Post Office Protocol
RFC 5034 - The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism
RFC 3206 - SYS and AUTH POP Response Codes
RFC 2595 - Using TLS with IMAP, POP3 and ACAP
RFC 2449 - POP3 Extension Mechanism
RFC 2384 - POP URL Scheme
RFC 2195 - IMAP/POP AUTHorize Extension for Simple Challenge/Response
RFC 1957 - Some Observations on Implementations of POP3