Post Office Protocol (POP) is a client/server protocol that allows client e-mail software to retrieve e-mail from a server that receives and stores it. The first version of the protocol was issued in October 1984, and the most recent version, POP3, was introduced in November 1988.

A POP3 server provides e-mail services to e-mail client software. When an e-mail client needs to use the services of a POP3 server, it opens a connection over the Internet and waits for a greeting from the server. The sequence of a POP3 session progresses through three states--the authorization state, the transaction state, and the update state.

In the authorization state, the client receives a greeting from the server and sends the user name and password to access a specific mailbox. If the user name and password match the server's records, the server authorizes the client to access the e-mail messages.

In the transaction state, the client requests to read the messages. The server opens the mailbox with an exclusive lock and transfers the messages to the computer running the e-mail client software. These messages can be read immediately or stored for future, offline reading.

The update state begins when the client software terminates the POP3 session. The server removes e-mail messages marked as "deleted" by the client software and releases the lock on the mailbox. This completes the POP3 session.