Post Office Protocol version 3, usuallyabbreviated to POP3, is defined in RFC 1939 and was designed…"to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it." (RFC 1939)
The process employed by POP3 is quite straightforward. A server starts the POP3 service by listening on TCP port 110. When a client wishes to use the POP3 service it establishes a connection with the server using that port. The POP3 server replies with a greeting and the client and server then exchange commands and responses until the connection is closed. The POP3 protocol has less than a dozen commands and its purpose is simply to allow for the retrieval of messages from a maildrop server.
The most important thing to remember about POP3 and SMTP is that SMTP is a mechanism used for sending and receiving messages and that POP3 is used for storing and retrieving messages. It should also be noted that an SMTP receiver receives all mail for a domain, whereas POP3 clients retrieve only mail from the user's individual mailbox.