Prepare 0.10.0-beta.3 (#578)

* Improve docs in lib.rs

* Typos

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-alpha.5"
+version = "0.10.0-beta.3"
 description = "Email client"
 readme = "README.md"
 homepage = "https://lettre.rs"
@@ -23,7 +23,7 @@ tracing = { version = "0.1.16", default-features = false, features = ["std"], op
 
 # builder
 hyperx = { version = "1", optional = true, features = ["headers"] }
-mime = { version = "0.3", optional = true }
+mime = { version = "0.3.4", optional = true }
 uuid = { version = "0.8", features = ["v4"] }
 rand = { version = "0.8", optional = true }
 quoted_printable = { version = "0.4", optional = true }
@@ -36,7 +36,7 @@ serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
 
 # smtp
-nom = { version = "6", default-features = false, features = ["alloc"], optional = true }
+nom = { version = "6", default-features = false, features = ["alloc", "std"], optional = true }
 r2d2 = { version = "0.8", optional = true } # feature
 hostname = { version = "0.3", optional = true } # feature
 
@@ -80,7 +80,7 @@ harness = false
 name = "transport_smtp"
 
 [features]
-default = ["file-transport", "file-transport-envelope", "smtp-transport", "native-tls", "hostname", "r2d2", "sendmail-transport", "builder"]
+default = ["smtp-transport", "native-tls", "hostname", "r2d2", "builder"]
 builder = ["mime", "base64", "hyperx", "rand", "quoted_printable"]
 
 # transports

README.md

@@ -28,8 +28,8 @@
 </div>
 
 <div align="center">
-  <a href="https://deps.rs/crate/lettre/0.10.0-alpha.4">
-    <img src="https://deps.rs/crate/lettre/0.10.0-alpha.4/status.svg"
+  <a href="https://deps.rs/crate/lettre/0.10.0-beta.3">
+    <img src="https://deps.rs/crate/lettre/0.10.0-beta.3/status.svg"
       alt="dependency status" />
   </a>
 </div>
@@ -66,7 +66,7 @@ To use this library, add the following to your `Cargo.toml`:
 
 ```toml
 [dependencies]
-lettre = "0.10.0-alpha.4"
+lettre = "0.10.0-beta.3"
 ```
 
 ```rust,no_run

examples/asyncstd1_smtp_starttls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Connector,
-    AsyncStd1Transport, Message,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
+    AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -18,10 +18,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail using STARTTLS
-    let mailer = AsyncSmtpTransport::<AsyncStd1Connector>::starttls_relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
+        AsyncSmtpTransport::<AsyncStd1Executor>::starttls_relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

examples/asyncstd1_smtp_tls.rs

@@ -1,6 +1,6 @@
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Connector,
-    AsyncStd1Transport, Message,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncStd1Executor,
+    AsyncTransport, Message,
 };
 
 #[async_std::main]
@@ -18,10 +18,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail
-    let mailer = AsyncSmtpTransport::<AsyncStd1Connector>::relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<AsyncStd1Executor> =
+        AsyncSmtpTransport::<AsyncStd1Executor>::relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

examples/basic_html.rs

@@ -1,5 +1,7 @@
-use lettre::message::{header, MultiPart, SinglePart};
-use lettre::{FileTransport, Message, Transport};
+use lettre::{
+    message::{header, MultiPart, SinglePart},
+    FileTransport, Message, Transport,
+};
 
 fn main() {
     // The html we want to send.

examples/maud_html.rs

@@ -1,5 +1,7 @@
-use lettre::message::{header, MultiPart, SinglePart};
-use lettre::{FileTransport, Message, Transport};
+use lettre::{
+    message::{header, MultiPart, SinglePart},
+    FileTransport, Message, Transport,
+};
 use maud::html;
 
 fn main() {

examples/smtp_selfsigned.rs

@@ -1,8 +1,10 @@
 use std::fs;
 
 use lettre::{
-    transport::smtp::authentication::Credentials,
-    transport::smtp::client::{Certificate, Tls, TlsParameters},
+    transport::smtp::{
+        authentication::Credentials,
+        client::{Certificate, Tls, TlsParameters},
+    },
     Message, SmtpTransport, Transport,
 };
 
@@ -20,9 +22,10 @@ fn main() {
     // 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 mut tls = TlsParameters::builder("smtp.server.com".to_string());
-    tls.add_root_certificate(cert);
-    let tls = tls.build().unwrap();
+    let tls = TlsParameters::builder("smtp.server.com".to_string())
+        .add_root_certificate(cert)
+        .build()
+        .unwrap();
 
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 

examples/tokio02_smtp_starttls.rs

@@ -4,8 +4,8 @@
 use tokio02_crate as tokio;
 
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, Message, Tokio02Connector,
-    Tokio02Transport,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    Tokio02Executor,
 };
 
 #[tokio::main]
@@ -23,10 +23,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail using STARTTLS
-    let mailer = AsyncSmtpTransport::<Tokio02Connector>::starttls_relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<Tokio02Executor> =
+        AsyncSmtpTransport::<Tokio02Executor>::starttls_relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

examples/tokio02_smtp_tls.rs

@@ -4,8 +4,8 @@
 use tokio02_crate as tokio;
 
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, Message, Tokio02Connector,
-    Tokio02Transport,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    Tokio02Executor,
 };
 
 #[tokio::main]
@@ -23,10 +23,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail
-    let mailer = AsyncSmtpTransport::<Tokio02Connector>::relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<Tokio02Executor> =
+        AsyncSmtpTransport::<Tokio02Executor>::relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

examples/tokio1_smtp_starttls.rs

@@ -4,8 +4,8 @@
 use tokio1_crate as tokio;
 
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, Message, Tokio1Connector,
-    Tokio1Transport,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    Tokio1Executor,
 };
 
 #[tokio::main]
@@ -23,10 +23,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail using STARTTLS
-    let mailer = AsyncSmtpTransport::<Tokio1Connector>::starttls_relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<Tokio1Executor> =
+        AsyncSmtpTransport::<Tokio1Executor>::starttls_relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

examples/tokio1_smtp_tls.rs

@@ -4,8 +4,8 @@
 use tokio1_crate as tokio;
 
 use lettre::{
-    transport::smtp::authentication::Credentials, AsyncSmtpTransport, Message, Tokio1Connector,
-    Tokio1Transport,
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, AsyncTransport, Message,
+    Tokio1Executor,
 };
 
 #[tokio::main]
@@ -23,10 +23,11 @@ async fn main() {
     let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
 
     // Open a remote connection to gmail
-    let mailer = AsyncSmtpTransport::<Tokio1Connector>::relay("smtp.gmail.com")
-        .unwrap()
-        .credentials(creds)
-        .build();
+    let mailer: AsyncSmtpTransport<Tokio1Executor> =
+        AsyncSmtpTransport::<Tokio1Executor>::relay("smtp.gmail.com")
+            .unwrap()
+            .credentials(creds)
+            .build();
 
     // Send the email
     match mailer.send(email).await {

src/address/envelope.rs

@@ -27,15 +27,12 @@ impl Envelope {
     ///
     /// # Examples
     ///
-    /// ```
-    /// use std::str::FromStr;
-    /// # use lettre::Address;
-    /// # use lettre::address::Envelope;
-    ///
+    /// ```rust
+    /// # use lettre::address::{Address, Envelope};
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let sender = Address::from_str("from@email.com")?;
-    /// let recipients = vec![Address::from_str("to@email.com")?];
+    /// let sender = "sender@email.com".parse::<Address>()?;
+    /// let recipients = vec!["to@email.com".parse::<Address>()?];
     ///
     /// let envelope = Envelope::new(Some(sender), recipients);
     /// # Ok(())
@@ -59,15 +56,12 @@ impl Envelope {
     ///
     /// # Examples
     ///
-    /// ```
-    /// use std::str::FromStr;
-    /// # use lettre::Address;
-    /// # use lettre::address::Envelope;
-    ///
+    /// ```rust
+    /// # use lettre::address::{Address, Envelope};
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let sender = Address::from_str("from@email.com")?;
-    /// let recipients = vec![Address::from_str("to@email.com")?];
+    /// let sender = "from@email.com".parse::<Address>()?;
+    /// let recipients = vec!["to@email.com".parse::<Address>()?];
     ///
     /// let envelope = Envelope::new(Some(sender), recipients.clone())?;
     /// assert_eq!(envelope.to(), recipients.as_slice());
@@ -82,15 +76,12 @@ impl Envelope {
     ///
     /// # Examples
     ///
-    /// ```
-    /// use std::str::FromStr;
-    /// # use lettre::Address;
-    /// # use lettre::address::Envelope;
-    ///
+    /// ```rust
+    /// # use lettre::address::{Address, Envelope};
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let sender = Address::from_str("from@email.com")?;
-    /// let recipients = vec![Address::from_str("to@email.com")?];
+    /// let sender = "from@email.com".parse::<Address>()?;
+    /// let recipients = vec!["to@email.com".parse::<Address>()?];
     ///
     /// let envelope = Envelope::new(Some(sender), recipients.clone())?;
     /// assert!(envelope.from().is_some());
@@ -103,6 +94,15 @@ impl Envelope {
     pub fn from(&self) -> Option<&Address> {
         self.reverse_path.as_ref()
     }
+
+    #[cfg(feature = "smtp-transport")]
+    /// Check if any of the addresses in the envelope contains non-ascii chars
+    pub(crate) fn has_non_ascii_addresses(&self) -> bool {
+        self.reverse_path
+            .iter()
+            .chain(self.forward_path.iter())
+            .any(|a| !a.is_ascii())
+    }
 }
 
 #[cfg(feature = "builder")]

src/address/mod.rs

@@ -1,8 +1,12 @@
+//! Email addresses
+
 #[cfg(feature = "serde")]
 mod serde;
 
 mod envelope;
 mod types;
 
-pub use self::envelope::Envelope;
-pub use self::types::{Address, AddressError};
+pub use self::{
+    envelope::Envelope,
+    types::{Address, AddressError},
+};

src/address/types.rs

@@ -23,10 +23,13 @@ use std::{
 /// You can create an `Address` from a user and a domain:
 ///
 /// ```
-/// # use lettre::Address;
+/// use lettre::Address;
+///
 /// # use std::error::Error;
 /// # fn main() -> Result<(), Box<dyn Error>> {
-/// let address = Address::new("example", "email.com")?;
+/// let address = Address::new("user", "email.com")?;
+/// assert_eq!(address.user(), "user");
+/// assert_eq!(address.domain(), "email.com");
 /// # Ok(())
 /// # }
 /// ```
@@ -34,11 +37,13 @@ use std::{
 /// You can also create an `Address` from a string literal by parsing it:
 ///
 /// ```
-/// use std::str::FromStr;
-/// # use lettre::Address;
+/// use lettre::Address;
+///
 /// # use std::error::Error;
 /// # fn main() -> Result<(), Box<dyn Error>> {
-/// let address = Address::from_str("example@email.com")?;
+/// let address = "user@email.com".parse::<Address>()?;
+/// assert_eq!(address.user(), "user");
+/// assert_eq!(address.domain(), "email.com");
 /// # Ok(())
 /// # }
 /// ```
@@ -50,28 +55,6 @@ pub struct Address {
     at_start: usize,
 }
 
-impl<U, D> TryFrom<(U, D)> for Address
-where
-    U: AsRef<str>,
-    D: AsRef<str>,
-{
-    type Error = AddressError;
-
-    fn try_from((user, domain): (U, D)) -> Result<Self, Self::Error> {
-        let user = user.as_ref();
-        Address::check_user(user)?;
-
-        let domain = domain.as_ref();
-        Address::check_domain(domain)?;
-
-        let serialized = format!("{}@{}", user, domain);
-        Ok(Address {
-            serialized,
-            at_start: user.len(),
-        })
-    }
-}
-
 // Regex from the specs
 // https://html.spec.whatwg.org/multipage/forms.html#valid-e-mail-address
 // It will mark esoteric email addresses like quoted string as invalid
@@ -96,8 +79,8 @@ impl Address {
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let address = Address::new("example", "email.com")?;
-    /// let expected: Address = "example@email.com".parse()?;
+    /// let address = Address::new("user", "email.com")?;
+    /// let expected = "user@email.com".parse::<Address>()?;
     /// assert_eq!(expected, address);
     /// # Ok(())
     /// # }
@@ -115,8 +98,8 @@ impl Address {
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let address = Address::new("example", "email.com")?;
-    /// assert_eq!("example", address.user());
+    /// let address = Address::new("user", "email.com")?;
+    /// assert_eq!(address.user(), "user");
     /// # Ok(())
     /// # }
     /// ```
@@ -133,8 +116,8 @@ impl Address {
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
-    /// let address = Address::new("example", "email.com")?;
-    /// assert_eq!("email.com", address.domain());
+    /// let address = Address::new("user", "email.com")?;
+    /// assert_eq!(address.domain(), "email.com");
     /// # Ok(())
     /// # }
     /// ```
@@ -173,6 +156,12 @@ impl Address {
 
         Err(AddressError::InvalidDomain)
     }
+
+    #[cfg(feature = "smtp-transport")]
+    /// Check if the address contains non-ascii chars
+    pub(super) fn is_ascii(&self) -> bool {
+        self.serialized.is_ascii()
+    }
 }
 
 impl Display for Address {
@@ -198,6 +187,28 @@ impl FromStr for Address {
     }
 }
 
+impl<U, D> TryFrom<(U, D)> for Address
+where
+    U: AsRef<str>,
+    D: AsRef<str>,
+{
+    type Error = AddressError;
+
+    fn try_from((user, domain): (U, D)) -> Result<Self, Self::Error> {
+        let user = user.as_ref();
+        Address::check_user(user)?;
+
+        let domain = domain.as_ref();
+        Address::check_domain(domain)?;
+
+        let serialized = format!("{}@{}", user, domain);
+        Ok(Address {
+            serialized,
+            at_start: user.len(),
+        })
+    }
+}
+
 impl AsRef<str> for Address {
     fn as_ref(&self) -> &str {
         &self.serialized
@@ -211,6 +222,7 @@ impl AsRef<OsStr> for Address {
 }
 
 #[derive(Debug, PartialEq, Clone, Copy)]
+/// Errors in email addresses parsing
 pub enum AddressError {
     MissingParts,
     Unbalanced,

src/error.rs

@@ -1,3 +1,5 @@
+//! Error type for email messages
+
 use std::{
     error::Error as StdError,
     fmt::{self, Display, Formatter},

src/executor.rs

@@ -0,0 +1,272 @@
+use async_trait::async_trait;
+
+use std::fmt::Debug;
+#[cfg(feature = "file-transport")]
+use std::io::Result as IoResult;
+#[cfg(feature = "file-transport")]
+use std::path::Path;
+
+#[cfg(all(
+    feature = "smtp-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+use crate::transport::smtp::client::AsyncSmtpConnection;
+#[cfg(all(
+    feature = "smtp-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+use crate::transport::smtp::client::Tls;
+#[cfg(all(
+    feature = "smtp-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+use crate::transport::smtp::extension::ClientId;
+#[cfg(all(
+    feature = "smtp-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+use crate::transport::smtp::Error;
+
+/// Async executor abstraction trait
+///
+/// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
+/// in order to be able to work with different async runtimes.
+///
+/// [`AsyncSmtpTransport`]: crate::AsyncSmtpTransport
+/// [`AsyncSendmailTransport`]: crate::AsyncSendmailTransport
+/// [`AsyncFileTransport`]: crate::AsyncFileTransport
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
+#[async_trait]
+pub trait Executor: Debug + Send + Sync + private::Sealed {
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    async fn connect(
+        hostname: &str,
+        port: u16,
+        hello_name: &ClientId,
+        tls: &Tls,
+    ) -> Result<AsyncSmtpConnection, Error>;
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport-envelope")]
+    async fn fs_read(path: &Path) -> IoResult<Vec<u8>>;
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport")]
+    async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()>;
+}
+
+/// Async [`Executor`] using `tokio` `0.2.x`
+///
+/// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
+/// in order to be able to work with different async runtimes.
+///
+/// [`AsyncSmtpTransport`]: crate::AsyncSmtpTransport
+/// [`AsyncSendmailTransport`]: crate::AsyncSendmailTransport
+/// [`AsyncFileTransport`]: crate::AsyncFileTransport
+#[allow(missing_copy_implementations)]
+#[non_exhaustive]
+#[cfg(feature = "tokio02")]
+#[cfg_attr(docsrs, doc(cfg(feature = "tokio02")))]
+#[derive(Debug)]
+pub struct Tokio02Executor;
+
+#[async_trait]
+#[cfg(feature = "tokio02")]
+impl Executor for Tokio02Executor {
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    async fn connect(
+        hostname: &str,
+        port: u16,
+        hello_name: &ClientId,
+        tls: &Tls,
+    ) -> Result<AsyncSmtpConnection, Error> {
+        #[allow(clippy::match_single_binding)]
+        let tls_parameters = match tls {
+            #[cfg(any(feature = "tokio02-native-tls", feature = "tokio02-rustls-tls"))]
+            Tls::Wrapper(ref tls_parameters) => Some(tls_parameters.clone()),
+            _ => None,
+        };
+        #[allow(unused_mut)]
+        let mut conn =
+            AsyncSmtpConnection::connect_tokio02(hostname, port, hello_name, tls_parameters)
+                .await?;
+
+        #[cfg(any(feature = "tokio02-native-tls", feature = "tokio02-rustls-tls"))]
+        match tls {
+            Tls::Opportunistic(ref tls_parameters) => {
+                if conn.can_starttls() {
+                    conn.starttls(tls_parameters.clone(), hello_name).await?;
+                }
+            }
+            Tls::Required(ref tls_parameters) => {
+                conn.starttls(tls_parameters.clone(), hello_name).await?;
+            }
+            _ => (),
+        }
+
+        Ok(conn)
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport-envelope")]
+    async fn fs_read(path: &Path) -> IoResult<Vec<u8>> {
+        tokio02_crate::fs::read(path).await
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport")]
+    async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()> {
+        tokio02_crate::fs::write(path, contents).await
+    }
+}
+
+/// Async [`Executor`] using `tokio` `1.x`
+///
+/// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
+/// in order to be able to work with different async runtimes.
+///
+/// [`AsyncSmtpTransport`]: crate::AsyncSmtpTransport
+/// [`AsyncSendmailTransport`]: crate::AsyncSendmailTransport
+/// [`AsyncFileTransport`]: crate::AsyncFileTransport
+#[allow(missing_copy_implementations)]
+#[non_exhaustive]
+#[cfg(feature = "tokio1")]
+#[cfg_attr(docsrs, doc(cfg(feature = "tokio1")))]
+#[derive(Debug)]
+pub struct Tokio1Executor;
+
+#[async_trait]
+#[cfg(feature = "tokio1")]
+impl Executor for Tokio1Executor {
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    async fn connect(
+        hostname: &str,
+        port: u16,
+        hello_name: &ClientId,
+        tls: &Tls,
+    ) -> Result<AsyncSmtpConnection, Error> {
+        #[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()),
+            _ => None,
+        };
+        #[allow(unused_mut)]
+        let mut conn =
+            AsyncSmtpConnection::connect_tokio1(hostname, port, hello_name, tls_parameters).await?;
+
+        #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
+        match tls {
+            Tls::Opportunistic(ref tls_parameters) => {
+                if conn.can_starttls() {
+                    conn.starttls(tls_parameters.clone(), hello_name).await?;
+                }
+            }
+            Tls::Required(ref tls_parameters) => {
+                conn.starttls(tls_parameters.clone(), hello_name).await?;
+            }
+            _ => (),
+        }
+
+        Ok(conn)
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport-envelope")]
+    async fn fs_read(path: &Path) -> IoResult<Vec<u8>> {
+        tokio1_crate::fs::read(path).await
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport")]
+    async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()> {
+        tokio1_crate::fs::write(path, contents).await
+    }
+}
+
+/// Async [`Executor`] using `async-std` `1.x`
+///
+/// Used by [`AsyncSmtpTransport`], [`AsyncSendmailTransport`] and [`AsyncFileTransport`]
+/// in order to be able to work with different async runtimes.
+///
+/// [`AsyncSmtpTransport`]: crate::AsyncSmtpTransport
+/// [`AsyncSendmailTransport`]: crate::AsyncSendmailTransport
+/// [`AsyncFileTransport`]: crate::AsyncFileTransport
+#[allow(missing_copy_implementations)]
+#[non_exhaustive]
+#[cfg(feature = "async-std1")]
+#[cfg_attr(docsrs, doc(cfg(feature = "async-std1")))]
+#[derive(Debug)]
+pub struct AsyncStd1Executor;
+
+#[async_trait]
+#[cfg(feature = "async-std1")]
+impl Executor for AsyncStd1Executor {
+    #[doc(hidden)]
+    #[cfg(feature = "smtp-transport")]
+    async fn connect(
+        hostname: &str,
+        port: u16,
+        hello_name: &ClientId,
+        tls: &Tls,
+    ) -> 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()),
+            _ => None,
+        };
+        #[allow(unused_mut)]
+        let mut conn =
+            AsyncSmtpConnection::connect_asyncstd1(hostname, port, hello_name, tls_parameters)
+                .await?;
+
+        #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
+        match tls {
+            Tls::Opportunistic(ref tls_parameters) => {
+                if conn.can_starttls() {
+                    conn.starttls(tls_parameters.clone(), hello_name).await?;
+                }
+            }
+            Tls::Required(ref tls_parameters) => {
+                conn.starttls(tls_parameters.clone(), hello_name).await?;
+            }
+            _ => (),
+        }
+
+        Ok(conn)
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport-envelope")]
+    async fn fs_read(path: &Path) -> IoResult<Vec<u8>> {
+        async_std::fs::read(path).await
+    }
+
+    #[doc(hidden)]
+    #[cfg(feature = "file-transport")]
+    async fn fs_write(path: &Path, contents: &[u8]) -> IoResult<()> {
+        async_std::fs::write(path, contents).await
+    }
+}
+
+mod private {
+    use super::*;
+
+    pub trait Sealed {}
+
+    #[cfg(feature = "tokio02")]
+    impl Sealed for Tokio02Executor {}
+
+    #[cfg(feature = "tokio1")]
+    impl Sealed for Tokio1Executor {}
+
+    #[cfg(feature = "async-std1")]
+    impl Sealed for AsyncStd1Executor {}
+}

src/lib.rs

@@ -4,33 +4,102 @@
 //! * Pluggable email transports
 //! * Unicode support
 //! * Secure defaults
+//! * Async support
 //!
 //! Lettre requires Rust 1.45 or newer.
 //!
-//! ## Optional features
+//! ## Features
+//!
+//! This section lists each lettre feature and briefly explains it.
+//! More info about each module can be found in the corresponding module page.
+//!
+//! Features with `📫` near them are enabled by default.
+//!
+//! ### Typed message builder
+//!
+//! _Strongly typed [`message`] builder_
+//!
+//! * **builder** 📫: Enable the [`Message`] builder
+//! * **hostname** 📫: Try to use the actual system hostname in the `Message-ID` header
+//!
+//! ### SMTP transport
+//!
+//! _Send emails using [`SMTP`]_
+//!
+//! * **smtp-transport** 📫: Enable the SMTP transport
+//! * **r2d2** 📫: Connection pool for SMTP transport
+//! * **hostname** 📫: Try to use the actual system hostname for the SMTP `CLIENTID`
+//!
+//! #### SMTP over TLS via the native-tls crate
+//!
+//! _Secure SMTP connections using TLS from the `native-tls` crate_
+//!
+//! Uses schannel on Windows, Security-Framework on macOS, and OpenSSL on Linux.
+//!
+//! * **native-tls** 📫: TLS support for the synchronous version of the API
+//! * **tokio02-native-tls**: TLS support for the `tokio02` async version of the API
+//! * **tokio1-native-tls**: TLS support for the `tokio1` async version of the API
+//!
+//! NOTE: native-tls isn't supported with `async-std`
+//!
+//! #### SMTP over TLS via the rustls crate
+//!
+//! _Secure SMTP connections using TLS from the `rustls-tls` crate_
+//!
+//! Rustls uses [ring] as the cryptography implementation. As a result, [not all Rust's targets are supported][ring-support].
+//!
+//! * **rustls-tls**: TLS support for the synchronous version of the API
+//! * **tokio02-rustls-tls**: TLS support for the `tokio02` async version of the API
+//! * **tokio1-rustls-tls**: TLS support for the `tokio1` async version of the API
+//! * **async-std1-rustls-tls**: TLS support for the `async-std1` async version of the API
+//!
+//! ### Sendmail transport
+//!
+//! _Send emails using the [`sendmail`] command_
+//!
+//! * **sendmail-transport**: Enable the `sendmail` transport
+//!
+//! ### File transport
+//!
+//! _Save emails as an `.eml` [`file`]_
+//!
+//! * **file-transport**: Enable the file transport (saves emails into an `.eml` file)
+//! * **file-transport-envelope**: Allow writing the envelope into a JSON file (additionally saves envelopes into a `.json` file)
+//!
+//! ### Async execution runtimes
+//!
+//! _Use [tokio] or [async-std] as an async execution runtime for sending emails_
+//!
+//! The correct runtime version must be chosen in order for lettre to work correctly.
+//! For example, when sending emails from a Tokio 1.3.0 context, the Tokio 1.x executor
+//! ([`Tokio1Executor`]) must be used. Using a different version (for example Tokio 0.2.x),
+//! or async-std, would result in a runtime panic.
+//!
+//! * **tokio02**: Allow to asynchronously send emails using [Tokio 0.2.x]
+//! * **tokio1**: Allow to asynchronously send emails using [Tokio 1.x]
+//! * **async-std1**: Allow to asynchronously send emails using [async-std 1.x]
+//!
+//! NOTE: native-tls isn't supported with `async-std`
+//!
+//! ### Misc features
+//!
+//! _Additional features_
 //!
-//! * **builder**: Message builder
-//! * **file-transport**: Transport that write messages into a file
-//! * **file-transport-envelope**: Allow writing the envelope into a JSON file
-//! * **smtp-transport**: Transport over SMTP
-//! * **sendmail-transport**: Transport over SMTP
-//! * **rustls-tls**: TLS support with the `rustls` crate
-//! * **native-tls**: TLS support with the `native-tls` crate
-//! * **tokio02**: Allow to asyncronously send emails using tokio 0.2.x
-//! * **tokio02-rustls-tls**: Async TLS support with the `rustls` crate using tokio 0.2
-//! * **tokio02-native-tls**: Async TLS support with the `native-tls` crate using tokio 0.2
-//! * **tokio1**: Allow to asyncronously send emails using tokio 1.x
-//! * **tokio1-rustls-tls**: Async TLS support with the `rustls` crate using tokio 1.x
-//! * **tokio1-native-tls**: Async TLS support with the `native-tls` crate using tokio 1.x
-//! * **async-std1**: Allow to asynchronously send emails using async-std 1.x
-//! * NOTE: native-tls isn't supported with async-std at the moment
-//! * **async-std1-rustls-tls**: Async TLS support with the `rustls` crate using async-std 1.x
-//! * **r2d2**: Connection pool for SMTP transport
-//! * **tracing**: Logging using the `tracing` crate
 //! * **serde**: Serialization/Deserialization of entities
-//! * **hostname**: Ability to try to use actual hostname in SMTP transaction
+//! * **tracing**: Logging using the `tracing` crate
+//!
+//! [`SMTP`]: crate::transport::smtp
+//! [`sendmail`]: crate::transport::sendmail
+//! [`file`]: crate::transport::file
+//! [tokio]: https://docs.rs/tokio/1
+//! [async-std]: https://docs.rs/async-std/1
+//! [ring]: https://github.com/briansmith/ring#ring
+//! [ring-support]: https://github.com/briansmith/ring#online-automated-testing
+//! [Tokio 0.2.x]: https://docs.rs/tokio/0.2
+//! [Tokio 1.x]: https://docs.rs/tokio/1
+//! [async-std 1.x]: https://docs.rs/async-std/1
 
-#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0-alpha.5")]
+#![doc(html_root_url = "https://docs.rs/crate/lettre/0.10.0-beta.3")]
 #![doc(html_favicon_url = "https://lettre.rs/favicon.ico")]
 #![doc(html_logo_url = "https://avatars0.githubusercontent.com/u/15113230?v=4")]
 #![forbid(unsafe_code)]
@@ -46,6 +115,8 @@
 
 pub mod address;
 pub mod error;
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
+mod executor;
 #[cfg(feature = "builder")]
 #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
 pub mod message;
@@ -55,117 +126,53 @@ pub mod transport;
 #[macro_use]
 extern crate hyperx;
 
+#[cfg(feature = "async-std1")]
+pub use self::executor::AsyncStd1Executor;
+#[cfg(all(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))]
+pub use self::executor::Executor;
+#[cfg(feature = "tokio02")]
+pub use self::executor::Tokio02Executor;
+#[cfg(feature = "tokio1")]
+pub use self::executor::Tokio1Executor;
+#[cfg(all(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))]
+#[doc(inline)]
+pub use self::transport::AsyncTransport;
 pub use crate::address::Address;
-use crate::address::Envelope;
-use crate::error::Error;
 #[cfg(feature = "builder")]
+#[doc(inline)]
 pub use crate::message::Message;
+#[cfg(all(
+    feature = "file-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+#[doc(inline)]
+pub use crate::transport::file::AsyncFileTransport;
 #[cfg(feature = "file-transport")]
+#[doc(inline)]
 pub use crate::transport::file::FileTransport;
+#[cfg(all(
+    feature = "sendmail-transport",
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
+))]
+#[doc(inline)]
+pub use crate::transport::sendmail::AsyncSendmailTransport;
 #[cfg(feature = "sendmail-transport")]
+#[doc(inline)]
 pub use crate::transport::sendmail::SendmailTransport;
 #[cfg(all(
     feature = "smtp-transport",
-    any(feature = "tokio02", feature = "tokio1")
+    any(feature = "tokio02", feature = "tokio1", feature = "async-std1")
 ))]
 pub use crate::transport::smtp::AsyncSmtpTransport;
-#[cfg(all(feature = "smtp-transport", feature = "async-std1"))]
-pub use crate::transport::smtp::AsyncStd1Connector;
+#[doc(inline)]
+pub use crate::transport::Transport;
+use crate::{address::Envelope, error::Error};
+
 #[cfg(feature = "smtp-transport")]
 pub use crate::transport::smtp::SmtpTransport;
-#[cfg(all(feature = "smtp-transport", feature = "tokio02"))]
-pub use crate::transport::smtp::Tokio02Connector;
-#[cfg(all(feature = "smtp-transport", feature = "tokio1"))]
-pub use crate::transport::smtp::Tokio1Connector;
-#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
-use async_trait::async_trait;
+use std::error::Error as StdError;
 
-/// Blocking Transport method for emails
-pub trait Transport {
-    /// Response produced by the Transport
-    type Ok;
-    /// Error produced by the Transport
-    type Error;
-
-    /// Sends the email
-    #[cfg(feature = "builder")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
-    fn send(&self, message: &Message) -> Result<Self::Ok, Self::Error> {
-        let raw = message.formatted();
-        self.send_raw(message.envelope(), &raw)
-    }
-
-    fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-}
-
-/// async-std 1.x based Transport method for emails
-#[cfg(feature = "async-std1")]
-#[cfg_attr(docsrs, doc(cfg(feature = "async-std1")))]
-#[async_trait]
-pub trait AsyncStd1Transport {
-    /// Response produced by the Transport
-    type Ok;
-    /// Error produced by the Transport
-    type Error;
-
-    /// Sends the email
-    #[cfg(feature = "builder")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
-    // TODO take &Message
-    async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
-        let raw = message.formatted();
-        let envelope = message.envelope();
-        self.send_raw(&envelope, &raw).await
-    }
-
-    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-}
-
-/// tokio 0.2.x based Transport method for emails
-#[cfg(feature = "tokio02")]
-#[cfg_attr(docsrs, doc(cfg(feature = "tokio02")))]
-#[async_trait]
-pub trait Tokio02Transport {
-    /// Response produced by the Transport
-    type Ok;
-    /// Error produced by the Transport
-    type Error;
-
-    /// Sends the email
-    #[cfg(feature = "builder")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
-    // TODO take &Message
-    async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
-        let raw = message.formatted();
-        let envelope = message.envelope();
-        self.send_raw(&envelope, &raw).await
-    }
-
-    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-}
-
-/// tokio 1.x based Transport method for emails
-#[cfg(feature = "tokio1")]
-#[cfg_attr(docsrs, doc(cfg(feature = "tokio1")))]
-#[async_trait]
-pub trait Tokio1Transport {
-    /// Response produced by the Transport
-    type Ok;
-    /// Error produced by the Transport
-    type Error;
-
-    /// Sends the email
-    #[cfg(feature = "builder")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
-    // TODO take &Message
-    async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
-        let raw = message.formatted();
-        let envelope = message.envelope();
-        self.send_raw(&envelope, &raw).await
-    }
-
-    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
-}
+pub(crate) type BoxError = Box<dyn StdError + Send + Sync>;
 
 #[cfg(test)]
 #[cfg(feature = "builder")]

src/message/body.rs

@@ -1,5 +1,7 @@
-use std::io::{self, Write};
-use std::ops::Deref;
+use std::{
+    io::{self, Write},
+    ops::Deref,
+};
 
 use crate::message::header::ContentTransferEncoding;
 

src/message/header/content.rs

@@ -7,7 +7,10 @@ use std::{
     str::{from_utf8, FromStr},
 };
 
-header! { (ContentId, "Content-ID") => [String] }
+header! {
+    /// `Content-Id` header, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-7)
+    (ContentId, "Content-ID") => [String]
+}
 
 /// `Content-Transfer-Encoding` of the body
 ///

src/message/header/mod.rs

@@ -1,8 +1,4 @@
-/*!
-
-## Headers widely used in email messages
-
-*/
+//! Headers widely used in email messages
 
 mod content;
 mod mailbox;

src/message/header/special.rs

@@ -5,6 +5,7 @@ use hyperx::{
 use std::{fmt::Result as FmtResult, str::from_utf8};
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+/// Message format version, defined in [RFC2045](https://tools.ietf.org/html/rfc2045#section-4)
 pub struct MimeVersion {
     pub major: u8,
     pub minor: u8,

src/message/header/textual.rs

@@ -6,8 +6,9 @@ use hyperx::{
 use std::{fmt::Result as FmtResult, str::from_utf8};
 
 macro_rules! text_header {
-    ( $type_name: ident, $header_name: expr ) => {
+    ($(#[$attr:meta])* Header($type_name: ident, $header_name: expr )) => {
         #[derive(Debug, Clone, PartialEq)]
+        $(#[$attr])*
         pub struct $type_name(pub String);
 
         impl Header for $type_name {
@@ -33,13 +34,41 @@ macro_rules! text_header {
     };
 }
 
-text_header!(Subject, "Subject");
-text_header!(Comments, "Comments");
-text_header!(Keywords, "Keywords");
-text_header!(InReplyTo, "In-Reply-To");
-text_header!(References, "References");
-text_header!(MessageId, "Message-Id");
-text_header!(UserAgent, "User-Agent");
+text_header!(
+    /// `Subject` of the message, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Subject, "Subject")
+);
+text_header!(
+    /// `Comments` of the message, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Comments, "Comments")
+);
+text_header!(
+    /// `Keywords` header. Should contain a comma-separated list of one or more
+    /// words or quoted-strings, defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.5)
+    Header(Keywords, "Keywords")
+);
+text_header!(
+    /// `In-Reply-To` header. Contains one or more
+    /// unique message identifiers,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(InReplyTo, "In-Reply-To")
+);
+text_header!(
+    /// `References` header. Contains one or more
+    /// unique message identifiers,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(References, "References")
+);
+text_header!(
+    /// `Message-Id` header. Contains a unique message identifier,
+    /// defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.4)
+    Header(MessageId, "Message-Id")
+);
+text_header!(
+    /// `User-Agent` header. Contains information about the client,
+    /// defined in [draft-melnikov-email-user-agent-00](https://tools.ietf.org/html/draft-melnikov-email-user-agent-00#section-3)
+    Header(UserAgent, "User-Agent")
+);
 
 fn parse_text(raw: &[u8]) -> HyperResult<String> {
     if let Ok(src) = from_utf8(raw) {

src/message/mailbox/types.rs

@@ -54,7 +54,7 @@ impl Mailbox {
     /// # Examples
     ///
     /// ```
-    /// use lettre::{Address, message::Mailbox};
+    /// use lettre::{message::Mailbox, Address};
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
@@ -163,7 +163,10 @@ impl Mailboxes {
     /// # Examples
     ///
     /// ```
-    /// use lettre::{Address, message::{Mailbox, Mailboxes}};
+    /// use lettre::{
+    ///     message::{Mailbox, Mailboxes},
+    ///     Address,
+    /// };
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
@@ -182,7 +185,10 @@ impl Mailboxes {
     /// # Examples
     ///
     /// ```
-    /// use lettre::{Address, message::{Mailbox, Mailboxes}};
+    /// use lettre::{
+    ///     message::{Mailbox, Mailboxes},
+    ///     Address,
+    /// };
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
@@ -201,7 +207,10 @@ impl Mailboxes {
     /// # Examples
     ///
     /// ```
-    /// use lettre::{Address, message::{Mailbox, Mailboxes}};
+    /// use lettre::{
+    ///     message::{Mailbox, Mailboxes},
+    ///     Address,
+    /// };
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {
@@ -225,7 +234,10 @@ impl Mailboxes {
     /// # Examples
     ///
     /// ```
-    /// use lettre::{Address, message::{Mailbox, Mailboxes}};
+    /// use lettre::{
+    ///     message::{Mailbox, Mailboxes},
+    ///     Address,
+    /// };
     ///
     /// # use std::error::Error;
     /// # fn main() -> Result<(), Box<dyn Error>> {

src/message/mimebody.rs

@@ -6,15 +6,12 @@ use mime::Mime;
 use rand::Rng;
 
 /// MIME part variants
-///
 #[derive(Debug, Clone)]
 pub enum Part {
     /// Single part with content
-    ///
     Single(SinglePart),
 
     /// Multiple parts of content
-    ///
     Multi(MultiPart),
 }
 
@@ -37,11 +34,9 @@ impl Part {
 }
 
 /// Parts of multipart body
-///
 pub type Parts = Vec<Part>;
 
 /// Creates builder for single part
-///
 #[derive(Debug, Clone)]
 pub struct SinglePartBuilder {
     headers: Headers,
@@ -92,17 +87,16 @@ impl Default for SinglePartBuilder {
 /// # Example
 ///
 /// ```
-/// use lettre::message::{SinglePart, header};
+/// use lettre::message::{header, SinglePart};
 ///
 /// # use std::error::Error;
 /// # fn main() -> Result<(), Box<dyn Error>> {
 /// let part = SinglePart::builder()
-///      .header(header::ContentType("text/plain; charset=utf8".parse()?))
-///      .body(String::from("Текст письма в уникоде"));
+///     .header(header::ContentType("text/plain; charset=utf8".parse()?))
+///     .body(String::from("Текст письма в уникоде"));
 /// # Ok(())
 /// # }
 /// ```
-///
 #[derive(Debug, Clone)]
 pub struct SinglePart {
     headers: Headers,
@@ -116,30 +110,6 @@ impl SinglePart {
         SinglePartBuilder::new()
     }
 
-    #[doc(hidden)]
-    #[deprecated = "Replaced by SinglePart::builder(), which chooses the best Content-Transfer-Encoding based on the provided body"]
-    pub fn seven_bit() -> SinglePartBuilder {
-        Self::builder().header(ContentTransferEncoding::SevenBit)
-    }
-
-    #[doc(hidden)]
-    #[deprecated = "Replaced by SinglePart::builder(), which chooses the best Content-Transfer-Encoding based on the provided body"]
-    pub fn quoted_printable() -> SinglePartBuilder {
-        Self::builder().header(ContentTransferEncoding::QuotedPrintable)
-    }
-
-    #[doc(hidden)]
-    #[deprecated = "Replaced by SinglePart::builder(), which chooses the best Content-Transfer-Encoding based on the provided body"]
-    pub fn base64() -> SinglePartBuilder {
-        Self::builder().header(ContentTransferEncoding::Base64)
-    }
-
-    #[doc(hidden)]
-    #[deprecated = "Replaced by SinglePart::builder(), which chooses the best Content-Transfer-Encoding based on the provided body"]
-    pub fn eight_bit() -> SinglePartBuilder {
-        Self::builder().header(ContentTransferEncoding::EightBit)
-    }
-
     /// Get the headers from singlepart
     #[inline]
     pub fn headers(&self) -> &Headers {
@@ -170,7 +140,6 @@ impl EmailFormat for SinglePart {
 }
 
 /// The kind of multipart
-///
 #[derive(Debug, Clone)]
 pub enum MultiPartKind {
     /// Mixed kind to combine unrelated content parts
@@ -208,20 +177,19 @@ impl MultiPartKind {
     fn to_mime<S: Into<String>>(&self, boundary: Option<S>) -> Mime {
         let boundary = boundary.map_or_else(make_boundary, |s| s.into());
 
-        use self::MultiPartKind::*;
         format!(
             "multipart/{}; boundary=\"{}\"{}",
             match self {
-                Mixed => "mixed",
-                Alternative => "alternative",
-                Related => "related",
-                Encrypted { .. } => "encrypted",
-                Signed { .. } => "signed",
+                Self::Mixed => "mixed",
+                Self::Alternative => "alternative",
+                Self::Related => "related",
+                Self::Encrypted { .. } => "encrypted",
+                Self::Signed { .. } => "signed",
             },
             boundary,
             match self {
-                Encrypted { protocol } => format!("; protocol=\"{}\"", protocol),
-                Signed { protocol, micalg } =>
+                Self::Encrypted { protocol } => format!("; protocol=\"{}\"", protocol),
+                Self::Signed { protocol, micalg } =>
                     format!("; protocol=\"{}\"; micalg=\"{}\"", protocol, micalg),
                 _ => String::new(),
             }
@@ -231,18 +199,17 @@ impl MultiPartKind {
     }
 
     fn from_mime(m: &Mime) -> Option<Self> {
-        use self::MultiPartKind::*;
         match m.subtype().as_ref() {
-            "mixed" => Some(Mixed),
-            "alternative" => Some(Alternative),
-            "related" => Some(Related),
+            "mixed" => Some(Self::Mixed),
+            "alternative" => Some(Self::Alternative),
+            "related" => Some(Self::Related),
             "signed" => m.get_param("protocol").and_then(|p| {
-                m.get_param("micalg").map(|micalg| Signed {
+                m.get_param("micalg").map(|micalg| Self::Signed {
                     protocol: p.as_str().to_owned(),
                     micalg: micalg.as_str().to_owned(),
                 })
             }),
-            "encrypted" => m.get_param("protocol").map(|p| Encrypted {
+            "encrypted" => m.get_param("protocol").map(|p| Self::Encrypted {
                 protocol: p.as_str().to_owned(),
             }),
             _ => None,
@@ -257,7 +224,6 @@ impl From<MultiPartKind> for Mime {
 }
 
 /// Multipart builder
-///
 #[derive(Debug, Clone)]
 pub struct MultiPartBuilder {
     headers: Headers,
@@ -323,7 +289,6 @@ impl Default for MultiPartBuilder {
 }
 
 /// Multipart variant with parts
-///
 #[derive(Debug, Clone)]
 pub struct MultiPart {
     headers: Headers,

src/message/mod.rs

@@ -114,7 +114,6 @@
 //!
 //! <p><b>Hello</b>, <i>world</i>! <img src="cid:123"></p>
 //! --0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1--
-//!
 //! ```
 //! </details>
 //!
@@ -125,8 +124,8 @@
 //!
 //! ```rust
 //! # use std::error::Error;
+//! use lettre::message::{header, Body, Message, MultiPart, Part, SinglePart};
 //! use std::fs;
-//! use lettre::message::{Body, header, Message, MultiPart, Part, SinglePart};
 //!
 //! # fn main() -> Result<(), Box<dyn Error>> {
 //! let image = fs::read("docs/lettre.png")?;
@@ -236,7 +235,6 @@
 //!
 //! fn main() { println!("Hello, World!") }
 //! --0oVZ2r6AoLAhLlb0gPNSKy6BEqdS2IfwxrcbUuo1--
-//!
 //! ```
 //! </details>
 
@@ -252,8 +250,7 @@ mod mailbox;
 mod mimebody;
 mod utf8_b;
 
-use std::convert::TryFrom;
-use std::time::SystemTime;
+use std::{convert::TryFrom, time::SystemTime};
 
 use uuid::Uuid;
 
@@ -265,7 +262,8 @@ use crate::{
 
 const DEFAULT_MESSAGE_ID_DOMAIN: &str = "localhost";
 
-pub trait EmailFormat {
+/// Something that can be formatted as an email message
+trait EmailFormat {
     // Use a writer?
     fn format(&self, out: &mut Vec<u8>);
 }
@@ -334,7 +332,7 @@ impl MessageBuilder {
 
     /// Set `Sender` header. Should be used when providing several `From` mailboxes.
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.header(header::Sender(mbox))`.
     pub fn sender(self, mbox: Mailbox) -> Self {
@@ -343,7 +341,7 @@ impl MessageBuilder {
 
     /// Set or add mailbox to `From` header
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.mailbox(header::From(mbox))`.
     pub fn from(self, mbox: Mailbox) -> Self {
@@ -352,7 +350,7 @@ impl MessageBuilder {
 
     /// Set or add mailbox to `ReplyTo` header
     ///
-    /// https://tools.ietf.org/html/rfc5322#section-3.6.2
+    /// Defined in [RFC5322](https://tools.ietf.org/html/rfc5322#section-3.6.2).
     ///
     /// Shortcut for `self.mailbox(header::ReplyTo(mbox))`.
     pub fn reply_to(self, mbox: Mailbox) -> Self {
@@ -494,6 +492,7 @@ impl MessageBuilder {
 }
 
 /// Email message which can be formatted
+#[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
 #[derive(Clone, Debug)]
 pub struct Message {
     headers: Headers,

src/transport/file/error.rs

@@ -1,61 +1,96 @@
 //! Error and result type for file transport
 
-use self::Error::*;
-use std::{
-    error::Error as StdError,
-    fmt::{self, Display, Formatter},
-    io,
-};
+use crate::BoxError;
+use std::{error::Error as StdError, fmt};
 
-/// An enum of all error kinds.
-#[derive(Debug)]
-pub enum Error {
-    /// Internal client error
-    Client(&'static str),
-    /// IO error
-    Io(io::Error),
-    /// JSON error
-    #[cfg(feature = "file-transport-envelope")]
-    Json(serde_json::Error),
+/// The Errors that may occur when sending an email over SMTP
+pub struct Error {
+    inner: Box<Inner>,
 }
 
-impl Display for Error {
-    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), fmt::Error> {
-        match *self {
-            Client(err) => fmt.write_str(err),
-            Io(ref err) => err.fmt(fmt),
-            #[cfg(feature = "file-transport-envelope")]
-            Json(ref err) => err.fmt(fmt),
+struct Inner {
+    kind: Kind,
+    source: Option<BoxError>,
+}
+
+impl Error {
+    pub(crate) fn new<E>(kind: Kind, source: Option<E>) -> Error
+    where
+        E: Into<BoxError>,
+    {
+        Error {
+            inner: Box::new(Inner {
+                kind,
+                source: source.map(Into::into),
+            }),
         }
     }
+
+    /// Returns true if the error is a file I/O error
+    pub fn is_io(&self) -> bool {
+        matches!(self.inner.kind, Kind::Io)
+    }
+
+    /// Returns true if the error is an envelope serialization or deserialization error
+    #[cfg(feature = "file-transport-envelope")]
+    pub fn is_envelope(&self) -> bool {
+        matches!(self.inner.kind, Kind::Envelope)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) enum Kind {
+    /// File I/O error
+    Io,
+    /// Envelope serialization/deserialization error
+    #[cfg(feature = "file-transport-envelope")]
+    Envelope,
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = f.debug_struct("lettre::transport::file::Error");
+
+        builder.field("kind", &self.inner.kind);
+
+        if let Some(ref source) = self.inner.source {
+            builder.field("source", source);
+        }
+
+        builder.finish()
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.inner.kind {
+            Kind::Io => f.write_str("response error")?,
+            #[cfg(feature = "file-transport-envelope")]
+            Kind::Envelope => f.write_str("internal client error")?,
+        };
+
+        if let Some(ref e) = self.inner.source {
+            write!(f, ": {}", e)?;
+        }
+
+        Ok(())
+    }
 }
 
 impl StdError for Error {
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
-        match *self {
-            Io(ref err) => Some(&*err),
-            #[cfg(feature = "file-transport-envelope")]
-            Json(ref err) => Some(&*err),
-            _ => None,
-        }
+        self.inner.source.as_ref().map(|e| {
+            let r: &(dyn std::error::Error + 'static) = &**e;
+            r
+        })
     }
 }
 
-impl From<io::Error> for Error {
-    fn from(err: io::Error) -> Error {
-        Error::Io(err)
-    }
+pub(crate) fn io<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Io, Some(e))
 }
 
 #[cfg(feature = "file-transport-envelope")]
-impl From<serde_json::Error> for Error {
-    fn from(err: serde_json::Error) -> Error {
-        Error::Json(err)
-    }
-}
-
-impl From<&'static str> for Error {
-    fn from(string: &'static str) -> Error {
-        Error::Client(string)
-    }
+pub(crate) fn envelope<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Envelope, Some(e))
 }

src/transport/file/mod.rs

@@ -6,11 +6,11 @@
 //!
 //! ```rust
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "file-transport", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
+//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
-//! use lettre::{Transport, Message, FileTransport};
 //!
 //! // Write to the local temp directory
 //! let sender = FileTransport::new(temp_dir());
@@ -38,11 +38,11 @@
 //!
 //! ```rust
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "file-transport-envelope", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
+//! use lettre::{FileTransport, Message, Transport};
 //! use std::env::temp_dir;
-//! use lettre::{Transport, Message, FileTransport};
 //!
 //! // Write to the local temp directory
 //! let sender = FileTransport::with_envelope(temp_dir());
@@ -66,14 +66,14 @@
 //!
 //! ```rust,no_run
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "tokio1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{Tokio1Transport, Message, FileTransport};
+//! use lettre::{AsyncTransport, Tokio1Executor, Message, AsyncFileTransport};
 //!
 //! // Write to the local temp directory
-//! let sender = FileTransport::new(temp_dir());
+//! let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
@@ -91,14 +91,14 @@
 //!
 //! ```rust,no_run
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "async-std1", feature = "file-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
 //! use std::env::temp_dir;
-//! use lettre::{AsyncStd1Transport, Message, FileTransport};
+//! use lettre::{AsyncTransport, AsyncStd1Executor, Message, AsyncFileTransport};
 //!
 //! // Write to the local temp directory
-//! let sender = FileTransport::new(temp_dir());
+//! let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
 //!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
@@ -133,16 +133,13 @@
 //! ```
 
 pub use self::error::Error;
-use crate::address::Envelope;
-#[cfg(feature = "async-std1")]
-use crate::AsyncStd1Transport;
-#[cfg(feature = "tokio02")]
-use crate::Tokio02Transport;
-#[cfg(feature = "tokio1")]
-use crate::Tokio1Transport;
-use crate::Transport;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+use crate::{AsyncTransport, Executor};
 #[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
 use async_trait::async_trait;
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+use std::marker::PhantomData;
 use std::{
     path::{Path, PathBuf},
     str,
@@ -156,12 +153,26 @@ type Id = String;
 /// Writes the content and the envelope information to a file
 #[derive(Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(docsrs, doc(cfg(feature = "file-transport")))]
 pub struct FileTransport {
     path: PathBuf,
     #[cfg(feature = "file-transport-envelope")]
     save_envelope: bool,
 }
 
+/// Asynchronously writes the content and the envelope information to a file
+#[derive(Debug)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+pub struct AsyncFileTransport<E: Executor> {
+    inner: FileTransport,
+    marker_: PhantomData<E>,
+}
+
 impl FileTransport {
     /// Creates a new transport to the given directory
     ///
@@ -187,11 +198,71 @@ impl FileTransport {
         }
     }
 
+    /// Read a message that was written using the file transport.
+    ///
+    /// Reads the envelope and the raw message content.
+    #[cfg(feature = "file-transport-envelope")]
+    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 = 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 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))
     }
 }
 
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+impl<E> AsyncFileTransport<E>
+where
+    E: Executor,
+{
+    /// Creates a new transport to the given directory
+    ///
+    /// Writes the email content in eml format.
+    pub fn new<P: AsRef<Path>>(path: P) -> Self {
+        Self {
+            inner: FileTransport::new(path),
+            marker_: PhantomData,
+        }
+    }
+
+    /// Creates a new transport to the given directory
+    ///
+    /// Writes the email content in eml format and the envelope
+    /// in json format.
+    #[cfg(feature = "file-transport-envelope")]
+    pub fn with_envelope<P: AsRef<Path>>(path: P) -> Self {
+        Self {
+            inner: FileTransport::with_envelope(path),
+            marker_: PhantomData,
+        }
+    }
+
+    /// Read a message that was written using the file transport.
+    ///
+    /// 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 = E::fs_read(&eml_file).await.map_err(error::io)?;
+
+        let json_file = self.inner.path.join(format!("{}.json", email_id));
+        let json = E::fs_read(&json_file).await.map_err(error::io)?;
+        let envelope = serde_json::from_slice(&json).map_err(error::envelope)?;
+
+        Ok((envelope, eml))
+    }
+}
+
 impl Transport for FileTransport {
     type Ok = Id;
     type Error = Error;
@@ -202,13 +273,14 @@ impl Transport for FileTransport {
         let email_id = Uuid::new_v4();
 
         let file = self.path(&email_id, "eml");
-        fs::write(file, email)?;
+        fs::write(file, email).map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]
         {
             if self.save_envelope {
                 let file = self.path(&email_id, "json");
-                fs::write(file, serde_json::to_string(&envelope)?)?;
+                let buf = serde_json::to_string(&envelope).map_err(error::envelope)?;
+                fs::write(file, buf).map_err(error::io)?;
             }
         }
         // use envelope anyway
@@ -218,80 +290,27 @@ impl Transport for FileTransport {
     }
 }
 
-#[cfg(feature = "async-std1")]
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
 #[async_trait]
-impl AsyncStd1Transport for FileTransport {
+impl<E> AsyncTransport for AsyncFileTransport<E>
+where
+    E: Executor,
+{
     type Ok = Id;
     type Error = Error;
 
     async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
-        use async_std::fs;
-
         let email_id = Uuid::new_v4();
 
-        let file = self.path(&email_id, "eml");
-        fs::write(file, email).await?;
+        let file = self.inner.path(&email_id, "eml");
+        E::fs_write(&file, email).await.map_err(error::io)?;
 
         #[cfg(feature = "file-transport-envelope")]
         {
-            if self.save_envelope {
-                let file = self.path(&email_id, "json");
-                fs::write(file, serde_json::to_string(&envelope)?).await?;
-            }
-        }
-        // use envelope anyway
-        let _ = envelope;
-
-        Ok(email_id.to_string())
-    }
-}
-
-#[cfg(feature = "tokio02")]
-#[async_trait]
-impl Tokio02Transport for FileTransport {
-    type Ok = Id;
-    type Error = Error;
-
-    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
-        use tokio02_crate::fs;
-
-        let email_id = Uuid::new_v4();
-        let file = self.path(&email_id, "eml");
-        fs::write(file, email).await?;
-
-        #[cfg(feature = "file-transport-envelope")]
-        {
-            if self.save_envelope {
-                let file = self.path(&email_id, "json");
-                fs::write(file, serde_json::to_string(&envelope)?).await?;
-            }
-        }
-        // use envelope anyway
-        let _ = envelope;
-
-        Ok(email_id.to_string())
-    }
-}
-
-#[cfg(feature = "tokio1")]
-#[async_trait]
-impl Tokio1Transport for FileTransport {
-    type Ok = Id;
-    type Error = Error;
-
-    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
-        use tokio1_crate::fs;
-
-        let email_id = Uuid::new_v4();
-
-        let file = self.path(&email_id, "eml");
-        fs::write(file, email).await?;
-
-        #[cfg(feature = "file-transport-envelope")]
-        {
-            if self.save_envelope {
-                let file = self.path(&email_id, "json");
-                fs::write(file, serde_json::to_string(&envelope)?).await?;
+            if self.inner.save_envelope {
+                let file = self.inner.path(&email_id, "json");
+                let buf = serde_json::to_vec(&envelope).map_err(error::envelope)?;
+                E::fs_write(&file, &buf).await.map_err(error::io)?;
             }
         }
         // use envelope anyway

src/transport/mod.rs

@@ -1,20 +1,84 @@
-//! ### Sending Messages
+//! ## Transports for sending emails
 //!
-//! This section explains how to manipulate emails you have created.
-//!
-//! This mailer contains several different transports for your emails. To be sendable, the
-//! emails have to implement `Email`, which is the case for emails created with `lettre::builder`.
+//! This module contains `Transport`s for sending emails. A `Transport` implements a high-level API
+//! for sending emails. It automatically manages the underlying resources and doesn't require any
+//! specific knowledge of email protocols in order to be used.
 //!
 //! The following transports are available:
 //!
-//! * The `SmtpTransport` uses the SMTP protocol to send the message over the network. It is
-//!   the preferred way of sending emails.
-//! * The `SendmailTransport` uses the sendmail command to send messages. It is an alternative to
-//!   the SMTP transport.
-//! * The `FileTransport` creates a file containing the email content to be sent. It can be used
-//!   for debugging or if you want to keep all sent emails.
-//! * The `StubTransport` is useful for debugging, and only prints the content of the email in the
-//!   logs.
+//! | Module       | Protocol | Sync API              | Async API                  | Description                                             |
+//! | ------------ | -------- | --------------------- | -------------------------- | ------------------------------------------------------- |
+//! | [`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                  |
+//!
+//! ## Building an email
+//!
+//! Emails can either be built though [`Message`], which is a typed API for constructing emails
+//! (find out more about it by going over the [`message`][crate::message] module),
+//! or via external means.
+//!
+//! [`Message`]s can be sent via [`Transport::send`] or [`AsyncTransport::send`], while messages
+//! built without lettre's [`message`][crate::message] APIs can be sent via [`Transport::send_raw`]
+//! or [`AsyncTransport::send_raw`].
+//!
+//! ## Brief example
+//!
+//! This example shows how to build an email and send it via an SMTP relay server.
+//! It is in no way a complete example, but it shows how to get started with lettre.
+//! More examples can be found by looking at the specific modules, linked in the _Module_ column
+//! of the [table above](#transports-for-sending-emails).
+//!
+//! ```rust,no_run
+//! # use std::error::Error;
+//! #
+//! # #[cfg(all(feature = "builder", feature = "smtp-transport"))]
+//! # fn main() -> Result<(), Box<dyn Error>> {
+//! use lettre::transport::smtp::authentication::Credentials;
+//! use lettre::{Message, SmtpTransport, Transport};
+//!
+//! let email = Message::builder()
+//!     .from("NoBody <nobody@domain.tld>".parse()?)
+//!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
+//!     .to("Hei <hei@domain.tld>".parse()?)
+//!     .subject("Happy new year")
+//!     .body(String::from("Be happy!"))?;
+//!
+//! let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+//!
+//! // Open a remote connection to the SMTP relay server
+//! let mailer = SmtpTransport::relay("smtp.gmail.com")?
+//!     .credentials(creds)
+//!     .build();
+//!
+//! // Send the email
+//! match mailer.send(&email) {
+//!     Ok(_) => println!("Email sent successfully!"),
+//!     Err(e) => panic!("Could not send email: {:?}", e),
+//! }
+//! # Ok(())
+//! # }
+//! # #[cfg(not(all(feature = "builder", feature = "smtp-transport")))]
+//! # fn main() {}
+//! ```
+//!
+//! [`Message`]: crate::Message
+//! [`file`]: self::file
+//! [`SmtpTransport`]: crate::SmtpTransport
+//! [`AsyncSmtpTransport`]: crate::AsyncSmtpTransport
+//! [`SendmailTransport`]: crate::SendmailTransport
+//! [`AsyncSendmailTransport`]: crate::AsyncSendmailTransport
+//! [`FileTransport`]: crate::FileTransport
+//! [`AsyncFileTransport`]: crate::AsyncFileTransport
+//! [`StubTransport`]: crate::transport::stub::StubTransport
+
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+use async_trait::async_trait;
+
+use crate::Envelope;
+#[cfg(feature = "builder")]
+use crate::Message;
 
 #[cfg(feature = "file-transport")]
 #[cfg_attr(docsrs, doc(cfg(feature = "file-transport")))]
@@ -26,3 +90,47 @@ pub mod sendmail;
 #[cfg_attr(docsrs, doc(cfg(feature = "smtp-transport")))]
 pub mod smtp;
 pub mod stub;
+
+/// Blocking Transport method for emails
+pub trait Transport {
+    /// Response produced by the Transport
+    type Ok;
+    /// Error produced by the Transport
+    type Error;
+
+    /// Sends the email
+    #[cfg(feature = "builder")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
+    fn send(&self, message: &Message) -> Result<Self::Ok, Self::Error> {
+        let raw = message.formatted();
+        self.send_raw(message.envelope(), &raw)
+    }
+
+    fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
+}
+
+/// Async Transport method for emails
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
+#[async_trait]
+pub trait AsyncTransport {
+    /// Response produced by the Transport
+    type Ok;
+    /// Error produced by the Transport
+    type Error;
+
+    /// Sends the email
+    #[cfg(feature = "builder")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "builder")))]
+    // TODO take &Message
+    async fn send(&self, message: Message) -> Result<Self::Ok, Self::Error> {
+        let raw = message.formatted();
+        let envelope = message.envelope();
+        self.send_raw(&envelope, &raw).await
+    }
+
+    async fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error>;
+}

src/transport/sendmail/error.rs

@@ -1,52 +1,92 @@
 //! Error and result type for sendmail transport
 
-use self::Error::*;
-use std::{
-    error::Error as StdError,
-    fmt::{self, Display, Formatter},
-    io,
-    string::FromUtf8Error,
-};
+use crate::BoxError;
+use std::{error::Error as StdError, fmt};
 
-/// An enum of all error kinds.
-#[derive(Debug)]
-pub enum Error {
-    /// Internal client error
-    Client(String),
-    /// Error parsing UTF8 in response
-    Utf8Parsing(FromUtf8Error),
-    /// IO error
-    Io(io::Error),
+/// The Errors that may occur when sending an email over sendmail
+pub struct Error {
+    inner: Box<Inner>,
 }
 
-impl Display for Error {
-    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), fmt::Error> {
-        match *self {
-            Client(ref err) => err.fmt(fmt),
-            Utf8Parsing(ref err) => err.fmt(fmt),
-            Io(ref err) => err.fmt(fmt),
+struct Inner {
+    kind: Kind,
+    source: Option<BoxError>,
+}
+
+impl Error {
+    pub(crate) fn new<E>(kind: Kind, source: Option<E>) -> Error
+    where
+        E: Into<BoxError>,
+    {
+        Error {
+            inner: Box::new(Inner {
+                kind,
+                source: source.map(Into::into),
+            }),
         }
     }
+
+    /// Returns true if the error is from client
+    pub fn is_client(&self) -> bool {
+        matches!(self.inner.kind, Kind::Client)
+    }
+
+    /// Returns true if the error comes from the response
+    pub fn is_response(&self) -> bool {
+        matches!(self.inner.kind, Kind::Response)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) enum Kind {
+    /// Error parsing a response
+    Response,
+    /// Internal client error
+    Client,
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = f.debug_struct("lettre::transport::sendmail::Error");
+
+        builder.field("kind", &self.inner.kind);
+
+        if let Some(ref source) = self.inner.source {
+            builder.field("source", source);
+        }
+
+        builder.finish()
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.inner.kind {
+            Kind::Response => f.write_str("response error")?,
+            Kind::Client => f.write_str("internal client error")?,
+        };
+
+        if let Some(ref e) = self.inner.source {
+            write!(f, ": {}", e)?;
+        }
+
+        Ok(())
+    }
 }
 
 impl StdError for Error {
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
-        match *self {
-            Io(ref err) => Some(&*err),
-            Utf8Parsing(ref err) => Some(&*err),
-            _ => None,
-        }
+        self.inner.source.as_ref().map(|e| {
+            let r: &(dyn std::error::Error + 'static) = &**e;
+            r
+        })
     }
 }
 
-impl From<io::Error> for Error {
-    fn from(err: io::Error) -> Error {
-        Error::Io(err)
-    }
+pub(crate) fn response<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Response, Some(e))
 }
 
-impl From<FromUtf8Error> for Error {
-    fn from(err: FromUtf8Error) -> Error {
-        Utf8Parsing(err)
-    }
+pub(crate) fn client<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Client, Some(e))
 }

src/transport/sendmail/mod.rs

@@ -1,13 +1,13 @@
-//! The sendmail transport sends the email using the local sendmail command.
+//! The sendmail transport sends the email using the local `sendmail` command.
 //!
 //! ## Sync example
 //!
 //! ```rust
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "sendmail-transport", feature = "builder"))]
 //! # fn main() -> Result<(), Box<dyn Error>> {
-//! # use lettre::{Message, Transport, SendmailTransport};
+//! use lettre::{Message, SendmailTransport, Transport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -30,10 +30,10 @@
 //!
 //! ```rust,no_run
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "tokio02", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, Tokio02Transport, SendmailTransport};
+//! use lettre::{Message, AsyncTransport, Tokio02Executor, AsyncSendmailTransport, SendmailTransport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -42,7 +42,7 @@
 //!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let sender = SendmailTransport::new();
+//! let sender = AsyncSendmailTransport::<Tokio02Executor>::new();
 //! let result = sender.send(email).await;
 //! assert!(result.is_ok());
 //! # Ok(())
@@ -53,10 +53,10 @@
 //!
 //! ```rust,no_run
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "tokio1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, Tokio1Transport, SendmailTransport};
+//! use lettre::{Message, AsyncTransport, Tokio1Executor, AsyncSendmailTransport, SendmailTransport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -65,7 +65,7 @@
 //!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let sender = SendmailTransport::new();
+//! let sender = AsyncSendmailTransport::<Tokio1Executor>::new();
 //! let result = sender.send(email).await;
 //! assert!(result.is_ok());
 //! # Ok(())
@@ -76,10 +76,10 @@
 //!
 //!```rust,no_run
 //! # use std::error::Error;
-//!
+//! #
 //! # #[cfg(all(feature = "async-std1", feature = "sendmail-transport", feature = "builder"))]
 //! # async fn run() -> Result<(), Box<dyn Error>> {
-//! use lettre::{Message, AsyncStd1Transport, SendmailTransport};
+//! use lettre::{Message, AsyncTransport, AsyncStd1Executor, AsyncSendmailTransport};
 //!
 //! let email = Message::builder()
 //!     .from("NoBody <nobody@domain.tld>".parse()?)
@@ -88,7 +88,7 @@
 //!     .subject("Happy new year")
 //!     .body(String::from("Be happy!"))?;
 //!
-//! let sender = SendmailTransport::new();
+//! let sender = AsyncSendmailTransport::<AsyncStd1Executor>::new();
 //! let result = sender.send(email).await;
 //! assert!(result.is_ok());
 //! # Ok(())
@@ -96,38 +96,55 @@
 //! ```
 
 pub use self::error::Error;
-use crate::address::Envelope;
 #[cfg(feature = "async-std1")]
-use crate::AsyncStd1Transport;
+use crate::AsyncStd1Executor;
 #[cfg(feature = "tokio02")]
-use crate::Tokio02Transport;
+use crate::Tokio02Executor;
 #[cfg(feature = "tokio1")]
-use crate::Tokio1Transport;
-use crate::Transport;
+use crate::Tokio1Executor;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+use crate::{AsyncTransport, Executor};
 #[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
 use async_trait::async_trait;
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+use std::marker::PhantomData;
 use std::{
     ffi::OsString,
-    io::prelude::*,
+    io::Write,
     process::{Command, Stdio},
 };
 
 mod error;
 
-const DEFAUT_SENDMAIL: &str = "/usr/sbin/sendmail";
+const DEFAULT_SENDMAIL: &str = "/usr/sbin/sendmail";
 
-/// Sends an email using the `sendmail` command
+/// Sends emails using the `sendmail` command
 #[derive(Debug, Clone)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(docsrs, doc(cfg(feature = "sendmail-transport")))]
 pub struct SendmailTransport {
     command: OsString,
 }
 
+/// Asynchronously sends emails using the `sendmail` command
+#[derive(Debug, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
+pub struct AsyncSendmailTransport<E: Executor> {
+    inner: SendmailTransport,
+    marker_: PhantomData<E>,
+}
+
 impl SendmailTransport {
     /// Creates a new transport with the default `/usr/sbin/sendmail` command
     pub fn new() -> SendmailTransport {
         SendmailTransport {
-            command: DEFAUT_SENDMAIL.into(),
+            command: DEFAULT_SENDMAIL.into(),
         }
     }
 
@@ -151,12 +168,34 @@ impl SendmailTransport {
             .stderr(Stdio::piped());
         c
     }
+}
+
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+impl<E> AsyncSendmailTransport<E>
+where
+    E: Executor,
+{
+    /// Creates a new transport with the default `/usr/sbin/sendmail` command
+    pub fn new() -> Self {
+        Self {
+            inner: SendmailTransport::new(),
+            marker_: PhantomData,
+        }
+    }
+
+    /// Creates a new transport to the given sendmail command
+    pub fn new_with_command<S: Into<OsString>>(command: S) -> Self {
+        Self {
+            inner: SendmailTransport::new_with_command(command),
+            marker_: PhantomData,
+        }
+    }
 
     #[cfg(feature = "tokio02")]
     fn tokio02_command(&self, envelope: &Envelope) -> tokio02_crate::process::Command {
         use tokio02_crate::process::Command;
 
-        let mut c = Command::new(&self.command);
+        let mut c = Command::new(&self.inner.command);
         c.kill_on_drop(true);
         c.arg("-i");
         if let Some(from) = envelope.from() {
@@ -174,7 +213,7 @@ impl SendmailTransport {
     fn tokio1_command(&self, envelope: &Envelope) -> tokio1_crate::process::Command {
         use tokio1_crate::process::Command;
 
-        let mut c = Command::new(&self.command);
+        let mut c = Command::new(&self.inner.command);
         c.kill_on_drop(true);
         c.arg("-i");
         if let Some(from) = envelope.from() {
@@ -192,7 +231,7 @@ impl SendmailTransport {
     fn async_std_command(&self, envelope: &Envelope) -> async_std::process::Command {
         use async_std::process::Command;
 
-        let mut c = Command::new(&self.command);
+        let mut c = Command::new(&self.inner.command);
         // TODO: figure out why enabling this kills it earlier
         // c.kill_on_drop(true);
         c.arg("-i");
@@ -214,28 +253,44 @@ impl Default for SendmailTransport {
     }
 }
 
+#[cfg(any(feature = "async-std1", feature = "tokio02", feature = "tokio1"))]
+impl<E> Default for AsyncSendmailTransport<E>
+where
+    E: Executor,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Transport for SendmailTransport {
     type Ok = ();
     type Error = Error;
 
     fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         // Spawn the sendmail command
-        let mut process = self.command(envelope).spawn()?;
+        let mut process = self.command(envelope).spawn().map_err(error::client)?;
 
-        process.stdin.as_mut().unwrap().write_all(email)?;
-        let output = process.wait_with_output()?;
+        process
+            .stdin
+            .as_mut()
+            .unwrap()
+            .write_all(email)
+            .map_err(error::client)?;
+        let output = process.wait_with_output().map_err(error::client)?;
 
         if output.status.success() {
             Ok(())
         } else {
-            Err(error::Error::Client(String::from_utf8(output.stderr)?))
+            let stderr = String::from_utf8(output.stderr).map_err(error::response)?;
+            Err(error::client(stderr))
         }
     }
 }
 
 #[cfg(feature = "async-std1")]
 #[async_trait]
-impl AsyncStd1Transport for SendmailTransport {
+impl AsyncTransport for AsyncSendmailTransport<AsyncStd1Executor> {
     type Ok = ();
     type Error = Error;
 
@@ -245,22 +300,29 @@ impl AsyncStd1Transport for SendmailTransport {
         let mut command = self.async_std_command(envelope);
 
         // Spawn the sendmail command
-        let mut process = command.spawn()?;
+        let mut process = command.spawn().map_err(error::client)?;
 
-        process.stdin.as_mut().unwrap().write_all(&email).await?;
-        let output = process.output().await?;
+        process
+            .stdin
+            .as_mut()
+            .unwrap()
+            .write_all(&email)
+            .await
+            .map_err(error::client)?;
+        let output = process.output().await.map_err(error::client)?;
 
         if output.status.success() {
             Ok(())
         } else {
-            Err(Error::Client(String::from_utf8(output.stderr)?))
+            let stderr = String::from_utf8(output.stderr).map_err(error::response)?;
+            Err(error::client(stderr))
         }
     }
 }
 
 #[cfg(feature = "tokio02")]
 #[async_trait]
-impl Tokio02Transport for SendmailTransport {
+impl AsyncTransport for AsyncSendmailTransport<Tokio02Executor> {
     type Ok = ();
     type Error = Error;
 
@@ -270,22 +332,29 @@ impl Tokio02Transport for SendmailTransport {
         let mut command = self.tokio02_command(envelope);
 
         // Spawn the sendmail command
-        let mut process = command.spawn()?;
+        let mut process = command.spawn().map_err(error::client)?;
 
-        process.stdin.as_mut().unwrap().write_all(&email).await?;
-        let output = process.wait_with_output().await?;
+        process
+            .stdin
+            .as_mut()
+            .unwrap()
+            .write_all(&email)
+            .await
+            .map_err(error::client)?;
+        let output = process.wait_with_output().await.map_err(error::client)?;
 
         if output.status.success() {
             Ok(())
         } else {
-            Err(Error::Client(String::from_utf8(output.stderr)?))
+            let stderr = String::from_utf8(output.stderr).map_err(error::response)?;
+            Err(error::client(stderr))
         }
     }
 }
 
 #[cfg(feature = "tokio1")]
 #[async_trait]
-impl Tokio1Transport for SendmailTransport {
+impl AsyncTransport for AsyncSendmailTransport<Tokio1Executor> {
     type Ok = ();
     type Error = Error;
 
@@ -295,15 +364,22 @@ impl Tokio1Transport for SendmailTransport {
         let mut command = self.tokio1_command(envelope);
 
         // Spawn the sendmail command
-        let mut process = command.spawn()?;
+        let mut process = command.spawn().map_err(error::client)?;
 
-        process.stdin.as_mut().unwrap().write_all(&email).await?;
-        let output = process.wait_with_output().await?;
+        process
+            .stdin
+            .as_mut()
+            .unwrap()
+            .write_all(&email)
+            .await
+            .map_err(error::client)?;
+        let output = process.wait_with_output().await.map_err(error::client)?;
 
         if output.status.success() {
             Ok(())
         } else {
-            Err(Error::Client(String::from_utf8(output.stderr)?))
+            let stderr = String::from_utf8(output.stderr).map_err(error::response)?;
+            Err(error::client(stderr))
         }
     }
 }

src/transport/smtp/async_transport.rs

@@ -1,28 +1,34 @@
+use std::fmt::{self, Debug};
+use std::marker::PhantomData;
+
 use async_trait::async_trait;
 
-#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
-use super::Tls;
 use super::{
     client::AsyncSmtpConnection, ClientId, Credentials, Error, Mechanism, Response, SmtpInfo,
 };
 #[cfg(feature = "async-std1")]
-use crate::AsyncStd1Transport;
-use crate::Envelope;
+use crate::AsyncStd1Executor;
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
+use crate::AsyncTransport;
 #[cfg(feature = "tokio02")]
-use crate::Tokio02Transport;
+use crate::Tokio02Executor;
 #[cfg(feature = "tokio1")]
-use crate::Tokio1Transport;
+use crate::Tokio1Executor;
+use crate::{Envelope, Executor};
 
-#[allow(missing_debug_implementations)]
-#[derive(Clone)]
-pub struct AsyncSmtpTransport<C> {
+/// Asynchronously sends emails using the SMTP protocol
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
+pub struct AsyncSmtpTransport<E> {
     // TODO: pool
-    inner: AsyncSmtpClient<C>,
+    inner: AsyncSmtpClient<E>,
 }
 
 #[cfg(feature = "tokio02")]
 #[async_trait]
-impl Tokio02Transport for AsyncSmtpTransport<Tokio02Connector> {
+impl AsyncTransport for AsyncSmtpTransport<Tokio02Executor> {
     type Ok = Response;
     type Error = Error;
 
@@ -40,7 +46,7 @@ impl Tokio02Transport for AsyncSmtpTransport<Tokio02Connector> {
 
 #[cfg(feature = "tokio1")]
 #[async_trait]
-impl Tokio1Transport for AsyncSmtpTransport<Tokio1Connector> {
+impl AsyncTransport for AsyncSmtpTransport<Tokio1Executor> {
     type Ok = Response;
     type Error = Error;
 
@@ -58,7 +64,7 @@ impl Tokio1Transport for AsyncSmtpTransport<Tokio1Connector> {
 
 #[cfg(feature = "async-std1")]
 #[async_trait]
-impl AsyncStd1Transport for AsyncSmtpTransport<AsyncStd1Connector> {
+impl AsyncTransport for AsyncSmtpTransport<AsyncStd1Executor> {
     type Ok = Response;
     type Error = Error;
 
@@ -74,9 +80,9 @@ impl AsyncStd1Transport for AsyncSmtpTransport<AsyncStd1Connector> {
     }
 }
 
-impl<C> AsyncSmtpTransport<C>
+impl<E> AsyncSmtpTransport<E>
 where
-    C: AsyncSmtpConnector,
+    E: Executor,
 {
     /// Simple and secure transport, using TLS connections to communicate with the SMTP server
     ///
@@ -92,8 +98,18 @@ where
         feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(
+            feature = "tokio02-native-tls",
+            feature = "tokio02-rustls-tls",
+            feature = "tokio1-native-tls",
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
+        )))
+    )]
     pub fn relay(relay: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
-        use super::{TlsParameters, SUBMISSIONS_PORT};
+        use super::{Tls, TlsParameters, SUBMISSIONS_PORT};
 
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -121,8 +137,18 @@ where
         feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(
+            feature = "tokio02-native-tls",
+            feature = "tokio02-rustls-tls",
+            feature = "tokio1-native-tls",
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
+        )))
+    )]
     pub fn starttls_relay(relay: &str) -> Result<AsyncSmtpTransportBuilder, Error> {
-        use super::{TlsParameters, SUBMISSION_PORT};
+        use super::{Tls, TlsParameters, SUBMISSION_PORT};
 
         let tls_parameters = TlsParameters::new(relay.into())?;
 
@@ -134,7 +160,7 @@ where
     /// Creates a new local SMTP client to port 25
     ///
     /// Shortcut for local unencrypted relay (typical local email daemon that will handle relaying)
-    pub fn unencrypted_localhost() -> AsyncSmtpTransport<C> {
+    pub fn unencrypted_localhost() -> AsyncSmtpTransport<E> {
         Self::builder_dangerous("localhost").build()
     }
 
@@ -158,10 +184,32 @@ where
     }
 }
 
+impl<E> Debug for AsyncSmtpTransport<E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = f.debug_struct("AsyncSmtpTransport");
+        builder.field("inner", &self.inner);
+        builder.finish()
+    }
+}
+
+impl<E> Clone for AsyncSmtpTransport<E>
+where
+    E: Executor,
+{
+    fn clone(&self) -> Self {
+        Self {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
 /// Contains client configuration.
 /// Instances of this struct can be created using functions of [`AsyncSmtpTransport`].
-#[allow(missing_debug_implementations)]
-#[derive(Clone)]
+#[derive(Debug, Clone)]
+#[cfg_attr(
+    docsrs,
+    doc(cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1")))
+)]
 pub struct AsyncSmtpTransportBuilder {
     info: SmtpInfo,
 }
@@ -201,41 +249,49 @@ impl AsyncSmtpTransportBuilder {
         feature = "async-std1-native-tls",
         feature = "async-std1-rustls-tls"
     ))]
-    pub fn tls(mut self, tls: Tls) -> Self {
+    #[cfg_attr(
+        docsrs,
+        doc(cfg(any(
+            feature = "tokio02-native-tls",
+            feature = "tokio02-rustls-tls",
+            feature = "tokio1-native-tls",
+            feature = "tokio1-rustls-tls",
+            feature = "async-std1-rustls-tls"
+        )))
+    )]
+    pub fn tls(mut self, tls: super::Tls) -> Self {
         self.info.tls = tls;
         self
     }
 
-    /// Build the transport (with default pool if enabled)
-    pub fn build<C>(self) -> AsyncSmtpTransport<C>
+    /// Build the transport
+    pub fn build<E>(self) -> AsyncSmtpTransport<E>
     where
-        C: AsyncSmtpConnector,
+        E: Executor,
     {
-        let connector = Default::default();
         let client = AsyncSmtpClient {
-            connector,
             info: self.info,
+            marker_: PhantomData,
         };
         AsyncSmtpTransport { inner: client }
     }
 }
 
 /// Build client
-#[derive(Clone)]
-pub struct AsyncSmtpClient<C> {
-    connector: C,
+pub struct AsyncSmtpClient<E> {
     info: SmtpInfo,
+    marker_: PhantomData<E>,
 }
 
-impl<C> AsyncSmtpClient<C>
+impl<E> AsyncSmtpClient<E>
 where
-    C: AsyncSmtpConnector,
+    E: Executor,
 {
     /// Creates a new connection directly usable to send emails
     ///
     /// Handles encryption and authentication
     pub async fn connection(&self) -> Result<AsyncSmtpConnection, Error> {
-        let mut conn = C::connect(
+        let mut conn = E::connect(
             &self.info.server,
             self.info.port,
             &self.info.hello_name,
@@ -250,152 +306,22 @@ where
     }
 }
 
-#[async_trait]
-pub trait AsyncSmtpConnector: Default + private::Sealed {
-    async fn connect(
-        hostname: &str,
-        port: u16,
-        hello_name: &ClientId,
-        tls: &Tls,
-    ) -> Result<AsyncSmtpConnection, Error>;
-}
-
-#[derive(Debug, Copy, Clone, Default)]
-#[cfg(feature = "tokio02")]
-#[cfg_attr(docsrs, doc(cfg(feature = "tokio02")))]
-pub struct Tokio02Connector;
-
-#[async_trait]
-#[cfg(feature = "tokio02")]
-impl AsyncSmtpConnector for Tokio02Connector {
-    async fn connect(
-        hostname: &str,
-        port: u16,
-        hello_name: &ClientId,
-        tls: &Tls,
-    ) -> Result<AsyncSmtpConnection, Error> {
-        #[allow(clippy::match_single_binding)]
-        let tls_parameters = match tls {
-            #[cfg(any(feature = "tokio02-native-tls", feature = "tokio02-rustls-tls"))]
-            Tls::Wrapper(ref tls_parameters) => Some(tls_parameters.clone()),
-            _ => None,
-        };
-        #[allow(unused_mut)]
-        let mut conn =
-            AsyncSmtpConnection::connect_tokio02(hostname, port, hello_name, tls_parameters)
-                .await?;
-
-        #[cfg(any(feature = "tokio02-native-tls", feature = "tokio02-rustls-tls"))]
-        match tls {
-            Tls::Opportunistic(ref tls_parameters) => {
-                if conn.can_starttls() {
-                    conn.starttls(tls_parameters.clone(), hello_name).await?;
-                }
-            }
-            Tls::Required(ref tls_parameters) => {
-                conn.starttls(tls_parameters.clone(), hello_name).await?;
-            }
-            _ => (),
-        }
-
-        Ok(conn)
+impl<E> Debug for AsyncSmtpClient<E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = f.debug_struct("AsyncSmtpClient");
+        builder.field("info", &self.info);
+        builder.finish()
     }
 }
 
-#[derive(Debug, Copy, Clone, Default)]
-#[cfg(feature = "tokio1")]
-#[cfg_attr(docsrs, doc(cfg(feature = "tokio1")))]
-pub struct Tokio1Connector;
-
-#[async_trait]
-#[cfg(feature = "tokio1")]
-impl AsyncSmtpConnector for Tokio1Connector {
-    async fn connect(
-        hostname: &str,
-        port: u16,
-        hello_name: &ClientId,
-        tls: &Tls,
-    ) -> Result<AsyncSmtpConnection, Error> {
-        #[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()),
-            _ => None,
-        };
-        #[allow(unused_mut)]
-        let mut conn =
-            AsyncSmtpConnection::connect_tokio1(hostname, port, hello_name, tls_parameters).await?;
-
-        #[cfg(any(feature = "tokio1-native-tls", feature = "tokio1-rustls-tls"))]
-        match tls {
-            Tls::Opportunistic(ref tls_parameters) => {
-                if conn.can_starttls() {
-                    conn.starttls(tls_parameters.clone(), hello_name).await?;
-                }
-            }
-            Tls::Required(ref tls_parameters) => {
-                conn.starttls(tls_parameters.clone(), hello_name).await?;
-            }
-            _ => (),
+impl<E> AsyncSmtpClient<E>
+where
+    E: Executor,
+{
+    fn clone(&self) -> Self {
+        Self {
+            info: self.info.clone(),
+            marker_: PhantomData,
         }
-
-        Ok(conn)
     }
 }
-
-#[derive(Debug, Copy, Clone, Default)]
-#[cfg(feature = "async-std1")]
-#[cfg_attr(docsrs, doc(cfg(feature = "async-std1")))]
-pub struct AsyncStd1Connector;
-
-#[async_trait]
-#[cfg(feature = "async-std1")]
-impl AsyncSmtpConnector for AsyncStd1Connector {
-    async fn connect(
-        hostname: &str,
-        port: u16,
-        hello_name: &ClientId,
-        tls: &Tls,
-    ) -> 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()),
-            _ => None,
-        };
-        #[allow(unused_mut)]
-        let mut conn =
-            AsyncSmtpConnection::connect_asyncstd1(hostname, port, hello_name, tls_parameters)
-                .await?;
-
-        #[cfg(any(feature = "async-std1-native-tls", feature = "async-std1-rustls-tls"))]
-        match tls {
-            Tls::Opportunistic(ref tls_parameters) => {
-                if conn.can_starttls() {
-                    conn.starttls(tls_parameters.clone(), hello_name).await?;
-                }
-            }
-            Tls::Required(ref tls_parameters) => {
-                conn.starttls(tls_parameters.clone(), hello_name).await?;
-            }
-            _ => (),
-        }
-
-        Ok(conn)
-    }
-}
-
-mod private {
-    use super::*;
-
-    pub trait Sealed {}
-
-    #[cfg(feature = "tokio02")]
-    impl Sealed for Tokio02Connector {}
-
-    #[cfg(feature = "tokio1")]
-    impl Sealed for Tokio1Connector {}
-
-    #[cfg(feature = "async-std1")]
-    impl Sealed for AsyncStd1Connector {}
-}

src/transport/smtp/authentication.rs

@@ -1,14 +1,14 @@
 //! Provides limited SASL authentication mechanisms
 
-use crate::transport::smtp::error::Error;
-use std::fmt::{self, Display, Formatter};
+use crate::transport::smtp::error::{self, Error};
+use std::fmt::{self, Debug, Display, Formatter};
 
 /// Accepted authentication mechanisms
 /// Trying LOGIN last as it is deprecated.
 pub const DEFAULT_MECHANISMS: &[Mechanism] = &[Mechanism::Plain, Mechanism::Login];
 
 /// Contains user credentials
-#[derive(PartialEq, Eq, Clone, Hash, Debug)]
+#[derive(PartialEq, Eq, Clone, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Credentials {
     authentication_identity: String,
@@ -35,19 +35,26 @@ where
     }
 }
 
+impl Debug for Credentials {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Credentials").finish()
+    }
+}
+
 /// Represents authentication mechanisms
 #[derive(PartialEq, Eq, Copy, Clone, Hash, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Mechanism {
-    /// PLAIN authentication mechanism
-    /// RFC 4616: https://tools.ietf.org/html/rfc4616
+    /// PLAIN authentication mechanism, defined in
+    /// [RFC 4616](https://tools.ietf.org/html/rfc4616)
     Plain,
     /// LOGIN authentication mechanism
     /// Obsolete but needed for some providers (like office365)
-    /// https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt
+    ///
+    /// Defined in [draft-murchison-sasl-login-00](https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt).
     Login,
-    /// Non-standard XOAUTH2 mechanism
-    /// https://developers.google.com/gmail/imap/xoauth2-protocol
+    /// Non-standard XOAUTH2 mechanism, defined in
+    /// [xoauth2-protocol](https://developers.google.com/gmail/imap/xoauth2-protocol)
     Xoauth2,
 }
 
@@ -79,15 +86,15 @@ impl Mechanism {
     ) -> Result<String, Error> {
         match self {
             Mechanism::Plain => match challenge {
-                Some(_) => Err(Error::Client("This mechanism does not expect a challenge")),
+                Some(_) => Err(error::client("This mechanism does not expect a challenge")),
                 None => Ok(format!(
                     "\u{0}{}\u{0}{}",
                     credentials.authentication_identity, credentials.secret
                 )),
             },
             Mechanism::Login => {
-                let decoded_challenge =
-                    challenge.ok_or(Error::Client("This mechanism does expect a challenge"))?;
+                let decoded_challenge = challenge
+                    .ok_or_else(|| error::client("This mechanism does expect a challenge"))?;
 
                 if vec!["User Name", "Username:", "Username"].contains(&decoded_challenge) {
                     return Ok(credentials.authentication_identity.to_string());
@@ -97,10 +104,10 @@ impl Mechanism {
                     return Ok(credentials.secret.to_string());
                 }
 
-                Err(Error::Client("Unrecognized challenge"))
+                Err(error::client("Unrecognized challenge"))
             }
             Mechanism::Xoauth2 => match challenge {
-                Some(_) => Err(Error::Client("This mechanism does not expect a challenge")),
+                Some(_) => Err(error::client("This mechanism does not expect a challenge")),
                 None => Ok(format!(
                     "user={}\x01auth=Bearer {}\x01\x01",
                     credentials.authentication_identity, credentials.secret

src/transport/smtp/client/async_connection.rs

@@ -1,18 +1,17 @@
-use std::{fmt::Display, io};
-
-use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
-
 use super::{AsyncNetworkStream, ClientCodec, TlsParameters};
 use crate::{
     transport::smtp::{
         authentication::{Credentials, Mechanism},
         commands::*,
+        error,
         error::Error,
         extension::{ClientId, Extension, MailBodyParameter, MailParameter, ServerInfo},
         response::{parse_response, Response},
     },
     Envelope,
 };
+use futures_util::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+use std::fmt::Display;
 
 #[cfg(feature = "tracing")]
 use super::escape_crlf;
@@ -112,9 +111,32 @@ impl AsyncSmtpConnection {
         // Mail
         let mut mail_options = vec![];
 
-        if self.server_info().supports_feature(Extension::EightBitMime) {
+        // Internationalization handling
+        //
+        // * 8BITMIME: https://tools.ietf.org/html/rfc6152
+        // * SMTPUTF8: https://tools.ietf.org/html/rfc653
+
+        // Check for non-ascii addresses and use the SMTPUTF8 option if any.
+        if envelope.has_non_ascii_addresses() {
+            if !self.server_info().supports_feature(Extension::SmtpUtfEight) {
+                // don't try to send non-ascii addresses (per RFC)
+                return Err(error::client(
+                    "Envelope contains non-ascii chars but server does not support SMTPUTF8",
+                ));
+            }
+            mail_options.push(MailParameter::SmtpUtfEight);
+        }
+
+        // Check for non-ascii content in message
+        if !email.is_ascii() {
+            if !self.server_info().supports_feature(Extension::EightBitMime) {
+                return Err(error::client(
+                    "Message contains non-ascii chars but server does not support 8BITMIME",
+                ));
+            }
             mail_options.push(MailParameter::Body(MailBodyParameter::EightBitMime));
         }
+
         try_smtp!(
             self.command(Mail::new(envelope.from().cloned(), mail_options))
                 .await,
@@ -163,7 +185,7 @@ impl AsyncSmtpConnection {
             try_smtp!(self.ehlo(hello_name).await, self);
             Ok(())
         } else {
-            Err(Error::Client("STARTTLS is not supported on this server"))
+            Err(error::client("STARTTLS is not supported on this server"))
         }
     }
 
@@ -210,12 +232,10 @@ impl AsyncSmtpConnection {
         let mechanism = self
             .server_info
             .get_auth_mechanism(mechanisms)
-            .ok_or(Error::Client(
-                "No compatible authentication mechanism was found",
-            ))?;
+            .ok_or_else(|| error::client("No compatible authentication mechanism was found"))?;
 
         // Limit challenges to avoid blocking
-        let mut challenges = 10;
+        let mut challenges: u8 = 10;
         let mut response = self
             .command(Auth::new(mechanism, credentials.clone(), None)?)
             .await?;
@@ -234,7 +254,7 @@ impl AsyncSmtpConnection {
         }
 
         if challenges == 0 {
-            Err(Error::ResponseParsing("Unexpected number of challenges"))
+            Err(error::response("Unexpected number of challenges"))
         } else {
             Ok(response)
         }
@@ -258,8 +278,16 @@ impl AsyncSmtpConnection {
 
     /// Writes a string to the server
     async fn write(&mut self, string: &[u8]) -> Result<(), Error> {
-        self.stream.get_mut().write_all(string).await?;
-        self.stream.get_mut().flush().await?;
+        self.stream
+            .get_mut()
+            .write_all(string)
+            .await
+            .map_err(error::network)?;
+        self.stream
+            .get_mut()
+            .flush()
+            .await
+            .map_err(error::network)?;
 
         #[cfg(feature = "tracing")]
         tracing::debug!("Wrote: {}", escape_crlf(&String::from_utf8_lossy(string)));
@@ -270,27 +298,33 @@ impl AsyncSmtpConnection {
     pub async fn read_response(&mut self) -> Result<Response, Error> {
         let mut buffer = String::with_capacity(100);
 
-        while self.stream.read_line(&mut buffer).await? > 0 {
+        while self
+            .stream
+            .read_line(&mut buffer)
+            .await
+            .map_err(error::network)?
+            > 0
+        {
             #[cfg(feature = "tracing")]
             tracing::debug!("<< {}", escape_crlf(&buffer));
             match parse_response(&buffer) {
                 Ok((_remaining, response)) => {
-                    if response.is_positive() {
-                        return Ok(response);
+                    return if response.is_positive() {
+                        Ok(response)
+                    } else {
+                        Err(error::code(response.code))
                     }
-
-                    return Err(response.into());
                 }
                 Err(nom::Err::Failure(e)) => {
-                    return Err(Error::Parsing(e.code));
+                    return Err(error::response(e.to_string()));
                 }
                 Err(nom::Err::Incomplete(_)) => { /* read more */ }
                 Err(nom::Err::Error(e)) => {
-                    return Err(Error::Parsing(e.code));
+                    return Err(error::response(e.to_string()));
                 }
             }
         }
 
-        Err(io::Error::new(io::ErrorKind::Other, "incomplete").into())
+        Err(error::response("incomplete response"))
     }
 }

src/transport/smtp/client/async_net.rs

@@ -10,8 +10,10 @@ use std::{
     task::{Context, Poll},
 };
 
-use futures_io::{AsyncRead as FuturesAsyncRead, AsyncWrite as FuturesAsyncWrite};
-use futures_io::{Error as IoError, ErrorKind, Result as IoResult};
+use futures_io::{
+    AsyncRead as FuturesAsyncRead, AsyncWrite as FuturesAsyncWrite, Error as IoError, ErrorKind,
+    Result as IoResult,
+};
 #[cfg(feature = "tokio02")]
 use tokio02_crate::io::{AsyncRead as _, AsyncWrite as _};
 #[cfg(feature = "tokio1")]
@@ -48,7 +50,7 @@ use tokio1_rustls::client::TlsStream as Tokio1RustlsTlsStream;
 ))]
 use super::InnerTlsParameters;
 use super::TlsParameters;
-use crate::transport::smtp::Error;
+use crate::transport::smtp::{error, Error};
 
 /// A network stream
 pub struct AsyncNetworkStream {
@@ -142,7 +144,9 @@ impl AsyncNetworkStream {
         port: u16,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = Tokio02TcpStream::connect((hostname, port)).await?;
+        let tcp_stream = Tokio02TcpStream::connect((hostname, port))
+            .await
+            .map_err(error::connection)?;
 
         let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio02Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
@@ -157,7 +161,9 @@ impl AsyncNetworkStream {
         port: u16,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = Tokio1TcpStream::connect((hostname, port)).await?;
+        let tcp_stream = Tokio1TcpStream::connect((hostname, port))
+            .await
+            .map_err(error::connection)?;
 
         let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::Tokio1Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
@@ -172,7 +178,9 @@ impl AsyncNetworkStream {
         port: u16,
         tls_parameters: Option<TlsParameters>,
     ) -> Result<AsyncNetworkStream, Error> {
-        let tcp_stream = AsyncStd1TcpStream::connect((hostname, port)).await?;
+        let tcp_stream = AsyncStd1TcpStream::connect((hostname, port))
+            .await
+            .map_err(error::connection)?;
 
         let mut stream = AsyncNetworkStream::new(InnerAsyncNetworkStream::AsyncStd1Tcp(tcp_stream));
         if let Some(tls_parameters) = tls_parameters {
@@ -201,7 +209,9 @@ impl AsyncNetworkStream {
                     _ => unreachable!(),
                 };
 
-                self.inner = Self::upgrade_tokio02_tls(tcp_stream, tls_parameters).await?;
+                self.inner = Self::upgrade_tokio02_tls(tcp_stream, tls_parameters)
+                    .await
+                    .map_err(error::connection)?;
                 Ok(())
             }
             #[cfg(all(
@@ -222,7 +232,9 @@ impl AsyncNetworkStream {
                     _ => unreachable!(),
                 };
 
-                self.inner = Self::upgrade_tokio1_tls(tcp_stream, tls_parameters).await?;
+                self.inner = Self::upgrade_tokio1_tls(tcp_stream, tls_parameters)
+                    .await
+                    .map_err(error::connection)?;
                 Ok(())
             }
             #[cfg(all(
@@ -243,7 +255,9 @@ impl AsyncNetworkStream {
                     _ => unreachable!(),
                 };
 
-                self.inner = Self::upgrade_asyncstd1_tls(tcp_stream, tls_parameters).await?;
+                self.inner = Self::upgrade_asyncstd1_tls(tcp_stream, tls_parameters)
+                    .await
+                    .map_err(error::connection)?;
                 Ok(())
             }
             _ => Ok(()),
@@ -269,7 +283,10 @@ impl AsyncNetworkStream {
                     use tokio02_native_tls_crate::TlsConnector;
 
                     let connector = TlsConnector::from(connector);
-                    let stream = connector.connect(&domain, tcp_stream).await?;
+                    let stream = connector
+                        .connect(&domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::Tokio02NativeTls(stream))
                 };
             }
@@ -282,10 +299,14 @@ impl AsyncNetworkStream {
                 return {
                     use tokio02_rustls::{webpki::DNSNameRef, TlsConnector};
 
-                    let domain = DNSNameRef::try_from_ascii_str(&domain)?;
+                    let domain =
+                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
 
                     let connector = TlsConnector::from(Arc::new(config));
-                    let stream = connector.connect(domain, tcp_stream).await?;
+                    let stream = connector
+                        .connect(domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::Tokio02RustlsTls(stream))
                 };
             }
@@ -311,7 +332,10 @@ impl AsyncNetworkStream {
                     use tokio1_native_tls_crate::TlsConnector;
 
                     let connector = TlsConnector::from(connector);
-                    let stream = connector.connect(&domain, tcp_stream).await?;
+                    let stream = connector
+                        .connect(&domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::Tokio1NativeTls(stream))
                 };
             }
@@ -324,10 +348,14 @@ impl AsyncNetworkStream {
                 return {
                     use tokio1_rustls::{webpki::DNSNameRef, TlsConnector};
 
-                    let domain = DNSNameRef::try_from_ascii_str(&domain)?;
+                    let domain =
+                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
 
                     let connector = TlsConnector::from(Arc::new(config));
-                    let stream = connector.connect(domain, tcp_stream).await?;
+                    let stream = connector
+                        .connect(domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::Tokio1RustlsTls(stream))
                 };
             }
@@ -370,13 +398,16 @@ impl AsyncNetworkStream {
 
                 #[cfg(feature = "async-std1-rustls-tls")]
                 return {
-                    use async_rustls::webpki::DNSNameRef;
-                    use async_rustls::TlsConnector;
+                    use async_rustls::{webpki::DNSNameRef, TlsConnector};
 
-                    let domain = DNSNameRef::try_from_ascii_str(&domain)?;
+                    let domain =
+                        DNSNameRef::try_from_ascii_str(&domain).map_err(error::connection)?;
 
                     let connector = TlsConnector::from(Arc::new(config));
-                    let stream = connector.connect(domain, tcp_stream).await?;
+                    let stream = connector
+                        .connect(domain, tcp_stream)
+                        .await
+                        .map_err(error::connection)?;
                     Ok(InnerAsyncNetworkStream::AsyncStd1RustlsTls(stream))
                 };
             }

src/transport/smtp/client/connection.rs

@@ -6,13 +6,16 @@ use std::{
 };
 
 use super::{ClientCodec, NetworkStream, TlsParameters};
-use crate::address::Envelope;
-use crate::transport::smtp::{
-    authentication::{Credentials, Mechanism},
-    commands::*,
-    error::Error,
-    extension::{ClientId, Extension, MailBodyParameter, MailParameter, ServerInfo},
-    response::{parse_response, Response},
+use crate::{
+    address::Envelope,
+    transport::smtp::{
+        authentication::{Credentials, Mechanism},
+        commands::*,
+        error,
+        error::Error,
+        extension::{ClientId, Extension, MailBodyParameter, MailParameter, ServerInfo},
+        response::{parse_response, Response},
+    },
 };
 
 #[cfg(feature = "tracing")]
@@ -64,7 +67,7 @@ impl SmtpConnection {
             panic: false,
             server_info: ServerInfo::default(),
         };
-        conn.set_timeout(timeout)?;
+        conn.set_timeout(timeout).map_err(error::network)?;
         // TODO log
         let _response = conn.read_response()?;
 
@@ -80,9 +83,32 @@ impl SmtpConnection {
         // Mail
         let mut mail_options = vec![];
 
-        if self.server_info().supports_feature(Extension::EightBitMime) {
+        // Internationalization handling
+        //
+        // * 8BITMIME: https://tools.ietf.org/html/rfc6152
+        // * SMTPUTF8: https://tools.ietf.org/html/rfc653
+
+        // Check for non-ascii addresses and use the SMTPUTF8 option if any.
+        if envelope.has_non_ascii_addresses() {
+            if !self.server_info().supports_feature(Extension::SmtpUtfEight) {
+                // don't try to send non-ascii addresses (per RFC)
+                return Err(error::client(
+                    "Envelope contains non-ascii chars but server does not support SMTPUTF8",
+                ));
+            }
+            mail_options.push(MailParameter::SmtpUtfEight);
+        }
+
+        // Check for non-ascii content in message
+        if !email.is_ascii() {
+            if !self.server_info().supports_feature(Extension::EightBitMime) {
+                return Err(error::client(
+                    "Message contains non-ascii chars but server does not support 8BITMIME",
+                ));
+            }
             mail_options.push(MailParameter::Body(MailBodyParameter::EightBitMime));
         }
+
         try_smtp!(
             self.command(Mail::new(envelope.from().cloned(), mail_options)),
             self
@@ -131,7 +157,7 @@ impl SmtpConnection {
             // when a TLS library is enabled
             unreachable!("TLS support required but not supported");
         } else {
-            Err(Error::Client("STARTTLS is not supported on this server"))
+            Err(error::client("STARTTLS is not supported on this server"))
         }
     }
 
@@ -184,9 +210,7 @@ impl SmtpConnection {
         let mechanism = self
             .server_info
             .get_auth_mechanism(mechanisms)
-            .ok_or(Error::Client(
-                "No compatible authentication mechanism was found",
-            ))?;
+            .ok_or_else(|| error::client("No compatible authentication mechanism was found"))?;
 
         // Limit challenges to avoid blocking
         let mut challenges = 10;
@@ -205,7 +229,7 @@ impl SmtpConnection {
         }
 
         if challenges == 0 {
-            Err(Error::ResponseParsing("Unexpected number of challenges"))
+            Err(error::response("Unexpected number of challenges"))
         } else {
             Ok(response)
         }
@@ -229,8 +253,11 @@ impl SmtpConnection {
 
     /// Writes a string to the server
     fn write(&mut self, string: &[u8]) -> Result<(), Error> {
-        self.stream.get_mut().write_all(string)?;
-        self.stream.get_mut().flush()?;
+        self.stream
+            .get_mut()
+            .write_all(string)
+            .map_err(error::network)?;
+        self.stream.get_mut().flush().map_err(error::network)?;
 
         #[cfg(feature = "tracing")]
         tracing::debug!("Wrote: {}", escape_crlf(&String::from_utf8_lossy(string)));
@@ -241,27 +268,27 @@ impl SmtpConnection {
     pub fn read_response(&mut self) -> Result<Response, Error> {
         let mut buffer = String::with_capacity(100);
 
-        while self.stream.read_line(&mut buffer)? > 0 {
+        while self.stream.read_line(&mut buffer).map_err(error::network)? > 0 {
             #[cfg(feature = "tracing")]
             tracing::debug!("<< {}", escape_crlf(&buffer));
             match parse_response(&buffer) {
                 Ok((_remaining, response)) => {
-                    if response.is_positive() {
-                        return Ok(response);
-                    }
-
-                    return Err(response.into());
+                    return if response.is_positive() {
+                        Ok(response)
+                    } else {
+                        Err(error::code(response.code))
+                    };
                 }
                 Err(nom::Err::Failure(e)) => {
-                    return Err(Error::Parsing(e.code));
+                    return Err(error::response(e.to_string()));
                 }
                 Err(nom::Err::Incomplete(_)) => { /* read more */ }
                 Err(nom::Err::Error(e)) => {
-                    return Err(Error::Parsing(e.code));
+                    return Err(error::response(e.to_string()));
                 }
             }
         }
 
-        Err(io::Error::new(io::ErrorKind::Other, "incomplete").into())
+        Err(error::response("incomplete response"))
     }
 }

src/transport/smtp/client/mod.rs

@@ -52,7 +52,7 @@ mod tls;
 /// The codec used for transparency
 #[derive(Default, Clone, Copy, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
-pub struct ClientCodec {
+struct ClientCodec {
     escape_count: u8,
 }
 

src/transport/smtp/client/net.rs

@@ -15,7 +15,7 @@ use rustls::{ClientSession, StreamOwned};
 #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
 use super::InnerTlsParameters;
 use super::{MockStream, TlsParameters};
-use crate::transport::smtp::Error;
+use crate::transport::smtp::{error, Error};
 
 /// A network stream
 pub struct NetworkStream {
@@ -84,18 +84,18 @@ impl NetworkStream {
             server: T,
             timeout: Duration,
         ) -> Result<TcpStream, Error> {
-            let addrs = server.to_socket_addrs()?;
+            let addrs = server.to_socket_addrs().map_err(error::connection)?;
             for addr in addrs {
                 if let Ok(result) = TcpStream::connect_timeout(&addr, timeout) {
                     return Ok(result);
                 }
             }
-            Err(Error::Client("Could not connect"))
+            Err(error::connection("Could not connect"))
         }
 
         let tcp_stream = match timeout {
             Some(t) => try_connect_timeout(server, t)?,
-            None => TcpStream::connect(server)?,
+            None => TcpStream::connect(server).map_err(error::connection)?,
         };
 
         let mut stream = NetworkStream::new(InnerNetworkStream::Tcp(tcp_stream));
@@ -140,14 +140,15 @@ impl NetworkStream {
             InnerTlsParameters::NativeTls(connector) => {
                 let stream = connector
                     .connect(tls_parameters.domain(), tcp_stream)
-                    .map_err(|err| Error::Io(io::Error::new(io::ErrorKind::Other, err)))?;
+                    .map_err(error::connection)?;
                 InnerNetworkStream::NativeTls(stream)
             }
             #[cfg(feature = "rustls-tls")]
             InnerTlsParameters::RustlsTls(connector) => {
                 use webpki::DNSNameRef;
 
-                let domain = DNSNameRef::try_from_ascii_str(tls_parameters.domain())?;
+                let domain = DNSNameRef::try_from_ascii_str(tls_parameters.domain())
+                    .map_err(error::connection)?;
                 let stream = StreamOwned::new(
                     ClientSession::new(&Arc::new(connector.clone()), domain),
                     tcp_stream,

src/transport/smtp/client/tls.rs

@@ -1,15 +1,15 @@
-#[cfg(feature = "rustls-tls")]
-use std::sync::Arc;
-
+#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+use crate::transport::smtp::{error, Error};
 #[cfg(feature = "native-tls")]
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls-tls")]
 use rustls::{ClientConfig, RootCertStore, ServerCertVerified, ServerCertVerifier, TLSError};
+use std::fmt::{self, Debug};
+#[cfg(feature = "rustls-tls")]
+use std::sync::Arc;
 #[cfg(feature = "rustls-tls")]
 use webpki::DNSNameRef;
 
-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
@@ -33,9 +33,22 @@ pub enum Tls {
     Wrapper(TlsParameters),
 }
 
+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"))]
+            Self::Opportunistic(_) => f.pad("Opportunistic"),
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            Self::Required(_) => f.pad("Required"),
+            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+            Self::Wrapper(_) => f.pad("Wrapper"),
+        }
+    }
+}
+
 /// Parameters to use for secure clients
 #[derive(Clone)]
-#[allow(missing_debug_implementations)]
 pub struct TlsParameters {
     pub(crate) connector: InnerTlsParameters,
     /// The domain name which is expected in the TLS certificate from the server
@@ -43,7 +56,7 @@ pub struct TlsParameters {
 }
 
 /// Builder for `TlsParameters`
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct TlsParametersBuilder {
     domain: String,
     root_certs: Vec<Certificate>,
@@ -65,7 +78,7 @@ impl TlsParametersBuilder {
     /// Add a custom root certificate
     ///
     /// 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) -> &mut Self {
+    pub fn add_root_certificate(mut self, cert: Certificate) -> Self {
         self.root_certs.push(cert);
         self
     }
@@ -85,10 +98,7 @@ impl TlsParametersBuilder {
     /// Hostname verification can only be disabled with the `native-tls` TLS backend.
     #[cfg(feature = "native-tls")]
     #[cfg_attr(docsrs, doc(cfg(feature = "native-tls")))]
-    pub fn dangerous_accept_invalid_hostnames(
-        &mut self,
-        accept_invalid_hostnames: bool,
-    ) -> &mut Self {
+    pub fn dangerous_accept_invalid_hostnames(mut self, accept_invalid_hostnames: bool) -> Self {
         self.accept_invalid_hostnames = accept_invalid_hostnames;
         self
     }
@@ -109,7 +119,7 @@ impl TlsParametersBuilder {
     ///
     /// This method should only be used as a last resort, as it introduces
     /// significant vulnerabilities to man-in-the-middle attacks.
-    pub fn dangerous_accept_invalid_certs(&mut self, accept_invalid_certs: bool) -> &mut Self {
+    pub fn dangerous_accept_invalid_certs(mut self, accept_invalid_certs: bool) -> Self {
         self.accept_invalid_certs = accept_invalid_certs;
         self
     }
@@ -145,7 +155,7 @@ impl TlsParametersBuilder {
         tls_builder.danger_accept_invalid_certs(self.accept_invalid_certs);
 
         tls_builder.min_protocol_version(Some(DEFAULT_TLS_MIN_PROTOCOL));
-        let connector = tls_builder.build()?;
+        let connector = tls_builder.build().map_err(error::tls)?;
         Ok(TlsParameters {
             connector: InnerTlsParameters::NativeTls(connector),
             domain: self.domain,
@@ -162,9 +172,7 @@ impl TlsParametersBuilder {
 
         for cert in self.root_certs {
             for rustls_cert in cert.rustls {
-                tls.root_store
-                    .add(&rustls_cert)
-                    .map_err(|_| Error::InvalidCertificate)?;
+                tls.root_store.add(&rustls_cert).map_err(error::tls)?;
             }
         }
         if self.accept_invalid_certs {
@@ -230,12 +238,12 @@ pub struct Certificate {
     rustls: Vec<rustls::Certificate>,
 }
 
+#[cfg(any(feature = "native-tls", feature = "rustls-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::InvalidCertificate)?;
+        let native_tls_cert = native_tls::Certificate::from_der(&der).map_err(error::tls)?;
 
         Ok(Self {
             #[cfg(feature = "native-tls")]
@@ -248,8 +256,7 @@ impl Certificate {
     /// Create a `Certificate` from a PEM encoded certificate
     pub fn from_pem(pem: &[u8]) -> Result<Self, Error> {
         #[cfg(feature = "native-tls")]
-        let native_tls_cert =
-            native_tls::Certificate::from_pem(pem).map_err(|_| Error::InvalidCertificate)?;
+        let native_tls_cert = native_tls::Certificate::from_pem(pem).map_err(error::tls)?;
 
         #[cfg(feature = "rustls-tls")]
         let rustls_cert = {
@@ -257,7 +264,7 @@ impl Certificate {
             use std::io::Cursor;
 
             let mut pem = Cursor::new(pem);
-            pemfile::certs(&mut pem).map_err(|_| Error::InvalidCertificate)?
+            pemfile::certs(&mut pem).map_err(|_| error::tls("invalid certificates"))?
         };
 
         Ok(Self {
@@ -269,6 +276,12 @@ impl Certificate {
     }
 }
 
+impl Debug for Certificate {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Certificate").finish()
+    }
+}
+
 #[cfg(feature = "rustls-tls")]
 struct InvalidCertsVerifier;
 

src/transport/smtp/commands.rs

@@ -1,13 +1,13 @@
 //! SMTP commands
 
 use crate::{
+    address::Address,
     transport::smtp::{
         authentication::{Credentials, Mechanism},
-        error::Error,
+        error::{self, Error},
         extension::{ClientId, MailParameter, RcptParameter},
         response::Response,
     },
-    Address,
 };
 use std::fmt::{self, Display, Formatter};
 
@@ -261,16 +261,17 @@ impl Auth {
         response: &Response,
     ) -> Result<Auth, Error> {
         if !response.has_code(334) {
-            return Err(Error::ResponseParsing("Expecting a challenge"));
+            return Err(error::response("Expecting a challenge"));
         }
 
         let encoded_challenge = response
             .first_word()
-            .ok_or(Error::ResponseParsing("Could not read auth challenge"))?;
+            .ok_or_else(|| error::response("Could not read auth challenge"))?;
         #[cfg(feature = "tracing")]
         tracing::debug!("auth encoded challenge: {}", encoded_challenge);
 
-        let decoded_challenge = String::from_utf8(base64::decode(&encoded_challenge)?)?;
+        let decoded_base64 = 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);
 

src/transport/smtp/error.rs

@@ -1,162 +1,185 @@
 //! Error and result type for SMTP clients
 
-use self::Error::*;
-use crate::transport::smtp::response::{Response, Severity};
-use base64::DecodeError;
-use std::{
-    error::Error as StdError,
-    fmt::{self, Display, Formatter},
-    io,
-    string::FromUtf8Error,
+use crate::{
+    transport::smtp::response::{Code, Severity},
+    BoxError,
 };
+use std::{error::Error as StdError, fmt};
 
-/// An enum of all error kinds.
-#[derive(Debug)]
-pub enum Error {
-    /// Transient SMTP error, 4xx reply code
-    ///
-    /// [RFC 5321, section 4.2.1](https://tools.ietf.org/html/rfc5321#section-4.2.1)
-    Transient(Response),
-    /// Permanent SMTP error, 5xx reply code
-    ///
-    /// [RFC 5321, section 4.2.1](https://tools.ietf.org/html/rfc5321#section-4.2.1)
-    Permanent(Response),
-    /// Error parsing a response
-    ResponseParsing(&'static str),
-    /// Error parsing a base64 string in response
-    ChallengeParsing(DecodeError),
-    /// Error parsing UTF8 in response
-    Utf8Parsing(FromUtf8Error),
-    /// Internal client error
-    Client(&'static str),
-    /// DNS resolution error
-    Resolution,
-    /// IO error
-    Io(io::Error),
-    /// TLS error
-    #[cfg(feature = "native-tls")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "native-tls")))]
-    Tls(native_tls::Error),
-    /// Parsing error
-    Parsing(nom::error::ErrorKind),
-    /// Invalid hostname
-    #[cfg(feature = "rustls-tls")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "rustls-tls")))]
-    InvalidDNSName(webpki::InvalidDNSNameError),
-    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-    #[cfg_attr(docsrs, doc(cfg(any(feature = "native-tls", feature = "rustls-tls"))))]
-    InvalidCertificate,
-    #[cfg(feature = "r2d2")]
-    #[cfg_attr(docsrs, doc(cfg(feature = "r2d2")))]
-    Pool(r2d2::Error),
+// Inspired by https://github.com/seanmonstar/reqwest/blob/a8566383168c0ef06c21f38cbc9213af6ff6db31/src/error.rs
+
+/// The Errors that may occur when sending an email over SMTP
+pub struct Error {
+    inner: Box<Inner>,
 }
 
-impl Display for Error {
-    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), fmt::Error> {
-        match *self {
-            // Try to display the first line of the server's response that usually
-            // contains a short humanly readable error message
-            Transient(ref err) => fmt.write_str(
-                err.first_line()
-                    .unwrap_or("transient error during SMTP transaction"),
-            ),
-            Permanent(ref err) => fmt.write_str(
-                err.first_line()
-                    .unwrap_or("permanent error during SMTP transaction"),
-            ),
-            ResponseParsing(err) => fmt.write_str(err),
-            ChallengeParsing(ref err) => err.fmt(fmt),
-            Utf8Parsing(ref err) => err.fmt(fmt),
-            Resolution => fmt.write_str("could not resolve hostname"),
-            Client(err) => fmt.write_str(err),
-            Io(ref err) => err.fmt(fmt),
-            #[cfg(feature = "native-tls")]
-            Tls(ref err) => err.fmt(fmt),
-            Parsing(ref err) => fmt.write_str(err.description()),
-            #[cfg(feature = "rustls-tls")]
-            InvalidDNSName(ref err) => err.fmt(fmt),
-            #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-            InvalidCertificate => fmt.write_str("invalid certificate"),
-            #[cfg(feature = "r2d2")]
-            Pool(ref err) => err.fmt(fmt),
+struct Inner {
+    kind: Kind,
+    source: Option<BoxError>,
+}
+
+impl Error {
+    pub(crate) fn new<E>(kind: Kind, source: Option<E>) -> Error
+    where
+        E: Into<BoxError>,
+    {
+        Error {
+            inner: Box::new(Inner {
+                kind,
+                source: source.map(Into::into),
+            }),
         }
     }
-}
 
-impl StdError for Error {
-    fn source(&self) -> Option<&(dyn StdError + 'static)> {
-        match *self {
-            ChallengeParsing(ref err) => Some(&*err),
-            Utf8Parsing(ref err) => Some(&*err),
-            Io(ref err) => Some(&*err),
-            #[cfg(feature = "native-tls")]
-            Tls(ref err) => Some(&*err),
+    /// Returns true if the error is from response
+    pub fn is_response(&self) -> bool {
+        matches!(self.inner.kind, Kind::Response)
+    }
+
+    /// Returns true if the error is from client
+    pub fn is_client(&self) -> bool {
+        matches!(self.inner.kind, Kind::Client)
+    }
+
+    /// Returns true if the error is a transient SMTP error
+    pub fn is_transient(&self) -> bool {
+        matches!(self.inner.kind, Kind::Transient(_))
+    }
+
+    /// Returns true if the error is a permanent SMTP error
+    pub fn is_permanent(&self) -> bool {
+        matches!(self.inner.kind, Kind::Permanent(_))
+    }
+
+    /// Returns true if the error is caused by a timeout
+    pub fn is_timeout(&self) -> bool {
+        let mut source = self.source();
+
+        while let Some(err) = source {
+            if let Some(io_err) = err.downcast_ref::<std::io::Error>() {
+                return io_err.kind() == std::io::ErrorKind::TimedOut;
+            }
+
+            source = err.source();
+        }
+
+        false
+    }
+
+    /// 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"))))]
+    pub fn is_tls(&self) -> bool {
+        matches!(self.inner.kind, Kind::Tls)
+    }
+
+    /// Returns the status code, if the error was generated from a response.
+    pub fn status(&self) -> Option<Code> {
+        match self.inner.kind {
+            Kind::Transient(code) => Some(code),
+            Kind::Permanent(code) => Some(code),
             _ => None,
         }
     }
 }
 
-impl From<io::Error> for Error {
-    fn from(err: io::Error) -> Error {
-        Io(err)
+#[derive(Debug)]
+pub(crate) enum Kind {
+    /// Transient SMTP error, 4xx reply code
+    ///
+    /// [RFC 5321, section 4.2.1](https://tools.ietf.org/html/rfc5321#section-4.2.1)
+    Transient(Code),
+    /// Permanent SMTP error, 5xx reply code
+    ///
+    /// [RFC 5321, section 4.2.1](https://tools.ietf.org/html/rfc5321#section-4.2.1)
+    Permanent(Code),
+    /// Error parsing a response
+    Response,
+    /// Internal client error
+    Client,
+    /// Connection error
+    Connection,
+    /// 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"))]
+    Tls,
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = f.debug_struct("lettre::transport::smtp::Error");
+
+        builder.field("kind", &self.inner.kind);
+
+        if let Some(ref source) = self.inner.source {
+            builder.field("source", source);
+        }
+
+        builder.finish()
     }
 }
 
-#[cfg(feature = "native-tls")]
-impl From<native_tls::Error> for Error {
-    fn from(err: native_tls::Error) -> Error {
-        Tls(err)
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        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"))]
+            Kind::Tls => f.write_str("tls error")?,
+            Kind::Transient(ref code) => {
+                write!(f, "transient error ({})", code)?;
+            }
+            Kind::Permanent(ref code) => {
+                write!(f, "permanent error ({})", code)?;
+            }
+        };
+
+        if let Some(ref e) = self.inner.source {
+            write!(f, ": {}", e)?;
+        }
+
+        Ok(())
     }
 }
 
-impl From<nom::Err<nom::error::Error<&str>>> for Error {
-    fn from(err: nom::Err<nom::error::Error<&str>>) -> Error {
-        Parsing(match err {
-            nom::Err::Incomplete(_) => nom::error::ErrorKind::Complete,
-            nom::Err::Failure(e) => e.code,
-            nom::Err::Error(e) => e.code,
+impl StdError for Error {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        self.inner.source.as_ref().map(|e| {
+            let r: &(dyn std::error::Error + 'static) = &**e;
+            r
         })
     }
 }
 
-impl From<DecodeError> for Error {
-    fn from(err: DecodeError) -> Error {
-        ChallengeParsing(err)
+pub(crate) fn code(c: Code) -> Error {
+    match c.severity {
+        Severity::TransientNegativeCompletion => Error::new::<Error>(Kind::Transient(c), None),
+        Severity::PermanentNegativeCompletion => Error::new::<Error>(Kind::Permanent(c), None),
+        _ => client("Unknown error code"),
     }
 }
 
-impl From<FromUtf8Error> for Error {
-    fn from(err: FromUtf8Error) -> Error {
-        Utf8Parsing(err)
-    }
+pub(crate) fn response<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Response, Some(e))
 }
 
-#[cfg(feature = "rustls-tls")]
-impl From<webpki::InvalidDNSNameError> for Error {
-    fn from(err: webpki::InvalidDNSNameError) -> Error {
-        InvalidDNSName(err)
-    }
+pub(crate) fn client<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Client, Some(e))
 }
 
-#[cfg(feature = "r2d2")]
-impl From<r2d2::Error> for Error {
-    fn from(err: r2d2::Error) -> Error {
-        Pool(err)
-    }
+pub(crate) fn network<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Network, Some(e))
 }
 
-impl From<Response> for Error {
-    fn from(response: Response) -> Error {
-        match response.code.severity {
-            Severity::TransientNegativeCompletion => Transient(response),
-            Severity::PermanentNegativeCompletion => Permanent(response),
-            _ => Client("Unknown error code"),
-        }
-    }
+pub(crate) fn connection<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Connection, Some(e))
 }
 
-impl From<&'static str> for Error {
-    fn from(string: &'static str) -> Error {
-        Client(string)
-    }
+#[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+pub(crate) fn tls<E: Into<BoxError>>(e: E) -> Error {
+    Error::new(Kind::Tls, Some(e))
 }

src/transport/smtp/extension.rs

@@ -1,7 +1,10 @@
 //! ESMTP features
 
 use crate::transport::smtp::{
-    authentication::Mechanism, error::Error, response::Response, util::XText,
+    authentication::Mechanism,
+    error::{self, Error},
+    response::Response,
+    util::XText,
 };
 use std::{
     collections::HashSet,
@@ -58,6 +61,7 @@ impl Display for ClientId {
 }
 
 impl ClientId {
+    #[doc(hidden)]
     #[deprecated(since = "0.10.0", note = "Please use ClientId::Domain(domain) instead")]
     /// Creates a new `ClientId` from a fully qualified domain name
     pub fn new(domain: String) -> Self {
@@ -71,15 +75,15 @@ impl ClientId {
 pub enum Extension {
     /// 8BITMIME keyword
     ///
-    /// RFC 6152: https://tools.ietf.org/html/rfc6152
+    /// Defined in [RFC 6152](https://tools.ietf.org/html/rfc6152)
     EightBitMime,
     /// SMTPUTF8 keyword
     ///
-    /// RFC 6531: https://tools.ietf.org/html/rfc6531
+    /// Defined in [RFC 6531](https://tools.ietf.org/html/rfc6531)
     SmtpUtfEight,
     /// STARTTLS keyword
     ///
-    /// RFC 2487: https://tools.ietf.org/html/rfc2487
+    /// Defined in [RFC 2487](https://tools.ietf.org/html/rfc2487)
     StartTls,
     /// AUTH mechanism
     Authentication(Mechanism),
@@ -126,7 +130,7 @@ impl ServerInfo {
     pub fn from_response(response: &Response) -> Result<ServerInfo, Error> {
         let name = match response.first_word() {
             Some(name) => name,
-            None => return Err(Error::ResponseParsing("Could not read server name")),
+            None => return Err(error::response("Could not read server name")),
         };
 
         let mut features: HashSet<Extension> = HashSet::new();

src/transport/smtp/mod.rs

@@ -43,8 +43,7 @@
 //!     .body(String::from("Be happy!"))?;
 //!
 //! // Create TLS transport on port 465
-//! let sender = SmtpTransport::relay("smtp.example.com")
-//!     .expect("relay valid")
+//! let sender = SmtpTransport::relay("smtp.example.com")?
 //!     .build();
 //! // Send the email via remote relay
 //! let result = sender.send(&email);
@@ -52,118 +51,73 @@
 //! # Ok(())
 //! # }
 //! ```
-
-//! #### Complete example
 //!
-//! ```todo
-//! # #[cfg(feature = "smtp-transport")]
-//! # {
-//! use lettre::transport::smtp::authentication::{Credentials, Mechanism};
-//! use lettre::{Email, Envelope, Transport, SmtpClient};
-//! use lettre::transport::smtp::extension::ClientId;
+//! #### Authentication
 //!
-//! let email_1 = Email::new(
-//!     Envelope::new(
-//!         Some(EmailAddress::new("user@localhost".to_string())?),
-//!         vec![EmailAddress::new("root@localhost".to_string())?],
-//!     )?,
-//!     "id1".to_string(),
-//!     "Hello world".to_string().into_bytes(),
-//! );
+//! Example with authentication and connection pool:
 //!
-//! let email_2 = Email::new(
-//!     Envelope::new(
-//!         Some(EmailAddress::new("user@localhost".to_string())?),
-//!         vec![EmailAddress::new("root@localhost".to_string())?],
-//!     )?,
-//!     "id2".to_string(),
-//!     "Hello world a second time".to_string().into_bytes(),
-//! );
+//! ```rust,no_run
+//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
+//! # fn test() -> Result<(), Box<dyn std::error::Error>> {
+//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::{PoolConfig, authentication::{Credentials, Mechanism}}};
 //!
-//! // Connect to a remote server on a custom port
-//! let mut mailer = SmtpClient::new_simple("server.tld")?
-//!    // Set the name sent during EHLO/HELO, default is `localhost`
-//!    .hello_name(ClientId::Domain("my.hostname.tld".to_string()))
-//!    // Add credentials for authentication
-//!    .credentials(Credentials::new("username".to_string(), "password".to_string()))
-//!    // Enable SMTPUTF8 if the server supports it
-//!    .smtp_utf8(true)
-//!    // Configure expected authentication mechanism
-//!    .authentication_mechanism(Mechanism::Plain)
-//!    // Enable connection reuse
-//!    .connection_reuse(ConnectionReuseParameters::ReuseUnlimited).transport();
+//! let email = Message::builder()
+//!     .from("NoBody <nobody@domain.tld>".parse()?)
+//!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
+//!     .to("Hei <hei@domain.tld>".parse()?)
+//!     .subject("Happy new year")
+//!     .body(String::from("Be happy!"))?;
 //!
-//! let result_1 = mailer.send(&email_1);
-//! assert!(result_1.is_ok());
+//! // Create TLS transport on port 587 with STARTTLS
+//! let sender = SmtpTransport::starttls_relay("smtp.example.com")?
+//!     // Add credentials for authentication
+//!     .credentials(Credentials::new("username".to_string(), "password".to_string()))
+//!     // Configure expected authentication mechanism
+//!     .authentication(vec![Mechanism::Plain])
+//!     // Connection pool settings
+//!     .pool_config( PoolConfig::new().max_size(20))
+//!     .build();
 //!
-//! // The second email will use the same connection
-//! let result_2 = mailer.send(&email_2);
-//! assert!(result_2.is_ok());
-//!
-//! // Explicitly close the SMTP transaction as we enabled connection reuse
-//! mailer.close();
+//! // Send the email via remote relay
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
+//! # Ok(())
 //! # }
 //! ```
 //!
 //! You can specify custom TLS settings:
 //!
-//! ```todo
-//! # #[cfg(feature = "native-tls")]
-//! # {
-//! use lettre::{
-//!     ClientSecurity, ClientTlsParameters, EmailAddress, Envelope,
-//!     Email, SmtpClient, Transport,
-//! };
-//! use lettre::transport::smtp::authentication::{Credentials, Mechanism};
-//! use lettre::transport::smtp::ConnectionReuseParameters;
-//! use native_tls::{Protocol, TlsConnector};
+//! ```rust,no_run
+//! # #[cfg(all(feature = "builder", any(feature = "native-tls", feature = "rustls-tls")))]
+//! # fn test() -> Result<(), Box<dyn std::error::Error>> {
+//! use lettre::{Message, Transport, SmtpTransport, transport::smtp::client::{TlsParameters, Tls}};
 //!
-//!     let email = Email::new(
-//!         Envelope::new(
-//!             Some(EmailAddress::new("user@localhost".to_string())?),
-//!             vec![EmailAddress::new("root@localhost".to_string())?],
-//!         )?,
-//!         "message_id".to_string(),
-//!         "Hello world".to_string().into_bytes(),
-//!     );
+//! let email = Message::builder()
+//!     .from("NoBody <nobody@domain.tld>".parse()?)
+//!     .reply_to("Yuin <yuin@domain.tld>".parse()?)
+//!     .to("Hei <hei@domain.tld>".parse()?)
+//!     .subject("Happy new year")
+//!     .body(String::from("Be happy!"))?;
 //!
-//!     let mut tls_builder = TlsConnector::builder();
-//!     tls_builder.min_protocol_version(Some(Protocol::Tlsv10));
-//!     let tls_parameters =
-//!         ClientTlsParameters::new(
-//!             "smtp.example.com".to_string(),
-//!             tls_builder.build()?
-//!         );
+//! // Custom TLS configuration
+//! let tls = TlsParameters::builder("smtp.example.com".to_string())
+//!           .dangerous_accept_invalid_certs(true).build()?;
 //!
-//!     let mut mailer = SmtpClient::new(
-//!         ("smtp.example.com", 465), ClientSecurity::Wrapper(tls_parameters)
-//!     )?
-//!         .authentication_mechanism(Mechanism::Login)
-//!         .credentials(Credentials::new(
-//!             "example_username".to_string(), "example_password".to_string()
-//!         ))
-//!         .connection_reuse(ConnectionReuseParameters::ReuseUnlimited)
-//!         .transport();
+//! // Create TLS transport on port 465
+//! let sender = SmtpTransport::relay("smtp.example.com")?
+//!     // Custom TLS configuration
+//!     .tls(Tls::Required(tls))
+//!     .build();
 //!
-//!     let result = mailer.send(&email);
-//!
-//!     assert!(result.is_ok());
-//!
-//!     mailer.close();
+//! // Send the email via remote relay
+//! let result = sender.send(&email);
+//! assert!(result.is_ok());
+//! # Ok(())
 //! # }
 //! ```
-//!
 
-#[cfg(feature = "async-std1")]
-pub use self::async_transport::AsyncStd1Connector;
-#[cfg(feature = "tokio02")]
-pub use self::async_transport::Tokio02Connector;
-#[cfg(feature = "tokio1")]
-pub use self::async_transport::Tokio1Connector;
 #[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
-pub use self::async_transport::{
-    AsyncSmtpConnector, AsyncSmtpTransport, AsyncSmtpTransportBuilder,
-};
+pub use self::async_transport::{AsyncSmtpTransport, AsyncSmtpTransportBuilder};
 #[cfg(feature = "r2d2")]
 pub use self::pool::PoolConfig;
 #[cfg(feature = "r2d2")]
@@ -206,14 +160,13 @@ pub const SMTP_PORT: u16 = 25;
 pub const SUBMISSION_PORT: u16 = 587;
 /// Default submission over TLS port
 ///
-/// https://tools.ietf.org/html/rfc8314
+/// Defined in [RFC8314](https://tools.ietf.org/html/rfc8314)
 pub const SUBMISSIONS_PORT: u16 = 465;
 
 /// Default timeout
 pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(60);
 
-#[allow(missing_debug_implementations)]
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 struct SmtpInfo {
     /// Name sent during EHLO
     hello_name: ClientId,

src/transport/smtp/pool.rs

@@ -1,8 +1,8 @@
 use std::time::Duration;
 
-use crate::transport::smtp::{client::SmtpConnection, error::Error, SmtpClient};
+use crate::transport::smtp::{client::SmtpConnection, error, error::Error, SmtpClient};
 
-use r2d2::{ManageConnection, Pool};
+use r2d2::{CustomizeConnection, ManageConnection, Pool};
 
 /// Configuration for a connection pool
 #[derive(Debug, Clone)]
@@ -16,6 +16,11 @@ pub struct PoolConfig {
 }
 
 impl PoolConfig {
+    /// Create a new pool configuration with default values
+    pub fn new() -> Self {
+        Self::default()
+    }
+
     /// Minimum number of idle connections
     ///
     /// Defaults to `0`
@@ -28,7 +33,7 @@ impl PoolConfig {
     ///
     /// Defaults to `10`
     pub fn max_size(mut self, max_size: u32) -> Self {
-        self.min_idle = max_size;
+        self.max_size = max_size;
         self
     }
 
@@ -48,12 +53,16 @@ impl PoolConfig {
         self
     }
 
-    pub(crate) fn build<C: ManageConnection>(&self, client: C) -> Pool<C> {
+    pub(crate) fn build<C: ManageConnection<Connection = SmtpConnection, Error = Error>>(
+        &self,
+        client: C,
+    ) -> Pool<C> {
         Pool::builder()
             .min_idle(Some(self.min_idle))
             .max_size(self.max_size)
             .connection_timeout(self.connection_timeout)
             .idle_timeout(Some(self.idle_timeout))
+            .connection_customizer(Box::new(SmtpConnectionQuitter))
             .build_unchecked(client)
     }
 }
@@ -81,10 +90,22 @@ impl ManageConnection for SmtpClient {
         if conn.test_connected() {
             return Ok(());
         }
-        Err(Error::Client("is not connected anymore"))
+        Err(error::network("is not connected anymore"))
     }
 
     fn has_broken(&self, conn: &mut Self::Connection) -> bool {
         conn.has_broken()
     }
 }
+
+#[derive(Copy, Clone, Debug)]
+struct SmtpConnectionQuitter;
+
+impl CustomizeConnection<SmtpConnection, Error> for SmtpConnectionQuitter {
+    fn on_release(&self, conn: SmtpConnection) {
+        let mut conn = conn;
+        if !conn.has_broken() {
+            let _quit = conn.quit();
+        }
+    }
+}

src/transport/smtp/response.rs

@@ -1,7 +1,7 @@
 //! SMTP response, containing a mandatory return code and an optional text
 //! message
 
-use crate::transport::smtp::Error;
+use crate::transport::smtp::{error, Error};
 use nom::{
     branch::alt,
     bytes::streaming::{tag, take_until},
@@ -120,6 +120,14 @@ impl Code {
             detail,
         }
     }
+
+    /// Tells if the response is positive
+    pub fn is_positive(&self) -> bool {
+        matches!(
+            self.severity,
+            Severity::PositiveCompletion | Severity::PositiveIntermediate
+        )
+    }
 }
 
 /// Contains an SMTP reply, with separated code and message
@@ -139,7 +147,9 @@ impl FromStr for Response {
     type Err = Error;
 
     fn from_str(s: &str) -> result::Result<Response, Error> {
-        parse_response(s).map(|(_, r)| r).map_err(|e| e.into())
+        parse_response(s)
+            .map(|(_, r)| r)
+            .map_err(|e| error::response(e.to_string()))
     }
 }
 
@@ -151,10 +161,7 @@ impl Response {
 
     /// Tells if the response is positive
     pub fn is_positive(&self) -> bool {
-        matches!(
-            self.code.severity,
-            Severity::PositiveCompletion | Severity::PositiveIntermediate
-        )
+        self.code.is_positive()
     }
 
     /// Tests code equality

src/transport/smtp/transport.rs

@@ -5,13 +5,13 @@ use r2d2::Pool;
 
 #[cfg(feature = "r2d2")]
 use super::PoolConfig;
-use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
 #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
-use crate::address::Envelope;
-use crate::Transport;
+use super::{error, Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
+use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
+use crate::{address::Envelope, Transport};
 
-#[allow(missing_debug_implementations)]
+/// Sends emails using the SMTP protocol
+#[cfg_attr(docsrs, doc(cfg(feature = "smtp-transport")))]
 #[derive(Clone)]
 pub struct SmtpTransport {
     #[cfg(feature = "r2d2")]
@@ -27,7 +27,7 @@ impl Transport for SmtpTransport {
     /// Sends an email
     fn send_raw(&self, envelope: &Envelope, email: &[u8]) -> Result<Self::Ok, Self::Error> {
         #[cfg(feature = "r2d2")]
-        let mut conn = self.inner.get()?;
+        let mut conn = self.inner.get().map_err(error::client)?;
         #[cfg(not(feature = "r2d2"))]
         let mut conn = self.inner.connection()?;
 
@@ -111,8 +111,7 @@ impl SmtpTransport {
 
 /// Contains client configuration.
 /// Instances of this struct can be created using functions of [`SmtpTransport`].
-#[allow(missing_debug_implementations)]
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct SmtpTransportBuilder {
     info: SmtpInfo,
     #[cfg(feature = "r2d2")]
@@ -184,7 +183,7 @@ impl SmtpTransportBuilder {
 }
 
 /// Build client
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct SmtpClient {
     info: SmtpInfo,
 }

src/transport/stub/mod.rs

@@ -9,8 +9,7 @@
 //! ```rust
 //! # #[cfg(feature = "builder")]
 //! # {
-//! use lettre::{Message, Transport};
-//! use lettre::transport::stub::StubTransport;
+//! use lettre::{transport::stub::StubTransport, Message, Transport};
 //!
 //! # use std::error::Error;
 //! # fn main() -> Result<(), Box<dyn Error>> {
@@ -29,13 +28,10 @@
 //! # }
 //! ```
 
-use crate::address::Envelope;
-#[cfg(feature = "async-std1")]
-use crate::AsyncStd1Transport;
-#[cfg(feature = "tokio02")]
-use crate::Tokio02Transport;
-use crate::Transport;
-#[cfg(any(feature = "async-std1", feature = "tokio02"))]
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
+use crate::AsyncTransport;
+use crate::{address::Envelope, Transport};
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
 use async_trait::async_trait;
 use std::{error::Error as StdError, fmt};
 
@@ -84,20 +80,9 @@ impl Transport for StubTransport {
     }
 }
 
-#[cfg(feature = "async-std1")]
+#[cfg(any(feature = "tokio02", feature = "tokio1", feature = "async-std1"))]
 #[async_trait]
-impl AsyncStd1Transport for StubTransport {
-    type Ok = ();
-    type Error = Error;
-
-    async fn send_raw(&self, _envelope: &Envelope, _email: &[u8]) -> Result<Self::Ok, Self::Error> {
-        self.response
-    }
-}
-
-#[cfg(feature = "tokio02")]
-#[async_trait]
-impl Tokio02Transport for StubTransport {
+impl AsyncTransport for StubTransport {
     type Ok = ();
     type Error = Error;
 

tests/transport_file.rs

@@ -1,18 +1,14 @@
 #[cfg(test)]
 #[cfg(all(feature = "file-transport", feature = "builder"))]
-mod test {
-    use lettre::{transport::file::FileTransport, Message};
+mod sync {
+    use lettre::{FileTransport, Message, Transport};
     use std::{
         env::temp_dir,
         fs::{read_to_string, remove_file},
     };
 
-    #[cfg(feature = "tokio02")]
-    use tokio02_crate as tokio;
-
     #[test]
     fn file_transport() {
-        use lettre::Transport;
         let sender = FileTransport::new(temp_dir());
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
@@ -48,7 +44,6 @@ mod test {
     #[test]
     #[cfg(feature = "file-transport-envelope")]
     fn file_transport_with_envelope() {
-        use lettre::Transport;
         let sender = FileTransport::with_envelope(temp_dir());
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
@@ -81,58 +76,131 @@ mod test {
                 "Be happy!"
             )
         );
-        remove_file(eml_file).unwrap();
 
         assert_eq!(
             json,
             "{\"forward_path\":[\"hei@domain.tld\"],\"reverse_path\":\"nobody@domain.tld\"}"
         );
+
+        let (e, m) = sender.read(&id).unwrap();
+
+        assert_eq!(&e, email.envelope());
+        assert_eq!(m, email.formatted());
+
+        remove_file(eml_file).unwrap();
         remove_file(json_file).unwrap();
     }
+}
 
-    #[cfg(feature = "async-std1")]
-    #[async_std::test]
-    async fn file_transport_asyncstd1() {
-        use lettre::AsyncStd1Transport;
+#[cfg(test)]
+#[cfg(all(feature = "file-transport", feature = "builder", feature = "tokio02"))]
+mod tokio_02 {
+    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio02Executor};
+    use std::{
+        env::temp_dir,
+        fs::{read_to_string, remove_file},
+    };
 
-        let sender = FileTransport::new(temp_dir());
-        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")
-            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
-            .body(String::from("Be happy!"))
-            .unwrap();
+    use tokio02_crate as tokio;
 
-        let result = sender.send(email).await;
-        let id = result.unwrap();
-
-        let eml_file = temp_dir().join(format!("{}.eml", id));
-        let eml = read_to_string(&eml_file).unwrap();
-
-        assert_eq!(
-            eml,
-            concat!(
-                "From: NoBody <nobody@domain.tld>\r\n",
-                "Reply-To: Yuin <yuin@domain.tld>\r\n",
-                "To: Hei <hei@domain.tld>\r\n",
-                "Subject: Happy new year\r\n",
-                "Date: Tue, 15 Nov 1994 08:12:31 GMT\r\n",
-                "Content-Transfer-Encoding: 7bit\r\n",
-                "\r\n",
-                "Be happy!"
-            )
-        );
-        remove_file(eml_file).unwrap();
-    }
-
-    #[cfg(feature = "tokio02")]
     #[tokio::test]
     async fn file_transport_tokio02() {
-        use lettre::Tokio02Transport;
-
-        let sender = FileTransport::new(temp_dir());
+        let sender = AsyncFileTransport::<Tokio02Executor>::new(temp_dir());
+        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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let result = sender.send(email).await;
+        let id = result.unwrap();
+
+        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml = read_to_string(&eml_file).unwrap();
+
+        assert_eq!(
+            eml,
+            concat!(
+                "From: NoBody <nobody@domain.tld>\r\n",
+                "Reply-To: Yuin <yuin@domain.tld>\r\n",
+                "To: Hei <hei@domain.tld>\r\n",
+                "Subject: Happy new year\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 GMT\r\n",
+                "Content-Transfer-Encoding: 7bit\r\n",
+                "\r\n",
+                "Be happy!"
+            )
+        );
+        remove_file(eml_file).unwrap();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(feature = "file-transport", feature = "builder", feature = "tokio1"))]
+mod tokio_1 {
+    use lettre::{AsyncFileTransport, AsyncTransport, Message, Tokio1Executor};
+    use std::{
+        env::temp_dir,
+        fs::{read_to_string, remove_file},
+    };
+
+    use tokio1_crate as tokio;
+
+    #[cfg(feature = "tokio02")]
+    #[tokio::test]
+    async fn file_transport_tokio1() {
+        let sender = AsyncFileTransport::<Tokio1Executor>::new(temp_dir());
+        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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let result = sender.send(email).await;
+        let id = result.unwrap();
+
+        let eml_file = temp_dir().join(format!("{}.eml", id));
+        let eml = read_to_string(&eml_file).unwrap();
+
+        assert_eq!(
+            eml,
+            concat!(
+                "From: NoBody <nobody@domain.tld>\r\n",
+                "Reply-To: Yuin <yuin@domain.tld>\r\n",
+                "To: Hei <hei@domain.tld>\r\n",
+                "Subject: Happy new year\r\n",
+                "Date: Tue, 15 Nov 1994 08:12:31 GMT\r\n",
+                "Content-Transfer-Encoding: 7bit\r\n",
+                "\r\n",
+                "Be happy!"
+            )
+        );
+        remove_file(eml_file).unwrap();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(
+    feature = "file-transport",
+    feature = "builder",
+    feature = "async-std1"
+))]
+mod asyncstd_1 {
+    use lettre::{AsyncFileTransport, AsyncStd1Executor, AsyncTransport, Message};
+    use std::{
+        env::temp_dir,
+        fs::{read_to_string, remove_file},
+    };
+
+    #[async_std::test]
+    async fn file_transport_asyncstd1() {
+        let sender = AsyncFileTransport::<AsyncStd1Executor>::new(temp_dir());
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
             .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())

tests/transport_sendmail.rs

@@ -1,14 +1,10 @@
 #[cfg(test)]
 #[cfg(all(feature = "sendmail-transport", feature = "builder"))]
-mod test {
-    use lettre::{transport::sendmail::SendmailTransport, Message};
-
-    #[cfg(feature = "tokio02")]
-    use tokio02_crate as tokio;
+mod sync {
+    use lettre::{Message, SendmailTransport, Transport};
 
     #[test]
     fn sendmail_transport() {
-        use lettre::Transport;
         let sender = SendmailTransport::new();
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
@@ -22,33 +18,76 @@ mod test {
         println!("{:?}", result);
         assert!(result.is_ok());
     }
+}
 
-    #[cfg(feature = "async-std1")]
-    #[async_std::test]
-    async fn sendmail_transport_asyncstd1() {
-        use lettre::AsyncStd1Transport;
+#[cfg(test)]
+#[cfg(all(
+    feature = "sendmail-transport",
+    feature = "builder",
+    feature = "tokio02"
+))]
+mod tokio_02 {
+    use lettre::{AsyncSendmailTransport, AsyncTransport, Message, Tokio02Executor};
+    use tokio02_crate as tokio;
 
-        let sender = SendmailTransport::new();
-        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")
-            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
-            .body(String::from("Be happy!"))
-            .unwrap();
-
-        let result = sender.send(email).await;
-        println!("{:?}", result);
-        assert!(result.is_ok());
-    }
-
-    #[cfg(feature = "tokio02")]
     #[tokio::test]
     async fn sendmail_transport_tokio02() {
-        use lettre::Tokio02Transport;
-
-        let sender = SendmailTransport::new();
+        let sender = AsyncSendmailTransport::<Tokio02Executor>::new();
+        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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let result = sender.send(email).await;
+        println!("{:?}", result);
+        assert!(result.is_ok());
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(
+    feature = "sendmail-transport",
+    feature = "builder",
+    feature = "tokio1"
+))]
+mod tokio_1 {
+    use lettre::{AsyncSendmailTransport, AsyncTransport, Message, Tokio1Executor};
+    use tokio1_crate as tokio;
+
+    #[tokio::test]
+    async fn sendmail_transport_tokio1() {
+        let sender = AsyncSendmailTransport::<Tokio1Executor>::new();
+        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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let result = sender.send(email).await;
+        println!("{:?}", result);
+        assert!(result.is_ok());
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(
+    feature = "sendmail-transport",
+    feature = "builder",
+    feature = "async-std1"
+))]
+mod asyncstd_1 {
+    use lettre::{AsyncSendmailTransport, AsyncStd1Executor, AsyncTransport, Message};
+
+    #[async_std::test]
+    async fn sendmail_transport_asyncstd1() {
+        let sender = AsyncSendmailTransport::<AsyncStd1Executor>::new();
         let email = Message::builder()
             .from("NoBody <nobody@domain.tld>".parse().unwrap())
             .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())

tests/transport_smtp.rs

@@ -1,6 +1,6 @@
 #[cfg(test)]
 #[cfg(all(feature = "smtp-transport", feature = "builder"))]
-mod test {
+mod sync {
     use lettre::{Message, SmtpTransport, Transport};
 
     #[test]
@@ -12,10 +12,87 @@ mod test {
             .subject("Happy new year")
             .body(String::from("Be happy!"))
             .unwrap();
-        SmtpTransport::builder_dangerous("127.0.0.1")
+
+        let sender = SmtpTransport::builder_dangerous("127.0.0.1")
             .port(2525)
-            .build()
-            .send(&email)
-            .unwrap();
+            .build();
+        sender.send(&email).unwrap();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(feature = "smtp-transport", feature = "builder", feature = "tokio02"))]
+mod tokio_02 {
+    use lettre::{AsyncSmtpTransport, AsyncTransport, Message, Tokio02Executor};
+
+    use tokio02_crate as tokio;
+
+    #[tokio::test]
+    async fn smtp_transport_simple_tokio02() {
+        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")
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let sender: AsyncSmtpTransport<Tokio02Executor> =
+            AsyncSmtpTransport::<Tokio02Executor>::builder_dangerous("127.0.0.1")
+                .port(2525)
+                .build();
+        sender.send(email).await.unwrap();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(feature = "smtp-transport", feature = "builder", feature = "tokio1"))]
+mod tokio_1 {
+    use lettre::{AsyncSmtpTransport, AsyncTransport, Message, Tokio1Executor};
+
+    use tokio1_crate as tokio;
+
+    #[tokio::test]
+    async fn smtp_transport_simple_tokio1() {
+        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")
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let sender: AsyncSmtpTransport<Tokio1Executor> =
+            AsyncSmtpTransport::<Tokio1Executor>::builder_dangerous("127.0.0.1")
+                .port(2525)
+                .build();
+        sender.send(email).await.unwrap();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(
+    feature = "smtp-transport",
+    feature = "builder",
+    feature = "async-std1"
+))]
+mod asyncstd_1 {
+    use lettre::{AsyncSmtpTransport, AsyncStd1Executor, AsyncTransport, Message};
+
+    #[async_std::test]
+    async fn smtp_transport_simple_asyncstd1() {
+        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")
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        let sender: AsyncSmtpTransport<AsyncStd1Executor> =
+            AsyncSmtpTransport::<AsyncStd1Executor>::builder_dangerous("127.0.0.1")
+                .port(2525)
+                .build();
+        sender.send(email).await.unwrap();
     }
 }

tests/transport_smtp_pool.rs

@@ -1,8 +1,6 @@
 #[cfg(all(test, feature = "smtp-transport", feature = "r2d2"))]
-mod test {
-    use lettre::address::Envelope;
-    use lettre::{SmtpTransport, Transport};
-
+mod sync {
+    use lettre::{address::Envelope, SmtpTransport, Transport};
     use std::{sync::mpsc, thread};
 
     fn envelope() -> Envelope {

tests/transport_stub.rs

@@ -1,14 +1,10 @@
 #[cfg(test)]
 #[cfg(feature = "builder")]
-mod test {
-    use lettre::{transport::stub::StubTransport, Message};
-
-    #[cfg(feature = "tokio02")]
-    use tokio02_crate as tokio;
+mod sync {
+    use lettre::{transport::stub::StubTransport, Message, Transport};
 
     #[test]
     fn stub_transport() {
-        use lettre::Transport;
         let sender_ok = StubTransport::new_ok();
         let sender_ko = StubTransport::new_error();
         let email = Message::builder()
@@ -22,32 +18,65 @@ mod test {
         sender_ok.send(&email).unwrap();
         sender_ko.send(&email).unwrap_err();
     }
+}
 
-    #[cfg(feature = "async-std1")]
-    #[async_std::test]
-    async fn stub_transport_asyncstd1() {
-        use lettre::AsyncStd1Transport;
+#[cfg(test)]
+#[cfg(all(feature = "builder", feature = "tokio02"))]
+mod tokio_02 {
+    use lettre::{transport::stub::StubTransport, AsyncTransport, Message};
 
-        let sender_ok = StubTransport::new_ok();
-        let sender_ko = StubTransport::new_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")
-            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
-            .body(String::from("Be happy!"))
-            .unwrap();
+    use tokio02_crate as tokio;
 
-        sender_ok.send(email.clone()).await.unwrap();
-        sender_ko.send(email).await.unwrap_err();
-    }
-
-    #[cfg(feature = "tokio02")]
     #[tokio::test]
     async fn stub_transport_tokio02() {
-        use lettre::Tokio02Transport;
-
+        let sender_ok = StubTransport::new_ok();
+        let sender_ko = StubTransport::new_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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        sender_ok.send(email.clone()).await.unwrap();
+        sender_ko.send(email).await.unwrap_err();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(feature = "builder", feature = "tokio1"))]
+mod tokio_1 {
+    use lettre::{transport::stub::StubTransport, AsyncTransport, Message};
+
+    use tokio1_crate as tokio;
+
+    #[tokio::test]
+    async fn stub_transport_tokio1() {
+        let sender_ok = StubTransport::new_ok();
+        let sender_ko = StubTransport::new_error();
+        let 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")
+            .date("Tue, 15 Nov 1994 08:12:31 GMT".parse().unwrap())
+            .body(String::from("Be happy!"))
+            .unwrap();
+
+        sender_ok.send(email.clone()).await.unwrap();
+        sender_ko.send(email).await.unwrap_err();
+    }
+}
+
+#[cfg(test)]
+#[cfg(all(feature = "builder", feature = "async-std1"))]
+mod asyncstd_1 {
+    use lettre::{transport::stub::StubTransport, AsyncTransport, Message};
+
+    #[async_std::test]
+    async fn stub_transport_asyncstd1() {
         let sender_ok = StubTransport::new_ok();
         let sender_ko = StubTransport::new_error();
         let email = Message::builder()