History
POP (POP1) is specified in RFC 918 (1984), POP2 by RFC 937 (1985). The original specification of POP3 is RFC 1081 (1988). Its current specification is RFC 1939, updated with an extension mechanism, RFC 2449 and an authentication mechanism in RFC 1734.
POP2 has been assigned well-known port 109.
The original POP3 specification supported only an unencrypted USER/PASS login mechanism or Berkeley .rhosts access control. POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail. Most are provided by the POP3 extension mechanisms. POP3 clients support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version.
RFC 1460 introduced APOP into the core protocol. APOP is a challenge/response protocol which uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera Mail, Eudora, KMail, Novell Evolution, RimArts' Becky!, Windows Live Mail, PowerMail, Apple Mail, and Mutt.
An informal proposal had been outlined for a "POP4" specification, complete with a working server implementation. This "POP4" proposal added basic folder management, multipart message support, as well as message flag management, allowing for a light protocol which supports some popular IMAP features which POP3 currently lacks. However, in doing so, it shared with IMAP the embedding in a communication protocol a specific model of a mailbox, which, although common, is not universal. No progress has been observed in this "POP4" proposal since 2003. `
Read more about this topic: Post Office Protocol
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