The Post Office Protocol (POP) is a standard protocol specified in several RFCs for transferring e-mails from the mail server to the user's mail client. The POP protocol works in conjunction with the SMTP protocol and has been superseded by the more efficient IMAP protocol.
Most PC-based mail clients use the Post Office Protocol (POP) to download email from the server to the workstation. The e-mails are cached by the sender in the POP server of the sender's provider, loaded from there into the POP server of the recipient's provider and cached until they are picked up by the recipient. To transfer the mail, the workstation establishes the connection to the POP server and retrieves the mail.
The Post Office Protocol exists in the versions POP2 and POP3. The current POP3 version has the advantage that the e-mails are temporarily stored until the user picks them up. The e-mails loaded onto his workstation can then be read offline, i.e. without an existing Internet connection. For authentication, POP3 uses the user name and password written in plain text.Authenticated Post Office Protocol (APOP), which cryptographically encrypts authentication to protect against unauthorized eavesdropping.