POP & POP3: Post Office Protocol (version 3)

The Post Office Protocol (POP) is designed to allow a workstation(PC) to dynamically access a maildrop on a server host. POP3 is the version 3 (the latest version) of the Post Office Protocol. In other words, POP3 allows a workstation to retrieve mail that the server is holding for it. POP3 transmissions appear as data messages between stations. The messages are either command or reply messages.

There are several different technologies and approaches to building a distributed electronic mail infrastructure. Among them: POP (Post Office Protocol), DMSP (Distributed Mail System Protocol), and IMAP (Internet Message Access Protocol). Of the three, POP is the oldest and consequently the best known. DMSP is largely limited to a single application, PCMAIL, and is known primarily for its excellent support of "disconnected" operation. IMAP offers a superset of POP and DMSP capabilities, and provides good support for all three modes of remote mailbox access: offline, online, and disconnected.

POP was designed to support "offline" mail processing, in which, mail is delivered to a server, and a personal computer user periodically invokes a mail "client" program that connects to the server and downloads all of the pending mail to the user's own machine. The offline access mode is a kind of store-and-forward service, intended to move mail (on demand) from the mail server (drop point) to a single destination machine, usually a PC or Mac. Once delivered to the PC or Mac, the messages are then deleted from the mail server.

Optional POP3 Commands:

APOP name digest 

valid in the AUTHORIZATION state

TOP msg n  

valid in the TRANSACTION state

UIDL [msg]

POP3 Replies:   

+OK  -ERR

Reference