- [Post Office Protocol - Version 3, RFC 1939](https://tools.ietf.org/html/rfc1939)
- [Simple Mail Transfer Protocol, RFC 5321](https://tools.ietf.org/html/rfc5321)
- [SMTP Service Extension for Secure SMTP over Transport Layer Security, RFC 3207](https://tools.ietf.org/html/rfc3207)
+- [POP3 Extension Mechanism, RFC 2449](https://tools.ietf.org/html/rfc2449)