Post Office Protocol, POP – POP3

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In computer terms, Post Office Protocol (POP) is a protocol used by e-mail clients to retrieve e-mail from a server by a TCP/IP connection. Another e-mail retrieving protocol is Internet Message Access Protocol (IMAP). Basically all modem e-mail clients and servers use and support both protocols. The POP protocol has several versions with POP3 as the current version.

POP (POP1) was enacted in 1984, where users have an option to leave e-mail on a server after download or have the option to delete the message. The typical scenario is the user will connect, retrieve messages and store them on the user’s PC, delete them from the server, then disconnect.

Deletion of the message is marked so when the user disconnects the message will be deleted from the server.

POP2 was initiated in 1985.

POP3 was initiated in 1988 and is the current protocol. The original POP3 used an unencrypted login with a user and password access system. Today the POP3 protocol supports several different authentication methods to access a user’s e-mail.

A proposal for POP4 has been outlined but since 2003 has made no progress.

Another proposal for the POP3 was to offer extensions with POP3 but this was discouraged and it was established that the role of POP3 was mainly for users to retrieve, download, and delete messages within their mailbox.

A couple of extensions are as follows:

STARTTLS: This allows the use of Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on the POP3 port instead of another.

SDPS: This allows multiple accounts per domain known as Standard Dial-up POP3 Service (SDPS).