The Post Office Protocol (POP) is a standard protocol specified in several RFCs for transferring e-mail 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 The POP protocol works in conjunction with the SMTP protocol and has been superseded by the more efficient IMAP protocol.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 load e-mail 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 picks up the mail.
The Post Office Protocol exists in the versions POP2 and POP3. The current version POP3 has the advantage that the e-mails are stored temporarily 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.
An improved level of security is provided by the POP3S protocol. This is an enhanced POP3 protocol with Secure Socket Layer( SSL) and Transport Layer Security( TLS). Another POP protocol with enhanced security is the Authenticated Post Office Protocol( APOP), which cryptographically encrypts authentication to protect against unauthorized eavesdropping.