Implement creating SmtpTransport using STARTTLS

Cargo.toml

@@ -71,5 +71,9 @@ name = "smtp"
 required-features = ["smtp-transport"]
 
 [[example]]
-name = "smtp_gmail"
+name = "smtp_tls"
+required-features = ["smtp-transport", "native-tls"]
+
+[[example]]
+name = "smtp_starttls"
 required-features = ["smtp-transport", "native-tls"]

examples/smtp_starttls.rs

@@ -0,0 +1,26 @@
+use lettre::transport::smtp::authentication::Credentials;
+use lettre::{Message, SmtpTransport, Transport};
+
+fn main() {
+    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("Be happy!")
+        .unwrap();
+
+    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+
+    // Open a remote connection to gmail using STARTTLS
+    let mailer = SmtpTransport::starttls_relay("smtp.gmail.com")
+        .unwrap()
+        .credentials(creds)
+        .build();
+
+    // Send the email
+    match mailer.send(&email) {
+        Ok(_) => println!("Email sent successfully!"),
+        Err(e) => panic!("Could not send email: {:?}", e),
+    }
+}

src/transport/smtp/transport.rs

@@ -5,7 +5,7 @@ use r2d2::{Builder, Pool};
 
 use super::{ClientId, Credentials, Error, Mechanism, Response, SmtpConnection, SmtpInfo};
 #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
-use super::{Tls, TlsParameters, SUBMISSIONS_PORT};
+use super::{Tls, TlsParameters, SUBMISSIONS_PORT, SUBMISSION_PORT};
 use crate::{Envelope, Transport};
 
 #[allow(missing_debug_implementations)]
@@ -50,6 +50,19 @@ impl SmtpTransport {
             .tls(Tls::Wrapper(tls_parameters)))
     }
 
+    /// Simple and secure transport, should be used when the server doesn't support wrapped TLS connections.
+    /// Creates an encrypted transport over submissions port, by first connecting using an unencrypted
+    /// connection and then upgrading it with STARTTLS, using the provided domain to validate TLS certificates.
+    /// If the connection can't be upgraded it will fail connecting altogether.
+    #[cfg(any(feature = "native-tls", feature = "rustls-tls"))]
+    pub fn starttls_relay(relay: &str) -> Result<SmtpTransportBuilder, Error> {
+        let tls_parameters = TlsParameters::new(relay.into())?;
+
+        Ok(Self::builder_dangerous(relay)
+            .port(SUBMISSION_PORT)
+            .tls(Tls::Required(tls_parameters)))
+    }
+
     /// Creates a new local SMTP client to port 25
     ///
     /// Shortcut for local unencrypted relay (typical local email daemon that will handle relaying)