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.
POP3 is not designed to provide extensive manipulation operations of mail on the server; which are done by a more advanced (and complex) protocol IMAP4. POP3 uses TCP as the transport protocol.
Protocol Structure
POP3 Command Summary:
| Commands | Description |
| USER | Name of user |
| PASS | Password's users |
| STAT | Information on messages in the server |
| RETR | Number of message to get |
| DELE | Number of message to delete |
| LIST | Number of message to show |
| TOP <messageID> <nombredelignes> | Print X lines of the message starting from the beginning (header included) |
| QUIT | Exit to POP3's 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
Related Terms: SMTP, IMAP, TCP
Sponsor Source: POP3 is defined by IETF (http://www.ietf.org) RFC 1939.
Reference: http://www.javvin.com/protocol/rfc1939.pdf: Post Office Protocol - Version 3