The popd server is a fast, full, BSD-licensed RFC1939 implementation.

Popd allows mail for users in multiple domains to be served from
one server in `virtual' mode.  It allows for mail to be expired
from the server when users collect mail older than a specified
time, falsification of UIDL information and removal of old mail.

Additional features include SSL and proxy support, as well as
support for a variety of mailbox formats.  The bulletin feature
is nonfunctional in this release.