feat(mailer-smtp): add SCRAM-SHA-256 auth, Ed25519 DKIM, opportunistic TLS, SNI cert selection, pipelining and delivery/bridge improvements

rust/crates/mailer-smtp/src/command.rs

@@ -50,6 +50,7 @@ pub enum SmtpCommand {
 pub enum AuthMechanism {
     Plain,
     Login,
+    ScramSha256,
 }
 
 /// Errors that can occur during command parsing.
@@ -218,6 +219,7 @@ fn parse_auth(rest: &str) -> Result<SmtpCommand, ParseError> {
     let mechanism = match mech_str.to_ascii_uppercase().as_str() {
         "PLAIN" => AuthMechanism::Plain,
         "LOGIN" => AuthMechanism::Login,
+        "SCRAM-SHA-256" => AuthMechanism::ScramSha256,
         other => {
             return Err(ParseError::SyntaxError(format!(
                 "unsupported AUTH mechanism: {other}"