I can really really recommend qmail. http://www.qmail.org
Although it might take some time to get familiar with it, you will find it
very easy to configure. It performs very well, we managed lists with 100K+
deliveries a day on a simple Pentium/singe HDD box.
Concerning security, qmail is my first choice. As someone said on this list,
the qmail author (Dan Bernstein) "is so paranoid that he even avoids linking
against the standard C libraries" :-).
You will also like its modular architecture, which allows a lot of setups.
For example, you can use "lightweight" qmail setups on several machines in
order to tunnel all kind of mail to a central mail server.
POP3: Check out the vmailmgr package; it is designed to work with qmail's
maildirs. I have to admit that I am not actively reading the mailing list,
but vmailmgr seems to be actively supported and extended. I think there were
some postings on the list concerning IMAP with vmailmgr/qmail/?.
http://www.vmailmgr.org/
Also don't miss to check ezmlm for mailing list management.
Web based mail: Hm, forgot about that... I think I saw some stuff at
freshmeat, but I don't have any experience with it.
Hope this helps,
Matthias
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w e b f a c t o r y G m b H
Matthias Pigulla