RfCs interesting for a mail server (and used for implementing this one):

1652	SMTP Service Extension for 8bit-MIMEtransport
(1845)	SMTP Service Extension for Checkpoint/Restart			(this looks interesting, feel free to send a patch)
1870	SMTP Service Extension for Message Size Declaration
1893	Enhanced Mail System Status Codes
2034	SMTP Service Extension for Returning Enhanced Error Codes
2505	Anti-Spam Recommendations for SMTP MTAs
2554	SMTP Service Extension for Authentication
2821	Simple Mail Transfer Protocol
2822	Internet Message Format
2920	SMTP Service Extension for Command Pipelining
(3030)	SMTP Service Extensions for Transmission of Large and Binary MIME Messages	(looks good, waiting for a patch)
3207	SMTP Service Extension for Secure SMTP over Transport Layer Security
3848	ESMTP and LMTP Transmission Types Registration