Post Office Protocol

In computing, the Post Office Protocol version 3 (POP3) is an application-layer Internet standard protocol used by local e-mail clients to get e-mail from a remote mail server over a TCP/IP connection. POP3 and IMAP4 (Internet Message Access Protocol) are the two most widely accepted Internet standard protocols for e-mail retrieval. Almost all modern e-mail clients and servers support both.
Many years ago, a proposal was outlined for a POP4 specification, complete with a working server implementation. The proposed POP4 extension adds 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 lacks. No progress has been found in the POP4 specification since 2003.