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)