Postbus Supported RFC's This is the list of RFC's which are intended to be supported, shown in order of importance. RFC 5321 - Simple Mail Transfer Protocol RFC 3207 - SMTP Service Extension for Secure SMTP over Transport Layer Security RFC 6152 - SMTP Service Extension for 8-bit MIME Transport RFC 2920 - SMTP Service Extension for Command Pipelining RFC 6531 - SMTP Extension for Internationalized Email