From the DESIGN-document:

Well, the goals themselves are obvious; they're probably the same for most
other POP3 servers as well. It's their priority that differs. For popa3d,
the goals are:

1. Security (to the extent that is possible with POP3 at all, of course).
2. Reliability (again, as limited by the mailbox format and the protocol).
3. RFC compliance (slightly relaxed to work with real-world POP3 clients).
4. Performance (limited by the more important goals, above).

popa3d can be run from inetd, or as a stand-alone server.

Note that popa3d only uses flock(2) for locking and may not be safe
on NFS or with mail systems that only use lock files.

Homepage:
https://www.openwall.com/popa3d/
