POP3, defined in [RFC 1939], is an incredibly simple mail access protocol. Because the protocol is so simple, its functionality is rather limited. POP3 begins when the user agent (the client) opens a TCP connection to the the mail server (the server) on port 110. With the TCP connection established, POP3 progresses through three phases: authorization, transaction and update. During the first phase, authorization, the user agent sends a user name and a password to authenticate the user downloading the mail. During the second phase, transaction, the user agent retrieves messages; also during the transaction phase, the user agent can mark messages for deletion, can remove deletion marks and obtain mail statistics. The third phase, update, occurs after the client has issued the quit command ending the POP3 session; at this time, the mail server deletes the messages that were marked for deletion. In a POP3 transaction, the user agent simply downloads your mail from a server to your client. At this point the mail is then stored on your local machine.