Simple Mail Transfer Protocol.

• RFC 5321 – The Simple Mail Transfer Protocol (makes RFC 2821 and RFC 821 obsolete as of 2008)
• RFC 5322 – Internet (i.e. e-mail) Message Format (makes RFC 2822 and RFC 822 obsolete as of 2008)
• RFC 1869 – Defines the capability for SMTP service extensions, creating Extended SMTP, or ESMTP
• RFC 1891 – Delivery Status Notification (DSN) extension to SMTP
• RFC 1892 – The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages
• RFC 1894 – An Extensible Message Format for Delivery Status Notifications
• RFC 1895 – The Application/CALS-1840 Content-type
• RFC 3463 – Enhanced Status Codes for SMTP (makes RFC 1893 obsolete)
• RFC 1854 – SMTP service extension for Command Pipelining
• RFC 2487 – SMTP Service Extension for Secure SMTP over TLS
• RFC 2505 – Anti-Spam Recommendations for SMTP MTAs
• RFC 2554 – SMTP Service Extension for Authentication
• RFC 2920 – SMTP Service Extension for Command Pipelining
• RFC 3030 – SMTP Service Extensions for Transmission of Large and Binary MIME Messages
• RFC 3207 – SMTP Service Extension for Returning Enhanced Error