Post Office Protocol/POP3

In email and computing, POP and POP3 refer to the Post Office Protocol.  POP is an application-layer Internet standard protocol.  Local email clients use this standard protocol to retrieve email from a remote server over a TCI/IP (transmission control protocol/internet protocol) connection. POP is one of the two most prevalent standard protocols for email retrieval, alongside IMAP (internet message access protocol). POP has been through several iterations or versions; the current standard is version 3, or POP3.  Most web mail providers can use both IMAP and POP3 service.

POP retrieval supports simple maildrop, or download-delete requirements, necessary to access remote mailboxes. Typically, a POP service will connect to a server, retrieve all messages, and store them on the email user’s computer as new messages. A web user’s available messages are fixed when a POP session opens a maildrop.  When this occurs, messages are identified by a message-number that is specific to that session, or by a unique identifier that the POP server assigns to each message. These unique identifiers are permanent and allow users to access the same message in different sessions via an application service provider.