.escape_count == 3 {
-                        self.escape_count = 0;
-                        buf.extend_from_slice(&frame[start..idx]);
-                        buf.extend_from_slice(b".");
-                        start = idx;
-                    }
+                (b'\n', CodecStatus::StartingNewLine) => {
+                    self.status = CodecStatus::StartOfNewLine;
                 }
-                buf.extend_from_slice(&frame[start..]);
+                (_, CodecStatus::StartingNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                }
+                (b'.', CodecStatus::StartOfNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                    buf.push(b'.');
+                }
+                (_, CodecStatus::StartOfNewLine) => {
+                    self.status = CodecStatus::MiddleOfLine;
+                }
+                _ => {}
             }
         }
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+#[allow(clippy::enum_variant_names)]
+enum CodecStatus {
+    /// We are past the first character of the current line
+    MiddleOfLine,
+    /// We just read a `\r` character
+    StartingNewLine,
+    /// We are at the start of a new line
+    StartOfNewLine,
+}
+
 /// Returns the string replacing all the CRLF with "\<CRLF\>"
 /// Used for debug displays
 #[cfg(feature = "tracing")]
@@ -113,9 +114,10 @@ mod test {
 
     #[test]
     fn test_codec() {
+        let mut buf = Vec::new();
         let mut codec = ClientCodec::new();
-        let mut buf: Vec<u8> = vec![];
 
+        codec.encode(b".\r\n", &mut buf);
         codec.encode(b"test\r\n", &mut buf);
         codec.encode(b"test\r\n\r\n", &mut buf);
         codec.encode(b".\r\n", &mut buf);
@@ -126,14 +128,18 @@ mod test {
         codec.encode(b"test\n", &mut buf);
         codec.encode(b".test\n", &mut buf);
         codec.encode(b"test", &mut buf);
+        codec.encode(b"test", &mut buf);
+        codec.encode(b"test\r\n", &mut buf);
+        codec.encode(b".test\r\n", &mut buf);
+        codec.encode(b"test.\r\n", &mut buf);
         assert_eq!(
             String::from_utf8(buf).unwrap(),
-            "test\r\ntest\r\n\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntest"
+            "..\r\ntest\r\ntest\r\n\r\n..\r\n\r\ntestte\r\n..\r\nsttesttest.test\n.test\ntesttesttest\r\n..test\r\ntest.\r\n"
         );
     }
 
     #[test]
-    #[cfg(feature = "log")]
+    #[cfg(feature = "tracing")]
     fn test_escape_crlf() {
         assert_eq!(escape_crlf("\r\n"), "<CRLF>");
         assert_eq!(escape_crlf("EHLO my_name\r\n"), "EHLO my_name<CRLF>");

src/transport/smtp/client/net.rs

@@ -1,18 +1,21 @@
+#[cfg(feature = "rustls-tls")]
+use std::sync::Arc;
 use std::{
     io::{self, Read, Write},
     mem,
     net::{IpAddr, Ipv4Addr, Shutdown, SocketAddr, SocketAddrV4, TcpStream, ToSocketAddrs},
-    sync::Arc,
     time::Duration,
 };
 
+#[cfg(feature = "boring-tls")]
+use boring::ssl::SslStream;
 #[cfg(feature = "native-tls")]
 use native_tls::TlsStream;
 #[cfg(feature = "rustls-tls")]
-use rustls::{ClientConnection, ServerName, StreamOwned};
+use rustls::{pki_types::ServerName, ClientConnection, StreamOwned};
 use socket2::{Domain, Protocol, Type};
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
 use crate::transport::smtp::{error, Error};
@@ -35,6 +38,8 @@ enum InnerNetworkStream {
     /// Encrypted TCP stream
     #[cfg(feature = "rustls-tls")]
     RustlsTls(StreamOwned<ClientConnection, TcpStream>),
+    #[cfg(feature = "boring-tls")]
+    BoringTls(SslStream<TcpStream>),
     /// Can't be built
     None,
 }
@@ -50,12 +55,14 @@ impl NetworkStream {
 
     /// Returns peer's address
     pub fn peer_addr(&self) -> io::Result<SocketAddr> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref s) => s.peer_addr(),
+        match &self.inner {
+            InnerNetworkStream::Tcp(s) => s.peer_addr(),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref s) => s.get_ref().peer_addr(),
+            InnerNetworkStream::NativeTls(s) => s.get_ref().peer_addr(),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref s) => s.get_ref().peer_addr(),
+            InnerNetworkStream::RustlsTls(s) => s.get_ref().peer_addr(),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(s) => s.get_ref().peer_addr(),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(SocketAddr::V4(SocketAddrV4::new(
@@ -68,12 +75,14 @@ impl NetworkStream {
 
     /// Shutdowns the connection
     pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref s) => s.shutdown(how),
+        match &self.inner {
+            InnerNetworkStream::Tcp(s) => s.shutdown(how),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref s) => s.get_ref().shutdown(how),
+            InnerNetworkStream::NativeTls(s) => s.get_ref().shutdown(how),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref s) => s.get_ref().shutdown(how),
+            InnerNetworkStream::RustlsTls(s) => s.get_ref().shutdown(how),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(s) => s.get_ref().shutdown(how),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -137,13 +146,17 @@ impl NetworkStream {
 
     pub fn upgrade_tls(&mut self, tls_parameters: &TlsParameters) -> Result<(), Error> {
         match &self.inner {
-            #[cfg(not(any(feature = "native-tls", feature = "rustls-tls")))]
+            #[cfg(not(any(
+                feature = "native-tls",
+                feature = "rustls-tls",
+                feature = "boring-tls"
+            )))]
             InnerNetworkStream::Tcp(_) => {
                 let _ = tls_parameters;
                 panic!("Trying to upgrade an NetworkStream without having enabled either the native-tls or the rustls-tls feature");
             }
 
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             InnerNetworkStream::Tcp(_) => {
                 // get owned TcpStream
                 let tcp_stream = mem::replace(&mut self.inner, InnerNetworkStream::None);
@@ -159,7 +172,7 @@ impl NetworkStream {
         }
     }
 
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     fn upgrade_tls_impl(
         tcp_stream: TcpStream,
         tls_parameters: &TlsParameters,
@@ -176,21 +189,33 @@ impl NetworkStream {
             InnerTlsParameters::RustlsTls(connector) => {
                 let domain = ServerName::try_from(tls_parameters.domain())
                     .map_err(|_| error::connection("domain isn't a valid DNS name"))?;
-                let connection = ClientConnection::new(Arc::clone(connector), domain)
+                let connection = ClientConnection::new(Arc::clone(connector), domain.to_owned())
                     .map_err(error::connection)?;
                 let stream = StreamOwned::new(connection, tcp_stream);
                 InnerNetworkStream::RustlsTls(stream)
             }
+            #[cfg(feature = "boring-tls")]
+            InnerTlsParameters::BoringTls(connector) => {
+                let stream = connector
+                    .configure()
+                    .map_err(error::connection)?
+                    .verify_hostname(tls_parameters.accept_invalid_hostnames)
+                    .connect(tls_parameters.domain(), tcp_stream)
+                    .map_err(error::connection)?;
+                InnerNetworkStream::BoringTls(stream)
+            }
         })
     }
 
     pub fn is_encrypted(&self) -> bool {
-        match self.inner {
+        match &self.inner {
             InnerNetworkStream::Tcp(_) => false,
             #[cfg(feature = "native-tls")]
             InnerNetworkStream::NativeTls(_) => true,
             #[cfg(feature = "rustls-tls")]
             InnerNetworkStream::RustlsTls(_) => true,
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(_) => true,
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 false
@@ -198,7 +223,33 @@ impl NetworkStream {
         }
     }
 
-    #[cfg(any(feature = "native-tls", feature = "rustls-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-tls")]
+            InnerNetworkStream::RustlsTls(stream) => Ok(stream
+                .conn
+                .peer_certificates()
+                .unwrap()
+                .iter()
+                .map(|c| c.to_vec())
+                .collect()),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_cert_chain()
+                .unwrap()
+                .iter()
+                .map(|c| c.to_der().map_err(error::tls))
+                .collect::<Result<Vec<_>, _>>()?),
+            InnerNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
+        }
+    }
+
+    #[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")),
@@ -216,23 +267,27 @@ impl NetworkStream {
                 .unwrap()
                 .first()
                 .unwrap()
-                .clone()
-                .0),
+                .to_vec()),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(stream) => Ok(stream
+                .ssl()
+                .peer_certificate()
+                .unwrap()
+                .to_der()
+                .map_err(error::tls)?),
             InnerNetworkStream::None => panic!("InnerNetworkStream::None must never be built"),
         }
     }
 
     pub fn set_read_timeout(&mut self, duration: Option<Duration>) -> io::Result<()> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref mut stream) => stream.set_read_timeout(duration),
+        match &mut self.inner {
+            InnerNetworkStream::Tcp(stream) => stream.set_read_timeout(duration),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref mut stream) => {
-                stream.get_ref().set_read_timeout(duration)
-            }
+            InnerNetworkStream::NativeTls(stream) => stream.get_ref().set_read_timeout(duration),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref mut stream) => {
-                stream.get_ref().set_read_timeout(duration)
-            }
+            InnerNetworkStream::RustlsTls(stream) => stream.get_ref().set_read_timeout(duration),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(stream) => stream.get_ref().set_read_timeout(duration),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -242,18 +297,15 @@ impl NetworkStream {
 
     /// Set write timeout for IO calls
     pub fn set_write_timeout(&mut self, duration: Option<Duration>) -> io::Result<()> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref mut stream) => stream.set_write_timeout(duration),
+        match &mut self.inner {
+            InnerNetworkStream::Tcp(stream) => stream.set_write_timeout(duration),
 
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref mut stream) => {
-                stream.get_ref().set_write_timeout(duration)
-            }
+            InnerNetworkStream::NativeTls(stream) => stream.get_ref().set_write_timeout(duration),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref mut stream) => {
-                stream.get_ref().set_write_timeout(duration)
-            }
-
+            InnerNetworkStream::RustlsTls(stream) => stream.get_ref().set_write_timeout(duration),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(stream) => stream.get_ref().set_write_timeout(duration),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -264,12 +316,14 @@ impl NetworkStream {
 
 impl Read for NetworkStream {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref mut s) => s.read(buf),
+        match &mut self.inner {
+            InnerNetworkStream::Tcp(s) => s.read(buf),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref mut s) => s.read(buf),
+            InnerNetworkStream::NativeTls(s) => s.read(buf),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref mut s) => s.read(buf),
+            InnerNetworkStream::RustlsTls(s) => s.read(buf),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(s) => s.read(buf),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(0)
@@ -280,12 +334,14 @@ impl Read for NetworkStream {
 
 impl Write for NetworkStream {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref mut s) => s.write(buf),
+        match &mut self.inner {
+            InnerNetworkStream::Tcp(s) => s.write(buf),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref mut s) => s.write(buf),
+            InnerNetworkStream::NativeTls(s) => s.write(buf),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref mut s) => s.write(buf),
+            InnerNetworkStream::RustlsTls(s) => s.write(buf),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(s) => s.write(buf),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(0)
@@ -294,12 +350,14 @@ impl Write for NetworkStream {
     }
 
     fn flush(&mut self) -> io::Result<()> {
-        match self.inner {
-            InnerNetworkStream::Tcp(ref mut s) => s.flush(),
+        match &mut self.inner {
+            InnerNetworkStream::Tcp(s) => s.flush(),
             #[cfg(feature = "native-tls")]
-            InnerNetworkStream::NativeTls(ref mut s) => s.flush(),
+            InnerNetworkStream::NativeTls(s) => s.flush(),
             #[cfg(feature = "rustls-tls")]
-            InnerNetworkStream::RustlsTls(ref mut s) => s.flush(),
+            InnerNetworkStream::RustlsTls(s) => s.flush(),
+            #[cfg(feature = "boring-tls")]
+            InnerNetworkStream::BoringTls(s) => s.flush(),
             InnerNetworkStream::None => {
                 debug_assert!(false, "InnerNetworkStream::None must never be built");
                 Ok(())
@@ -309,7 +367,7 @@ 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 to the default
+/// 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
 fn bind_local_address(

src/transport/smtp/client/tls.rs

@@ -1,23 +1,63 @@
 use std::fmt::{self, Debug};
 #[cfg(feature = "rustls-tls")]
-use std::{sync::Arc, time::SystemTime};
+use std::{io, sync::Arc};
 
+#[cfg(feature = "boring-tls")]
+use boring::{
+    pkey::PKey,
+    ssl::{SslConnector, SslVersion},
+    x509::store::X509StoreBuilder,
+};
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls-tls")]
 use rustls::{
-    client::{ServerCertVerified, ServerCertVerifier, WebPkiVerifier},
-    ClientConfig, Error as TlsError, OwnedTrustAnchor, RootCertStore, ServerName,
+    client::danger::{HandshakeSignatureValid, ServerCertVerified, ServerCertVerifier},
+    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-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::{error, Error};
 
-/// Accepted protocols by default.
-/// This removes TLS 1.0 and 1.1 compared to tls-native defaults.
-// This is also rustls' default behavior
-#[cfg(feature = "native-tls")]
-const DEFAULT_TLS_MIN_PROTOCOL: Protocol = Protocol::Tlsv12;
+/// TLS protocol versions.
+#[derive(Debug, Copy, Clone)]
+#[non_exhaustive]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+pub enum TlsVersion {
+    /// TLS 1.0
+    ///
+    /// Should only be used when trying to support legacy
+    /// SMTP servers that haven't updated to
+    /// at least TLS 1.2 yet.
+    ///
+    /// Supported by `native-tls` and `boring-tls`.
+    Tlsv10,
+    /// TLS 1.1
+    ///
+    /// Should only be used when trying to support legacy
+    /// SMTP servers that haven't updated to
+    /// at least TLS 1.2 yet.
+    ///
+    /// Supported by `native-tls` and `boring-tls`.
+    Tlsv11,
+    /// TLS 1.2
+    ///
+    /// A good option for most SMTP servers.
+    ///
+    /// Supported by all TLS backends.
+    Tlsv12,
+    /// TLS 1.3
+    ///
+    /// The most secure option, although not supported by all SMTP servers.
+    ///
+    /// Although it is technically supported by all TLS backends,
+    /// trying to set it for `native-tls` will give a runtime error.
+    Tlsv13,
+}
 
 /// How to apply TLS to a client connection
 #[derive(Clone)]
@@ -26,16 +66,25 @@ pub enum Tls {
     /// Insecure connection only (for testing purposes)
     None,
     /// Start with insecure connection and use `STARTTLS` when available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Opportunistic(TlsParameters),
     /// Start with insecure connection and require `STARTTLS`
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Required(TlsParameters),
     /// Use TLS wrapped connection
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     Wrapper(TlsParameters),
 }
 
@@ -43,31 +92,61 @@ impl Debug for Tls {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match &self {
             Self::None => f.pad("None"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Self::Opportunistic(_) => f.pad("Opportunistic"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Self::Required(_) => f.pad("Required"),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Self::Wrapper(_) => f.pad("Wrapper"),
         }
     }
 }
 
+/// Source for the base set of root certificates to trust.
+#[allow(missing_copy_implementations)]
+#[derive(Clone, Debug, Default)]
+pub enum CertificateStore {
+    /// Use the default for the TLS backend.
+    ///
+    /// For native-tls, this will use the system certificate store on Windows, the keychain on
+    /// macOS, and OpenSSL directories on Linux (usually `/etc/ssl`).
+    ///
+    /// For rustls, this will also use the system store if the `rustls-native-certs` feature is
+    /// enabled, or will fall back to `webpki-roots`.
+    ///
+    /// The boring-tls backend uses the same logic as OpenSSL on all platforms.
+    #[default]
+    Default,
+    /// Use a hardcoded set of Mozilla roots via the `webpki-roots` crate.
+    ///
+    /// This option is only available in the rustls backend.
+    #[cfg(feature = "rustls-tls")]
+    WebpkiRoots,
+    /// Don't use any system certificates.
+    None,
+}
+
 /// Parameters to use for secure clients
 #[derive(Clone)]
 pub struct TlsParameters {
     pub(crate) connector: InnerTlsParameters,
     /// The domain name which is expected in the TLS certificate from the server
     pub(super) domain: String,
+    #[cfg(feature = "boring-tls")]
+    pub(super) accept_invalid_hostnames: bool,
 }
 
 /// Builder for `TlsParameters`
 #[derive(Debug, Clone)]
 pub struct TlsParametersBuilder {
     domain: String,
+    cert_store: CertificateStore,
     root_certs: Vec<Certificate>,
+    identity: Option<Identity>,
     accept_invalid_hostnames: bool,
     accept_invalid_certs: bool,
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    min_tls_version: TlsVersion,
 }
 
 impl TlsParametersBuilder {
@@ -75,20 +154,38 @@ impl TlsParametersBuilder {
     pub fn new(domain: String) -> Self {
         Self {
             domain,
+            cert_store: CertificateStore::Default,
             root_certs: Vec::new(),
+            identity: None,
             accept_invalid_hostnames: false,
             accept_invalid_certs: false,
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+            min_tls_version: TlsVersion::Tlsv12,
         }
     }
 
+    /// Set the source for the base set of root certificates to trust.
+    pub fn certificate_store(mut self, cert_store: CertificateStore) -> Self {
+        self.cert_store = cert_store;
+        self
+    }
+
     /// Add a custom root certificate
     ///
-    /// Can be used to safely connect to a server using a self signed certificate, for example.
+    /// Can be used to safely connect to a server using a self-signed certificate, for example.
     pub fn add_root_certificate(mut self, cert: Certificate) -> Self {
         self.root_certs.push(cert);
         self
     }
 
+    /// Add a client certificate
+    ///
+    /// Can be used to configure a client certificate to present to the server.
+    pub fn identify_with(mut self, identity: Identity) -> Self {
+        self.identity = Some(identity);
+        self
+    }
+
     /// Controls whether certificates with an invalid hostname are accepted
     ///
     /// Defaults to `false`.
@@ -100,15 +197,25 @@ impl TlsParametersBuilder {
     /// including those from other sites, are trusted.
     ///
     /// This method introduces significant vulnerabilities to man-in-the-middle attacks.
-    ///
-    /// Hostname verification can only be disabled with the `native-tls` TLS backend.
-    #[cfg(feature = "native-tls")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "native-tls")))]
+    #[cfg(any(feature = "native-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "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;
         self
     }
 
+    /// Controls which minimum TLS version is allowed
+    ///
+    /// Defaults to [`Tlsv12`][TlsVersion::Tlsv12].
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    pub fn set_min_tls_version(mut self, min_tls_version: TlsVersion) -> Self {
+        self.min_tls_version = min_tls_version;
+        self
+    }
+
     /// Controls whether invalid certificates are accepted
     ///
     /// Defaults to `false`.
@@ -130,16 +237,20 @@ impl TlsParametersBuilder {
         self
     }
 
-    /// Creates a new `TlsParameters` using native-tls or rustls
+    /// Creates a new `TlsParameters` using native-tls, boring-tls or rustls
     /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn build(self) -> Result<TlsParameters, Error> {
         #[cfg(feature = "rustls-tls")]
         return self.build_rustls();
-
-        #[cfg(not(feature = "rustls-tls"))]
+        #[cfg(all(not(feature = "rustls-tls"), feature = "native-tls"))]
         return self.build_native();
+        #[cfg(all(not(feature = "rustls-tls"), feature = "boring-tls"))]
+        return self.build_boring();
     }
 
     /// Creates a new `TlsParameters` using native-tls with the provided configuration
@@ -148,17 +259,106 @@ impl TlsParametersBuilder {
     pub fn build_native(self) -> Result<TlsParameters, Error> {
         let mut tls_builder = TlsConnector::builder();
 
+        match self.cert_store {
+            CertificateStore::Default => {}
+            CertificateStore::None => {
+                tls_builder.disable_built_in_roots(true);
+            }
+            #[allow(unreachable_patterns)]
+            other => {
+                return Err(error::tls(format!(
+                    "{other:?} is not supported in native tls"
+                )))
+            }
+        }
         for cert in self.root_certs {
             tls_builder.add_root_certificate(cert.native_tls);
         }
         tls_builder.danger_accept_invalid_hostnames(self.accept_invalid_hostnames);
         tls_builder.danger_accept_invalid_certs(self.accept_invalid_certs);
 
-        tls_builder.min_protocol_version(Some(DEFAULT_TLS_MIN_PROTOCOL));
+        let min_tls_version = match self.min_tls_version {
+            TlsVersion::Tlsv10 => Protocol::Tlsv10,
+            TlsVersion::Tlsv11 => Protocol::Tlsv11,
+            TlsVersion::Tlsv12 => Protocol::Tlsv12,
+            TlsVersion::Tlsv13 => {
+                return Err(error::tls(
+                    "min tls version Tlsv13 not supported in native tls",
+                ))
+            }
+        };
+
+        tls_builder.min_protocol_version(Some(min_tls_version));
+        if let Some(identity) = self.identity {
+            tls_builder.identity(identity.native_tls);
+        }
+
         let connector = tls_builder.build().map_err(error::tls)?;
         Ok(TlsParameters {
             connector: InnerTlsParameters::NativeTls(connector),
             domain: self.domain,
+            #[cfg(feature = "boring-tls")]
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
+        })
+    }
+
+    /// Creates a new `TlsParameters` using boring-tls with the provided configuration
+    #[cfg(feature = "boring-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "boring-tls")))]
+    pub fn build_boring(self) -> Result<TlsParameters, Error> {
+        use boring::ssl::{SslMethod, SslVerifyMode};
+
+        let mut tls_builder = SslConnector::builder(SslMethod::tls_client()).map_err(error::tls)?;
+
+        if self.accept_invalid_certs {
+            tls_builder.set_verify(SslVerifyMode::NONE);
+        } else {
+            match self.cert_store {
+                CertificateStore::Default => {}
+                CertificateStore::None => {
+                    // Replace the default store with an empty store.
+                    tls_builder
+                        .set_cert_store(X509StoreBuilder::new().map_err(error::tls)?.build());
+                }
+                #[allow(unreachable_patterns)]
+                other => {
+                    return Err(error::tls(format!(
+                        "{other:?} is not supported in boring tls"
+                    )))
+                }
+            }
+
+            let cert_store = tls_builder.cert_store_mut();
+
+            for cert in self.root_certs {
+                cert_store.add_cert(cert.boring_tls).map_err(error::tls)?;
+            }
+        }
+
+        if let Some(identity) = self.identity {
+            tls_builder
+                .set_certificate(identity.boring_tls.0.as_ref())
+                .map_err(error::tls)?;
+            tls_builder
+                .set_private_key(identity.boring_tls.1.as_ref())
+                .map_err(error::tls)?;
+        }
+
+        let min_tls_version = match self.min_tls_version {
+            TlsVersion::Tlsv10 => SslVersion::TLS1,
+            TlsVersion::Tlsv11 => SslVersion::TLS1_1,
+            TlsVersion::Tlsv12 => SslVersion::TLS1_2,
+            TlsVersion::Tlsv13 => SslVersion::TLS1_3,
+        };
+
+        tls_builder
+            .set_min_proto_version(Some(min_tls_version))
+            .map_err(error::tls)?;
+        let connector = tls_builder.build();
+        Ok(TlsParameters {
+            connector: InnerTlsParameters::BoringTls(connector),
+            domain: self.domain,
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
         })
     }
 
@@ -166,55 +366,112 @@ impl TlsParametersBuilder {
     #[cfg(feature = "rustls-tls")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
     pub fn build_rustls(self) -> Result<TlsParameters, Error> {
-        let tls = ClientConfig::builder();
-        let tls = tls.with_safe_defaults();
-
-        let tls = if self.accept_invalid_certs {
-            tls.with_custom_certificate_verifier(Arc::new(InvalidCertsVerifier {}))
-        } else {
-            let mut root_cert_store = RootCertStore::empty();
-            for cert in self.root_certs {
-                for rustls_cert in cert.rustls {
-                    root_cert_store.add(&rustls_cert).map_err(error::tls)?;
-                }
+        let just_version3 = &[&rustls::version::TLS13];
+        let supported_versions = match self.min_tls_version {
+            TlsVersion::Tlsv10 => {
+                return Err(error::tls("min tls version Tlsv10 not supported in rustls"))
             }
-            root_cert_store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(
-                |ta| {
-                    OwnedTrustAnchor::from_subject_spki_name_constraints(
-                        ta.subject,
-                        ta.spki,
-                        ta.name_constraints,
-                    )
-                },
-            ));
-
-            tls.with_custom_certificate_verifier(Arc::new(WebPkiVerifier::new(
-                root_cert_store,
-                None,
-            )))
+            TlsVersion::Tlsv11 => {
+                return Err(error::tls("min tls version Tlsv11 not supported in rustls"))
+            }
+            TlsVersion::Tlsv12 => rustls::ALL_VERSIONS,
+            TlsVersion::Tlsv13 => just_version3,
+        };
+
+        let tls = 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) -> Result<(), Error> {
+            let native_certs = rustls_native_certs::load_native_certs().map_err(error::tls)?;
+            let (added, ignored) = store.add_parsable_certificates(native_certs);
+            #[cfg(feature = "tracing")]
+            tracing::debug!(
+                "loaded platform certs with {added} valid and {ignored} ignored (invalid) certs"
+            );
+            Ok(())
+        }
+
+        #[cfg(feature = "rustls-tls")]
+        fn load_webpki_roots(store: &mut RootCertStore) {
+            store.extend(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
+        }
+
+        match self.cert_store {
+            CertificateStore::Default => {
+                #[cfg(feature = "rustls-native-certs")]
+                load_native_roots(&mut root_cert_store)?;
+                #[cfg(not(feature = "rustls-native-certs"))]
+                load_webpki_roots(&mut root_cert_store);
+            }
+            #[cfg(feature = "rustls-tls")]
+            CertificateStore::WebpkiRoots => {
+                load_webpki_roots(&mut root_cert_store);
+            }
+            CertificateStore::None => {}
+        }
+        for cert in self.root_certs {
+            for rustls_cert in cert.rustls {
+                root_cert_store.add(rustls_cert).map_err(error::tls)?;
+            }
+        }
+
+        let tls = if self.accept_invalid_certs || self.accept_invalid_hostnames {
+            let verifier = InvalidCertsVerifier {
+                ignore_invalid_hostnames: self.accept_invalid_hostnames,
+                ignore_invalid_certs: self.accept_invalid_certs,
+                roots: root_cert_store,
+                signature_algorithms,
+            };
+            tls.dangerous()
+                .with_custom_certificate_verifier(Arc::new(verifier))
+        } else {
+            tls.with_root_certificates(root_cert_store)
+        };
+
+        let tls = if let Some(identity) = self.identity {
+            let (client_certificates, private_key) = identity.rustls_tls;
+            tls.with_client_auth_cert(client_certificates, private_key)
+                .map_err(error::tls)?
+        } else {
+            tls.with_no_client_auth()
         };
-        let tls = tls.with_no_client_auth();
 
         Ok(TlsParameters {
             connector: InnerTlsParameters::RustlsTls(Arc::new(tls)),
             domain: self.domain,
+            #[cfg(feature = "boring-tls")]
+            accept_invalid_hostnames: self.accept_invalid_hostnames,
         })
     }
 }
 
 #[derive(Clone)]
+#[allow(clippy::enum_variant_names)]
 pub enum InnerTlsParameters {
     #[cfg(feature = "native-tls")]
     NativeTls(TlsConnector),
     #[cfg(feature = "rustls-tls")]
     RustlsTls(Arc<ClientConfig>),
+    #[cfg(feature = "boring-tls")]
+    BoringTls(SslConnector),
 }
 
 impl TlsParameters {
     /// Creates a new `TlsParameters` using native-tls or rustls
     /// depending on which one is available
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn new(domain: String) -> Result<Self, Error> {
         TlsParametersBuilder::new(domain).build()
     }
@@ -238,33 +495,47 @@ impl TlsParameters {
         TlsParametersBuilder::new(domain).build_rustls()
     }
 
+    /// Creates a new `TlsParameters` using boring
+    #[cfg(feature = "boring-tls")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "boring-tls")))]
+    pub fn new_boring(domain: String) -> Result<Self, Error> {
+        TlsParametersBuilder::new(domain).build_boring()
+    }
+
     pub fn domain(&self) -> &str {
         &self.domain
     }
 }
 
-/// A client certificate that can be used with [`TlsParametersBuilder::add_root_certificate`]
+/// A certificate that can be used with [`TlsParametersBuilder::add_root_certificate`]
 #[derive(Clone)]
 #[allow(missing_copy_implementations)]
 pub struct Certificate {
     #[cfg(feature = "native-tls")]
     native_tls: native_tls::Certificate,
     #[cfg(feature = "rustls-tls")]
-    rustls: Vec<rustls::Certificate>,
+    rustls: Vec<CertificateDer<'static>>,
+    #[cfg(feature = "boring-tls")]
+    boring_tls: boring::x509::X509,
 }
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 impl Certificate {
     /// Create a `Certificate` from a DER encoded certificate
     pub fn from_der(der: Vec<u8>) -> Result<Self, Error> {
         #[cfg(feature = "native-tls")]
         let native_tls_cert = native_tls::Certificate::from_der(&der).map_err(error::tls)?;
 
+        #[cfg(feature = "boring-tls")]
+        let boring_tls_cert = boring::x509::X509::from_der(&der).map_err(error::tls)?;
+
         Ok(Self {
             #[cfg(feature = "native-tls")]
             native_tls: native_tls_cert,
             #[cfg(feature = "rustls-tls")]
-            rustls: vec![rustls::Certificate(der)],
+            rustls: vec![der.into()],
+            #[cfg(feature = "boring-tls")]
+            boring_tls: boring_tls_cert,
         })
     }
 
@@ -273,16 +544,17 @@ impl Certificate {
         #[cfg(feature = "native-tls")]
         let native_tls_cert = native_tls::Certificate::from_pem(pem).map_err(error::tls)?;
 
+        #[cfg(feature = "boring-tls")]
+        let boring_tls_cert = boring::x509::X509::from_pem(pem).map_err(error::tls)?;
+
         #[cfg(feature = "rustls-tls")]
         let rustls_cert = {
             use std::io::Cursor;
 
             let mut pem = Cursor::new(pem);
             rustls_pemfile::certs(&mut pem)
+                .collect::<io::Result<Vec<_>>>()
                 .map_err(|_| error::tls("invalid certificates"))?
-                .into_iter()
-                .map(rustls::Certificate)
-                .collect::<Vec<_>>()
         };
 
         Ok(Self {
@@ -290,6 +562,8 @@ impl Certificate {
             native_tls: native_tls_cert,
             #[cfg(feature = "rustls-tls")]
             rustls: rustls_cert,
+            #[cfg(feature = "boring-tls")]
+            boring_tls: boring_tls_cert,
         })
     }
 }
@@ -300,20 +574,143 @@ impl Debug for Certificate {
     }
 }
 
+/// An identity that can be used with [`TlsParametersBuilder::identify_with`]
+#[allow(missing_copy_implementations)]
+pub struct Identity {
+    #[cfg(feature = "native-tls")]
+    native_tls: native_tls::Identity,
+    #[cfg(feature = "rustls-tls")]
+    rustls_tls: (Vec<CertificateDer<'static>>, PrivateKeyDer<'static>),
+    #[cfg(feature = "boring-tls")]
+    boring_tls: (boring::x509::X509, PKey<boring::pkey::Private>),
+}
+
+impl Debug for Identity {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Identity").finish()
+    }
+}
+
+impl Clone for Identity {
+    fn clone(&self) -> Self {
+        Identity {
+            #[cfg(feature = "native-tls")]
+            native_tls: self.native_tls.clone(),
+            #[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()),
+        }
+    }
+}
+
+#[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-tls")]
+            rustls_tls: Identity::from_pem_rustls_tls(pem, key)?,
+            #[cfg(feature = "boring-tls")]
+            boring_tls: Identity::from_pem_boring_tls(pem, key)?,
+        })
+    }
+
+    #[cfg(feature = "native-tls")]
+    fn from_pem_native_tls(pem: &[u8], key: &[u8]) -> Result<native_tls::Identity, Error> {
+        native_tls::Identity::from_pkcs8(pem, key).map_err(error::tls)
+    }
+
+    #[cfg(feature = "rustls-tls")]
+    fn from_pem_rustls_tls(
+        pem: &[u8],
+        key: &[u8],
+    ) -> Result<(Vec<CertificateDer<'static>>, PrivateKeyDer<'static>), Error> {
+        let mut key = key;
+        let key = rustls_pemfile::private_key(&mut key).unwrap().unwrap();
+        Ok((vec![pem.to_owned().into()], key))
+    }
+
+    #[cfg(feature = "boring-tls")]
+    fn from_pem_boring_tls(
+        pem: &[u8],
+        key: &[u8],
+    ) -> Result<(boring::x509::X509, PKey<boring::pkey::Private>), Error> {
+        let cert = boring::x509::X509::from_pem(pem).map_err(error::tls)?;
+        let key = boring::pkey::PKey::private_key_from_pem(key).map_err(error::tls)?;
+        Ok((cert, key))
+    }
+}
+
 #[cfg(feature = "rustls-tls")]
-struct InvalidCertsVerifier;
+#[derive(Debug)]
+struct InvalidCertsVerifier {
+    ignore_invalid_hostnames: bool,
+    ignore_invalid_certs: bool,
+    roots: RootCertStore,
+    signature_algorithms: WebPkiSupportedAlgorithms,
+}
 
 #[cfg(feature = "rustls-tls")]
 impl ServerCertVerifier for InvalidCertsVerifier {
     fn verify_server_cert(
         &self,
-        _end_entity: &rustls::Certificate,
-        _intermediates: &[rustls::Certificate],
-        _server_name: &ServerName,
-        _scts: &mut dyn Iterator<Item = &[u8]>,
+        end_entity: &CertificateDer<'_>,
+        intermediates: &[CertificateDer<'_>],
+        server_name: &ServerName<'_>,
         _ocsp_response: &[u8],
-        _now: SystemTime,
+        now: UnixTime,
     ) -> Result<ServerCertVerified, TlsError> {
+        let cert = ParsedCertificate::try_from(end_entity)?;
+
+        if !self.ignore_invalid_certs {
+            rustls::client::verify_server_cert_signed_by_trust_anchor(
+                &cert,
+                &self.roots,
+                intermediates,
+                now,
+                self.signature_algorithms.all,
+            )?;
+        }
+
+        if !self.ignore_invalid_hostnames {
+            rustls::client::verify_server_name(&cert, server_name)?;
+        }
         Ok(ServerCertVerified::assertion())
     }
+
+    fn verify_tls12_signature(
+        &self,
+        message: &[u8],
+        cert: &CertificateDer<'_>,
+        dss: &DigitallySignedStruct,
+    ) -> Result<HandshakeSignatureValid, TlsError> {
+        verify_tls12_signature(
+            message,
+            cert,
+            dss,
+            &rustls::crypto::ring::default_provider().signature_verification_algorithms,
+        )
+    }
+
+    fn verify_tls13_signature(
+        &self,
+        message: &[u8],
+        cert: &CertificateDer<'_>,
+        dss: &DigitallySignedStruct,
+    ) -> Result<HandshakeSignatureValid, TlsError> {
+        verify_tls13_signature(
+            message,
+            cert,
+            dss,
+            &rustls::crypto::ring::default_provider().signature_verification_algorithms,
+        )
+    }
+
+    fn supported_verify_schemes(&self) -> Vec<SignatureScheme> {
+        rustls::crypto::ring::default_provider()
+            .signature_verification_algorithms
+            .supported_schemes()
+    }
 }

src/transport/smtp/commands.rs

@@ -59,7 +59,7 @@ impl Display for Mail {
             self.sender.as_ref().map_or("", |s| s.as_ref())
         )?;
         for parameter in &self.parameters {
-            write!(f, " {}", parameter)?;
+            write!(f, " {parameter}")?;
         }
         f.write_str("\r\n")
     }
@@ -84,7 +84,7 @@ impl Display for Rcpt {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         write!(f, "RCPT TO:<{}>", self.recipient)?;
         for parameter in &self.parameters {
-            write!(f, " {}", parameter)?;
+            write!(f, " {parameter}")?;
         }
         f.write_str("\r\n")
     }
@@ -144,7 +144,7 @@ impl Display for Help {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         f.write_str("HELP")?;
         if let Some(argument) = &self.argument {
-            write!(f, " {}", argument)?;
+            write!(f, " {argument}")?;
         }
         f.write_str("\r\n")
     }
@@ -220,7 +220,7 @@ pub struct Auth {
 
 impl Display for Auth {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        let encoded_response = self.response.as_ref().map(base64::encode);
+        let encoded_response = self.response.as_ref().map(crate::base64::encode);
 
         if self.mechanism.supports_initial_response() {
             write!(f, "AUTH {} {}", self.mechanism, encoded_response.unwrap())?;
@@ -271,7 +271,7 @@ impl Auth {
         #[cfg(feature = "tracing")]
         tracing::debug!("auth encoded challenge: {}", encoded_challenge);
 
-        let decoded_base64 = base64::decode(&encoded_challenge).map_err(error::response)?;
+        let decoded_base64 = crate::base64::decode(encoded_challenge).map_err(error::response)?;
         let decoded_challenge = String::from_utf8(decoded_base64).map_err(error::response)?;
         #[cfg(feature = "tracing")]
         tracing::debug!("auth decoded challenge: {}", decoded_challenge);
@@ -296,15 +296,15 @@ mod test {
 
     #[test]
     fn test_display() {
-        let id = ClientId::Domain("localhost".to_string());
+        let id = ClientId::Domain("localhost".to_owned());
         let email = Address::from_str("test@example.com").unwrap();
         let mail_parameter = MailParameter::Other {
-            keyword: "TEST".to_string(),
-            value: Some("value".to_string()),
+            keyword: "TEST".to_owned(),
+            value: Some("value".to_owned()),
         };
         let rcpt_parameter = RcptParameter::Other {
-            keyword: "TEST".to_string(),
-            value: Some("value".to_string()),
+            keyword: "TEST".to_owned(),
+            value: Some("value".to_owned()),
         };
         assert_eq!(format!("{}", Ehlo::new(id)), "EHLO localhost\r\n");
         assert_eq!(
@@ -341,24 +341,18 @@ mod test {
             format!("{}", Rcpt::new(email, vec![rcpt_parameter])),
             "RCPT TO:<test@example.com> TEST=value\r\n"
         );
-        assert_eq!(format!("{}", Quit), "QUIT\r\n");
-        assert_eq!(format!("{}", Data), "DATA\r\n");
-        assert_eq!(format!("{}", Noop), "NOOP\r\n");
+        assert_eq!(format!("{Quit}"), "QUIT\r\n");
+        assert_eq!(format!("{Data}"), "DATA\r\n");
+        assert_eq!(format!("{Noop}"), "NOOP\r\n");
         assert_eq!(format!("{}", Help::new(None)), "HELP\r\n");
         assert_eq!(
-            format!("{}", Help::new(Some("test".to_string()))),
+            format!("{}", Help::new(Some("test".to_owned()))),
             "HELP test\r\n"
         );
-        assert_eq!(
-            format!("{}", Vrfy::new("test".to_string())),
-            "VRFY test\r\n"
-        );
-        assert_eq!(
-            format!("{}", Expn::new("test".to_string())),
-            "EXPN test\r\n"
-        );
-        assert_eq!(format!("{}", Rset), "RSET\r\n");
-        let credentials = Credentials::new("user".to_string(), "password".to_string());
+        assert_eq!(format!("{}", Vrfy::new("test".to_owned())), "VRFY test\r\n");
+        assert_eq!(format!("{}", Expn::new("test".to_owned())), "EXPN test\r\n");
+        assert_eq!(format!("{Rset}"), "RSET\r\n");
+        let credentials = Credentials::new("user".to_owned(), "password".to_owned());
         assert_eq!(
             format!(
                 "{}",

src/transport/smtp/connection_url.rs

@@ -0,0 +1,129 @@
+use url::Url;
+
+#[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;
+use super::{
+    authentication::Credentials, error, extension::ClientId, Error, SmtpTransportBuilder,
+    SMTP_PORT, SUBMISSIONS_PORT, SUBMISSION_PORT,
+};
+
+pub(crate) trait TransportBuilder {
+    fn new<T: Into<String>>(server: T) -> Self;
+    fn tls(self, tls: super::Tls) -> Self;
+    fn port(self, port: u16) -> Self;
+    fn credentials(self, credentials: Credentials) -> Self;
+    fn hello_name(self, name: ClientId) -> Self;
+}
+
+impl TransportBuilder for SmtpTransportBuilder {
+    fn new<T: Into<String>>(server: T) -> Self {
+        Self::new(server)
+    }
+
+    fn tls(self, tls: super::Tls) -> Self {
+        self.tls(tls)
+    }
+
+    fn port(self, port: u16) -> Self {
+        self.port(port)
+    }
+
+    fn credentials(self, credentials: Credentials) -> Self {
+        self.credentials(credentials)
+    }
+
+    fn hello_name(self, name: ClientId) -> Self {
+        self.hello_name(name)
+    }
+}
+
+#[cfg(any(feature = "tokio1", feature = "async-std1"))]
+impl TransportBuilder for AsyncSmtpTransportBuilder {
+    fn new<T: Into<String>>(server: T) -> Self {
+        Self::new(server)
+    }
+
+    fn tls(self, tls: super::Tls) -> Self {
+        self.tls(tls)
+    }
+
+    fn port(self, port: u16) -> Self {
+        self.port(port)
+    }
+
+    fn credentials(self, credentials: Credentials) -> Self {
+        self.credentials(credentials)
+    }
+
+    fn hello_name(self, name: ClientId) -> Self {
+        self.hello_name(name)
+    }
+}
+
+/// 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
+        .query_pairs()
+        .find(|(k, _)| k == "tls")
+        .map(|(_, v)| v.to_string());
+
+    let host = connection_url
+        .host_str()
+        .ok_or_else(|| error::connection("smtp host undefined"))?;
+
+    let mut builder = B::new(host);
+
+    match (connection_url.scheme(), tls.as_deref()) {
+        ("smtp", None) => {
+            builder = builder.port(connection_url.port().unwrap_or(SMTP_PORT));
+        }
+        #[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())?))
+        }
+        #[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())?))
+        }
+        #[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())?))
+        }
+        (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 {
+        let name = connection_url.path().trim_matches('/').to_owned();
+        builder = builder.hello_name(ClientId::Domain(name));
+    }
+
+    if let Some(password) = connection_url.password() {
+        let percent_decode = |s: &str| {
+            percent_encoding::percent_decode_str(s)
+                .decode_utf8()
+                .map(|cow| cow.into_owned())
+                .map_err(error::connection)
+        };
+        let credentials = Credentials::new(
+            percent_decode(connection_url.username())?,
+            percent_decode(password)?,
+        );
+        builder = builder.credentials(credentials);
+    }
+
+    Ok(builder)
+}

src/transport/smtp/error.rs

@@ -68,8 +68,11 @@ impl Error {
     }
 
     /// Returns true if the error is from TLS
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn is_tls(&self) -> bool {
         matches!(self.inner.kind, Kind::Tls)
     }
@@ -102,8 +105,11 @@ pub(crate) enum Kind {
     /// Underlying network i/o error
     Network,
     /// TLS error
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
     Tls,
 }
 
@@ -113,7 +119,7 @@ impl fmt::Debug for Error {
 
         builder.field("kind", &self.inner.kind);
 
-        if let Some(ref source) = self.inner.source {
+        if let Some(source) = &self.inner.source {
             builder.field("source", source);
         }
 
@@ -123,23 +129,23 @@ impl fmt::Debug for Error {
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.inner.kind {
+        match &self.inner.kind {
             Kind::Response => f.write_str("response error")?,
             Kind::Client => f.write_str("internal client error")?,
             Kind::Network => f.write_str("network error")?,
             Kind::Connection => f.write_str("Connection error")?,
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
             Kind::Tls => f.write_str("tls error")?,
-            Kind::Transient(ref code) => {
-                write!(f, "transient error ({})", code)?;
+            Kind::Transient(code) => {
+                write!(f, "transient error ({code})")?;
             }
-            Kind::Permanent(ref code) => {
-                write!(f, "permanent error ({})", code)?;
+            Kind::Permanent(code) => {
+                write!(f, "permanent error ({code})")?;
             }
         };
 
-        if let Some(ref e) = self.inner.source {
-            write!(f, ": {}", e)?;
+        if let Some(e) = &self.inner.source {
+            write!(f, ": {e}")?;
         }
 
         Ok(())
@@ -179,7 +185,7 @@ pub(crate) fn connection<E: Into<BoxError>>(e: E) -> Error {
     Error::new(Kind::Connection, Some(e))
 }
 
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 pub(crate) fn tls<E: Into<BoxError>>(e: E) -> Error {
     Error::new(Kind::Tls, Some(e))
 }

src/transport/smtp/extension.rs

@@ -4,7 +4,6 @@ use std::{
     collections::HashSet,
     fmt::{self, Display, Formatter},
     net::{Ipv4Addr, Ipv6Addr},
-    result::Result,
 };
 
 use crate::transport::smtp::{
@@ -53,10 +52,10 @@ impl Default for ClientId {
 
 impl Display for ClientId {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match *self {
-            Self::Domain(ref value) => f.write_str(value),
-            Self::Ipv4(ref value) => write!(f, "[{}]", value),
-            Self::Ipv6(ref value) => write!(f, "[IPv6:{}]", value),
+        match self {
+            Self::Domain(value) => f.write_str(value),
+            Self::Ipv4(value) => write!(f, "[{value}]"),
+            Self::Ipv6(value) => write!(f, "[IPv6:{value}]"),
         }
     }
 }
@@ -93,11 +92,11 @@ pub enum Extension {
 
 impl Display for Extension {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match *self {
+        match self {
             Extension::EightBitMime => f.write_str("8BITMIME"),
             Extension::SmtpUtfEight => f.write_str("SMTPUTF8"),
             Extension::StartTls => f.write_str("STARTTLS"),
-            Extension::Authentication(ref mechanism) => write!(f, "AUTH {}", mechanism),
+            Extension::Authentication(mechanism) => write!(f, "AUTH {mechanism}"),
         }
     }
 }
@@ -119,7 +118,7 @@ pub struct ServerInfo {
 impl Display for ServerInfo {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         let features = if self.features.is_empty() {
-            "no supported features".to_string()
+            "no supported features".to_owned()
         } else {
             format!("{:?}", self.features)
         };
@@ -174,7 +173,7 @@ impl ServerInfo {
         }
 
         Ok(ServerInfo {
-            name: name.to_string(),
+            name: name.to_owned(),
             features,
         })
     }
@@ -190,7 +189,7 @@ impl ServerInfo {
             .contains(&Extension::Authentication(mechanism))
     }
 
-    /// Gets a compatible mechanism from list
+    /// Gets a compatible mechanism from a list
     pub fn get_auth_mechanism(&self, mechanisms: &[Mechanism]) -> Option<Mechanism> {
         for mechanism in mechanisms {
             if self.supports_auth_mechanism(*mechanism) {
@@ -227,16 +226,16 @@ pub enum MailParameter {
 
 impl Display for MailParameter {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match *self {
-            MailParameter::Body(ref value) => write!(f, "BODY={}", value),
-            MailParameter::Size(size) => write!(f, "SIZE={}", size),
+        match self {
+            MailParameter::Body(value) => write!(f, "BODY={value}"),
+            MailParameter::Size(size) => write!(f, "SIZE={size}"),
             MailParameter::SmtpUtfEight => f.write_str("SMTPUTF8"),
             MailParameter::Other {
-                ref keyword,
-                value: Some(ref value),
+                keyword,
+                value: Some(value),
             } => write!(f, "{}={}", keyword, XText(value)),
             MailParameter::Other {
-                ref keyword,
+                keyword,
                 value: None,
             } => f.write_str(keyword),
         }
@@ -277,13 +276,13 @@ pub enum RcptParameter {
 
 impl Display for RcptParameter {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match *self {
+        match &self {
             RcptParameter::Other {
-                ref keyword,
-                value: Some(ref value),
-            } => write!(f, "{}={}", keyword, XText(value)),
+                keyword,
+                value: Some(value),
+            } => write!(f, "{keyword}={}", XText(value)),
             RcptParameter::Other {
-                ref keyword,
+                keyword,
                 value: None,
             } => f.write_str(keyword),
         }
@@ -292,33 +291,27 @@ impl Display for RcptParameter {
 
 #[cfg(test)]
 mod test {
-
-    use std::collections::HashSet;
-
     use super::*;
-    use crate::transport::smtp::{
-        authentication::Mechanism,
-        response::{Category, Code, Detail, Response, Severity},
-    };
+    use crate::transport::smtp::response::{Category, Code, Detail, Severity};
 
     #[test]
     fn test_clientid_fmt() {
         assert_eq!(
-            format!("{}", ClientId::Domain("test".to_string())),
-            "test".to_string()
+            format!("{}", ClientId::Domain("test".to_owned())),
+            "test".to_owned()
         );
-        assert_eq!(format!("{}", LOCALHOST_CLIENT), "[127.0.0.1]".to_string());
+        assert_eq!(format!("{LOCALHOST_CLIENT}"), "[127.0.0.1]".to_owned());
     }
 
     #[test]
     fn test_extension_fmt() {
         assert_eq!(
             format!("{}", Extension::EightBitMime),
-            "8BITMIME".to_string()
+            "8BITMIME".to_owned()
         );
         assert_eq!(
             format!("{}", Extension::Authentication(Mechanism::Plain)),
-            "AUTH PLAIN".to_string()
+            "AUTH PLAIN".to_owned()
         );
     }
 
@@ -331,11 +324,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: eightbitmime,
                 }
             ),
-            "name with {EightBitMime}".to_string()
+            "name with {EightBitMime}".to_owned()
         );
 
         let empty = HashSet::new();
@@ -344,11 +337,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: empty,
                 }
             ),
-            "name with no supported features".to_string()
+            "name with no supported features".to_owned()
         );
 
         let mut plain = HashSet::new();
@@ -358,11 +351,11 @@ mod test {
             format!(
                 "{}",
                 ServerInfo {
-                    name: "name".to_string(),
+                    name: "name".to_owned(),
                     features: plain,
                 }
             ),
-            "name with {Authentication(Plain)}".to_string()
+            "name with {Authentication(Plain)}".to_owned()
         );
     }
 
@@ -374,18 +367,14 @@ mod test {
                 Category::Unspecified4,
                 Detail::One,
             ),
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned()],
         );
 
         let mut features = HashSet::new();
         assert!(features.insert(Extension::EightBitMime));
 
         let server_info = ServerInfo {
-            name: "me".to_string(),
+            name: "me".to_owned(),
             features,
         };
 
@@ -401,10 +390,10 @@ mod test {
                 Detail::One,
             ),
             vec![
-                "me".to_string(),
-                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
+                "me".to_owned(),
+                "AUTH PLAIN CRAM-MD5 XOAUTH2 OTHER".to_owned(),
+                "8BITMIME".to_owned(),
+                "SIZE 42".to_owned(),
             ],
         );
 
@@ -414,7 +403,7 @@ mod test {
         assert!(features2.insert(Extension::Authentication(Mechanism::Xoauth2),));
 
         let server_info2 = ServerInfo {
-            name: "me".to_string(),
+            name: "me".to_owned(),
             features: features2,
         };
 

src/transport/smtp/mod.rs

@@ -77,8 +77,8 @@
 //! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
 //!     // Add credentials for authentication
 //!     .credentials(Credentials::new(
-//!         "username".to_string(),
-//!         "password".to_string(),
+//!         "username".to_owned(),
+//!         "password".to_owned(),
 //!     ))
 //!     // Configure expected authentication mechanism
 //!     .authentication(vec![Mechanism::Plain])
@@ -111,7 +111,7 @@
 //!     .body(String::from("Be happy!"))?;
 //!
 //! // Custom TLS configuration
-//! let tls = TlsParameters::builder("smtp.example.com".to_string())
+//! let tls = TlsParameters::builder("smtp.example.com".to_owned())
 //!     .dangerous_accept_invalid_certs(true)
 //!     .build()?;
 //!
@@ -140,7 +140,7 @@ pub use self::{
     error::Error,
     transport::{SmtpTransport, SmtpTransportBuilder},
 };
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use crate::transport::smtp::client::TlsParameters;
 use crate::transport::smtp::{
     authentication::{Credentials, Mechanism, DEFAULT_MECHANISMS},
@@ -154,6 +154,7 @@ mod async_transport;
 pub mod authentication;
 pub mod client;
 pub mod commands;
+mod connection_url;
 mod error;
 pub mod extension;
 #[cfg(feature = "pool")]
@@ -200,7 +201,7 @@ struct SmtpInfo {
 impl Default for SmtpInfo {
     fn default() -> Self {
         Self {
-            server: "localhost".to_string(),
+            server: "localhost".to_owned(),
             port: SMTP_PORT,
             hello_name: ClientId::default(),
             credentials: None,

src/transport/smtp/pool/async_impl.rs

@@ -2,7 +2,7 @@ use std::{
     fmt::{self, Debug},
     mem,
     ops::{Deref, DerefMut},
-    sync::Arc,
+    sync::{Arc, OnceLock},
     time::{Duration, Instant},
 };
 
@@ -10,7 +10,6 @@ use futures_util::{
     lock::Mutex,
     stream::{self, StreamExt},
 };
-use once_cell::sync::OnceCell;
 
 use super::{
     super::{client::AsyncSmtpConnection, Error},
@@ -22,7 +21,7 @@ pub struct Pool<E: Executor> {
     config: PoolConfig,
     connections: Mutex<Vec<ParkedConnection>>,
     client: AsyncSmtpClient<E>,
-    handle: OnceCell<E::Handle>,
+    handle: OnceLock<E::Handle>,
 }
 
 struct ParkedConnection {
@@ -41,7 +40,7 @@ impl<E: Executor> Pool<E> {
             config,
             connections: Mutex::new(Vec::new()),
             client,
-            handle: OnceCell::new(),
+            handle: OnceLock::new(),
         });
 
         {
@@ -203,7 +202,7 @@ impl<E: Executor> Debug for Pool<E> {
                 &match self.connections.try_lock() {
                     Some(connections) => format!("{} connections", connections.len()),
 
-                    None => "LOCKED".to_string(),
+                    None => "LOCKED".to_owned(),
                 },
             )
             .field("client", &self.client)

src/transport/smtp/pool/sync_impl.rs

@@ -186,8 +186,8 @@ impl Debug for Pool {
                 &match self.connections.try_lock() {
                     Ok(connections) => format!("{} connections", connections.len()),
 
-                    Err(TryLockError::WouldBlock) => "LOCKED".to_string(),
-                    Err(TryLockError::Poisoned(_)) => "POISONED".to_string(),
+                    Err(TryLockError::WouldBlock) => "LOCKED".to_owned(),
+                    Err(TryLockError::Poisoned(_)) => "POISONED".to_owned(),
                 },
             )
             .field("client", &self.client)

src/transport/smtp/response.rs

@@ -5,7 +5,6 @@ use std::{
     fmt::{Display, Formatter, Result},
     result,
     str::FromStr,
-    string::ToString,
 };
 
 use nom::{
@@ -19,7 +18,7 @@ use nom::{
 
 use crate::transport::smtp::{error, Error};
 
-/// First digit indicates severity
+/// The first digit indicates severity
 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Severity {
@@ -132,6 +131,12 @@ impl Code {
     }
 }
 
+impl From<Code> for u16 {
+    fn from(code: Code) -> Self {
+        code.detail as u16 + 10 * code.category as u16 + 100 * code.severity as u16
+    }
+}
+
 /// Contains an SMTP reply, with separated code and message
 ///
 /// The text message is optional, only the code is mandatory
@@ -151,7 +156,7 @@ impl FromStr for Response {
     fn from_str(s: &str) -> result::Result<Response, Error> {
         parse_response(s)
             .map(|(_, r)| r)
-            .map_err(|e| error::response(e.to_string()))
+            .map_err(|e| error::response(e.to_owned()))
     }
 }
 
@@ -317,6 +322,17 @@ mod test {
         assert_eq!(code.to_string(), "421");
     }
 
+    #[test]
+    fn test_code_to_u16() {
+        let code = Code {
+            severity: Severity::TransientNegativeCompletion,
+            category: Category::Connections,
+            detail: Detail::One,
+        };
+        let c: u16 = code.into();
+        assert_eq!(c, 421);
+    }
+
     #[test]
     fn test_response_from_str() {
         let raw_response = "250-me\r\n250-8BITMIME\r\n250-SIZE 42\r\n250 AUTH PLAIN CRAM-MD5\r\n";
@@ -329,10 +345,10 @@ mod test {
                     detail: Detail::Zero,
                 },
                 message: vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                    "AUTH PLAIN CRAM-MD5".to_string(),
+                    "me".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
+                    "AUTH PLAIN CRAM-MD5".to_owned(),
                 ],
             }
         );
@@ -352,11 +368,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .is_positive());
         assert!(!Response::new(
@@ -365,11 +377,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::Zero,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .is_positive());
     }
@@ -382,11 +390,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .has_code(451));
         assert!(!Response::new(
@@ -395,11 +399,7 @@ mod test {
                 category: Category::MailSystem,
                 detail: Detail::One,
             },
-            vec![
-                "me".to_string(),
-                "8BITMIME".to_string(),
-                "SIZE 42".to_string(),
-            ],
+            vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
         )
         .has_code(251));
     }
@@ -413,11 +413,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
             )
             .first_word(),
             Some("me")
@@ -430,9 +426,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
+                    "me mo".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_word(),
@@ -457,7 +453,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_word(),
             None
@@ -469,7 +465,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_word(),
             None
@@ -481,7 +477,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["".to_string()],
+                vec!["".to_owned()],
             )
             .first_word(),
             None
@@ -494,7 +490,7 @@ mod test {
         let res = parse_response(raw_response);
         match res {
             Err(nom::Err::Incomplete(_)) => {}
-            _ => panic!("Expected incomplete response, got {:?}", res),
+            _ => panic!("Expected incomplete response, got {res:?}"),
         }
     }
 
@@ -507,11 +503,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![
-                    "me".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
-                ],
+                vec!["me".to_owned(), "8BITMIME".to_owned(), "SIZE 42".to_owned(),],
             )
             .first_line(),
             Some("me")
@@ -524,9 +516,9 @@ mod test {
                     detail: Detail::One,
                 },
                 vec![
-                    "me mo".to_string(),
-                    "8BITMIME".to_string(),
-                    "SIZE 42".to_string(),
+                    "me mo".to_owned(),
+                    "8BITMIME".to_owned(),
+                    "SIZE 42".to_owned(),
                 ],
             )
             .first_line(),
@@ -551,7 +543,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec![" ".to_string()],
+                vec![" ".to_owned()],
             )
             .first_line(),
             Some(" ")
@@ -563,7 +555,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["  ".to_string()],
+                vec!["  ".to_owned()],
             )
             .first_line(),
             Some("  ")
@@ -575,7 +567,7 @@ mod test {
                     category: Category::MailSystem,
                     detail: Detail::One,
                 },
-                vec!["".to_string()],
+                vec!["".to_owned()],
             )
             .first_line(),
             Some("")

src/transport/smtp/transport.rs

@@ -1,13 +1,13 @@
 #[cfg(feature = "pool")]
 use std::sync::Arc;
-use std::time::Duration;
+use std::{fmt::Debug, time::Duration};
 
 #[cfg(feature = "pool")]
 use super::pool::sync_impl::Pool;
 #[cfg(feature = "pool")]
 use super::PoolConfig;
 use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
-#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+#[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
 use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{address::Envelope, Transport};
 
@@ -38,6 +38,14 @@ impl Transport for SmtpTransport {
     }
 }
 
+impl Debug for SmtpTransport {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut builder = f.debug_struct("SmtpTransport");
+        builder.field("inner", &self.inner);
+        builder.finish()
+    }
+}
+
 impl SmtpTransport {
     /// Simple and secure transport, using TLS connections to communicate with the SMTP server
     ///
@@ -45,8 +53,11 @@ impl SmtpTransport {
     ///
     /// Creates an encrypted transport over submissions port, using the provided domain
     /// to validate TLS certificates.
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -55,7 +66,7 @@ impl SmtpTransport {
             .tls(Tls::Wrapper(tls_parameters)))
     }
 
-    /// Simple an secure transport, using STARTTLS to obtain encrypted connections
+    /// Simple and secure transport, using STARTTLS to obtain encrypted connections
     ///
     /// Alternative to [`SmtpTransport::relay`](#method.relay), for SMTP servers
     /// that don't take SMTPS connections.
@@ -66,8 +77,11 @@ impl SmtpTransport {
     ///
     /// An error is returned if the connection can't be upgraded. No credentials
     /// or emails will be sent to the server, protecting from downgrade attacks.
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn starttls_relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -89,29 +103,106 @@ impl SmtpTransport {
     ///
     /// * No authentication
     /// * No TLS
-    /// * A 60 seconds timeout for smtp commands
+    /// * A 60-seconds timeout for smtp commands
     /// * Port 25
     ///
     /// Consider using [`SmtpTransport::relay`](#method.relay) or
     /// [`SmtpTransport::starttls_relay`](#method.starttls_relay) instead,
     /// if possible.
     pub fn builder_dangerous<T: Into<String>>(server: T) -> SmtpTransportBuilder {
-        let new = SmtpInfo {
-            server: server.into(),
-            ..Default::default()
-        };
+        SmtpTransportBuilder::new(server)
+    }
 
-        SmtpTransportBuilder {
-            info: new,
-            #[cfg(feature = "pool")]
-            pool_config: PoolConfig::default(),
-        }
+    /// Creates a `SmtpTransportBuilder` from a connection URL
+    ///
+    /// The protocol, credentials, host and port can be provided in a single URL.
+    /// Use the scheme `smtp` for an unencrypted relay (optionally in combination with the
+    /// `tls` parameter to allow/require STARTTLS) or `smtps` for SMTP over TLS.
+    /// The path section of the url can be used to set an alternative name for
+    /// the HELO / EHLO command.
+    /// For example `smtps://username:password@smtp.example.com/client.example.com:465`
+    /// will set the HELO / EHLO name `client.example.com`.
+    ///
+    /// <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::{
+    ///     message::header::ContentType, transport::smtp::authentication::Credentials, Message,
+    ///     SmtpTransport, Transport,
+    /// };
+    ///
+    /// let email = Message::builder()
+    ///     .from("NoBody <nobody@domain.tld>".parse().unwrap())
+    ///     .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();
+    ///
+    /// // Open a remote connection to example
+    /// let mailer = SmtpTransport::from_url("smtps://username:password@smtp.example.com:465")
+    ///     .unwrap()
+    ///     .build();
+    ///
+    /// // Send the email
+    /// match mailer.send(&email) {
+    ///     Ok(_) => println!("Email sent successfully!"),
+    ///     Err(e) => panic!("Could not send email: {e:?}"),
+    /// }
+    /// ```
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
+    pub fn from_url(connection_url: &str) -> Result<SmtpTransportBuilder, Error> {
+        super::connection_url::from_connection_url(connection_url)
     }
 
     /// Tests the SMTP connection
     ///
     /// `test_connection()` tests the connection by using the SMTP NOOP command.
-    /// The connection is closed afterwards if a connection pool is not used.
+    /// The connection is closed afterward if a connection pool is not used.
     pub fn test_connection(&self) -> Result<bool, Error> {
         let mut conn = self.inner.connection()?;
 
@@ -135,6 +226,20 @@ pub struct SmtpTransportBuilder {
 
 /// Builder for the SMTP `SmtpTransport`
 impl SmtpTransportBuilder {
+    // Create new builder with default parameters
+    pub(crate) fn new<T: Into<String>>(server: T) -> Self {
+        let new = SmtpInfo {
+            server: server.into(),
+            ..Default::default()
+        };
+
+        Self {
+            info: new,
+            #[cfg(feature = "pool")]
+            pool_config: PoolConfig::default(),
+        }
+    }
+
     /// Set the name used during EHLO
     pub fn hello_name(mut self, name: ClientId) -> Self {
         self.info.hello_name = name;
@@ -166,8 +271,11 @@ impl SmtpTransportBuilder {
     }
 
     /// Set the TLS settings to use
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls")))
+    )]
     pub fn tls(mut self, tls: Tls) -> Self {
         self.info.tls = tls;
         self
@@ -185,7 +293,7 @@ impl SmtpTransportBuilder {
 
     /// Build the transport
     ///
-    /// If the `pool` feature is enabled an `Arc` wrapped pool is be created.
+    /// If the `pool` feature is enabled, an `Arc` wrapped pool is created.
     /// Defaults can be found at [`PoolConfig`]
     pub fn build(self) -> SmtpTransport {
         let client = SmtpClient { info: self.info };
@@ -209,9 +317,9 @@ impl SmtpClient {
     /// Handles encryption and authentication
     pub fn connection(&self) -> Result<SmtpConnection, Error> {
         #[allow(clippy::match_single_binding)]
-        let tls_parameters = match self.info.tls {
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-            Tls::Wrapper(ref tls_parameters) => Some(tls_parameters),
+        let tls_parameters = match &self.info.tls {
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+            Tls::Wrapper(tls_parameters) => Some(tls_parameters),
             _ => None,
         };
 
@@ -224,14 +332,14 @@ impl SmtpClient {
             None,
         )?;
 
-        #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-        match self.info.tls {
-            Tls::Opportunistic(ref tls_parameters) => {
+        #[cfg(any(feature = "native-tls", feature = "rustls-tls", feature = "boring-tls"))]
+        match &self.info.tls {
+            Tls::Opportunistic(tls_parameters) => {
                 if conn.can_starttls() {
                     conn.starttls(tls_parameters, &self.info.hello_name)?;
                 }
             }
-            Tls::Required(ref tls_parameters) => {
+            Tls::Required(tls_parameters) => {
                 conn.starttls(tls_parameters, &self.info.hello_name)?;
             }
             _ => (),
@@ -243,3 +351,78 @@ impl SmtpClient {
         Ok(conn)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        transport::smtp::{authentication::Credentials, client::Tls},
+        SmtpTransport,
+    };
+
+    #[test]
+    fn transport_from_url() {
+        let builder = SmtpTransport::from_url("smtp://127.0.0.1:2525").unwrap();
+
+        assert_eq!(builder.info.port, 2525);
+        assert!(matches!(builder.info.tls, Tls::None));
+        assert_eq!(builder.info.server, "127.0.0.1");
+
+        let builder =
+            SmtpTransport::from_url("smtps://username:password@smtp.example.com:465").unwrap();
+
+        assert_eq!(builder.info.port, 465);
+        assert_eq!(
+            builder.info.credentials,
+            Some(Credentials::new(
+                "username".to_owned(),
+                "password".to_owned()
+            ))
+        );
+        assert!(matches!(builder.info.tls, Tls::Wrapper(_)));
+        assert_eq!(builder.info.server, "smtp.example.com");
+
+        let builder = SmtpTransport::from_url(
+            "smtps://user%40example.com:pa$$word%3F%22!@smtp.example.com:465",
+        )
+        .unwrap();
+
+        assert_eq!(builder.info.port, 465);
+        assert_eq!(
+            builder.info.credentials,
+            Some(Credentials::new(
+                "user@example.com".to_owned(),
+                "pa$$word?\"!".to_owned()
+            ))
+        );
+        assert!(matches!(builder.info.tls, Tls::Wrapper(_)));
+        assert_eq!(builder.info.server, "smtp.example.com");
+
+        let builder =
+            SmtpTransport::from_url("smtp://username:password@smtp.example.com:587?tls=required")
+                .unwrap();
+
+        assert_eq!(builder.info.port, 587);
+        assert_eq!(
+            builder.info.credentials,
+            Some(Credentials::new(
+                "username".to_owned(),
+                "password".to_owned()
+            ))
+        );
+        assert!(matches!(builder.info.tls, Tls::Required(_)));
+
+        let builder = SmtpTransport::from_url(
+            "smtp://username:password@smtp.example.com:587?tls=opportunistic",
+        )
+        .unwrap();
+
+        assert_eq!(builder.info.port, 587);
+        assert!(matches!(builder.info.tls, Tls::Opportunistic(_)));
+
+        let builder = SmtpTransport::from_url("smtps://smtp.example.com").unwrap();
+
+        assert_eq!(builder.info.port, 465);
+        assert_eq!(builder.info.credentials, None);
+        assert!(matches!(builder.info.tls, Tls::Wrapper(_)));
+    }
+}

src/transport/smtp/util.rs

@@ -41,7 +41,7 @@ mod tests {
         ]
         .iter()
         {
-            assert_eq!(format!("{}", XText(input)), (*expect).to_string());
+            assert_eq!(format!("{}", XText(input)), (*expect).to_owned());
         }
     }
 }

tests/transport_file.rs

@@ -33,7 +33,7 @@ mod sync {
         let result = sender.send(&email);
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(
@@ -68,10 +68,10 @@ mod sync {
         let result = sender.send(&email);
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
-        let json_file = temp_dir().join(format!("{}.json", id));
+        let json_file = temp_dir().join(format!("{id}.json"));
         let json = read_to_string(&json_file).unwrap();
 
         assert_eq!(
@@ -131,7 +131,7 @@ mod tokio_1 {
         let result = sender.send(email).await;
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(
@@ -182,7 +182,7 @@ mod asyncstd_1 {
         let result = sender.send(email).await;
         let id = result.unwrap();
 
-        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml_file = temp_dir().join(format!("{id}.eml"));
         let eml = read_to_string(&eml_file).unwrap();
 
         assert_eq!(

tests/transport_sendmail.rs

@@ -15,7 +15,7 @@ mod sync {
             .unwrap();
 
         let result = sender.send(&email);
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }
@@ -42,7 +42,7 @@ mod tokio_1 {
             .unwrap();
 
         let result = sender.send(email).await;
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }
@@ -68,7 +68,7 @@ mod asyncstd_1 {
             .unwrap();
 
         let result = sender.send(email).await;
-        println!("{:?}", result);
+        println!("{result:?}");
         assert!(result.is_ok());
     }
 }