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

(1845)	SMTP Service Extension for Checkpoint/Restart			(this looks interesting, feel free to send a patch)
1870	SMTP Service Extension for Message Size Declaration
2034	SMTP Service Extension for Returning Enhanced Error Codes
2045ff  Multipurpose Internet Mail Extensions (MIME)
2505	Anti-Spam Recommendations for SMTP MTAs
2822	Internet Message Format
2920	SMTP Service Extension for Command Pipelining
3030	SMTP Service Extensions for Transmission of Large and Binary MIME Messages	(CHUNKING is already supported)
3207	SMTP Service Extension for Secure SMTP over Transport Layer Security
3463	Enhanced Mail System Status Codes
3848	ESMTP and LMTP Transmission Types Registration
4409	Message Submission for Mail
4954	SMTP Service Extension for Authentication
5248	A Registry for SMTP Enhanced Mail System Status Codes
5321	Simple Mail Transfer Protocol
5782	DNS Blacklists and Whitelists
6152	SMTP Service Extension for 8bit-MIMEtransport
6698	The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA
7208	Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1
7218	Adding Acronyms to Simplify Conversations about DNS-Based Authentication of Named Entities (DANE)