(POP) A protocol designed to allow single-user computers to retrieve electronic mail from a POP server via TCP/IP. The default port is 110.
The POP server might be a computer with a permanent Internet connection whereas its clients might only connect to it occasionally, e.g. by modem.
There are (in 1994) three versions: POP, POP2, and POP3. Later versions are NOT compatible with earlier ones.