From The Free On-line Dictionary of Computing (30 December 2018) :
Post Office Protocol
POP server
(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.
[Details?]
(2007-03-07)