The Post Office Protocol version 3 (POP3) is one of the most common protocols to retrieve emails from a mail server. In opposition to the more modern IMAP4 protocol, it doesn't support the concept of folders.
POP3 allows receiving emails via a TCP/IP connection. It is much simpler structured than the IMAP protocol, so it is much easier to implement.
Normally clients retrieve all messages and then delete them from the server, though PHP3 also offers a way to keep the messages on the server once they are downloaded. Nearly all modern email servers and clients support the POP3 protocol.
POP was initially specified in 1984. The current version of the protocol is defined in RFC 1939 and has several extensions.