Sam Varshavchik has released his MTA, which is called Courier. See:
http://courier.sourceforge.net
(Is sourceforge always this slow?)
Sam is the author of maildrop, courier-imap, and sqwebmail.
Courier is still prerelease--though I don't know whether it's
considered alpha or beta--but it looks pretty interesting.
It borrows heavily from qmail (maildirs, extension addresses,
modularity, delivery instructions) but adds a lot (DSN, filtering,
IMAP, POP, webmail) and "fixes" things Sam thought qmail did wrong
(single-RCPT delivery, always-send-8, licensing, no bouncing of local
recipients during SMTP dialogue, not actively maintained).
I've grabbed it, but I haven't installed it anywhere yet. I don't have
a lot of experience with Sam's code: procmail has always been good
enough for me, and courier-imap was too functionally limited for the
one IMAP installation I set up. The Courier web pages don't say much
about security other than there are four setuid root modules.
Has anyone checked it out?
-Dave
