POPular is a suite of programs for setting up large POP3 server systems. It
consists of a POP3 proxy, a storage server and lots of utility programs. Note
that this program is intended to be used in large email systems and is
therefore quite complex to configure. If you only have a few thousand
mailboxes, you are probably better of with a different POP3 server.

WWW: http://www.remote.org/jochen/mail/popular/