This documentation provide summary and notes on implementation of SMTP service extension for secure SMTP over Transport Layer Security (TLS) as defined in RFC3207.
1. Service Extension
The EHLO keyword value associated with the extension is "STARTTLS" with no parameter.
A new SMTP command "STARTTLS" is defined.
A publicly-referenced SMTP server (on port 25) MUST NOT require use of the STARTTLS extension in order to deliver mail locally.
2. STARTTLS command
2.1.1. Success Response
"220" SP *text CRLF
After receiving a 220 response to a STARTTLS command, the client MUST start the TLS negotiation before giving any other SMTP commands. If, after having issued the STARTTLS command, the client finds out that some failure prevents it from actually starting a TLS handshake, then it SHOULD abort the connection.
2.1.2. Error Response
454 TLS not available due to temporary reason
501 Syntax error (no parameters allowed)
If the client receives the 454 response, the client must decide whether or not to continue the SMTP session.
A SMTP server that is not publicly referenced may choose to require that the client perform a TLS negotiation before accepting any commands. In this case, the server SHOULD return the reply code:
"530 Must issue a STARTTLS command first" CRLF
to every command other than NOOP, EHLO, STARTTLS, or QUIT. If the client and server are using the ENHANCEDSTATUSCODES ESMTP extension [RFC2034], the status code to be returned SHOULD be 5.7.0.
3. Post TLS Handshake
The client MUST discard any knowledge obtained from the server, such as the list of SMTP service extensions, which was not obtained from the TLS negotiation itself. The client SHOULD send an EHLO command as the first command after a successful TLS negotiation.
The list of SMTP service extensions returned in response to an EHLO command received after the TLS handshake MAY be different than the list returned before the TLS handshake.
A client MUST NOT attempt to start a TLS session if a TLS session is already active.
The server MUST discard any knowledge obtained from the client, such as the argument to the EHLO command, which was not obtained from the TLS negotiation itself.
A server MUST NOT return the STARTTLS extension in response to an EHLO command received after a TLS handshake has completed.
4. Security Considerations
If the SMTP client decides that the level of authentication or privacy is not high enough for it to continue, it SHOULD issue an SMTP QUIT command immediately after the TLS negotiation is complete.
If the SMTP server decides that the level of authentication or privacy is not high enough for it to continue, it SHOULD reply to every SMTP command from the client (other than a QUIT command) with,
"554 Command refused due to lack of security" CRLF
the server may choose to not accept any more SMTP commands.