Post Office Protocol version 3 (POP3) is a simple protocol that users can use to download emails from their mailbox on the mail server to local mailboxes on their PC. It is intended to work offline with the mail server. POP3 is specified in RFC 1939.
A client establishes connection with the well-known TCP port 110 of a server. After this connection has been established, the server introduces itself, and is in the Authentication state (i.e., waiting for an authentication from its user). For example:
+ OK QPOP (version 2.1.4-R4-b5a) at t1.pvt.cz starting <[email protected]>
Basic authentication is done with a username and password. If authentication is valid, communication proceeds to the Transaction state, where clients can work with messages in their mailbox on the server. Even if clients, for example, delete some messages from the mailbox on the server, such deletions are not permanent during the transaction state. At the end of the session, clients will have...