I have a setup at my school with proves to be pretty robust. For the MTA I use Qmail (for it's security and speed) and mail storage in the Maildir format. I use Maildir because it is a lot faster, and handles crashes better. Qmail comes with it's own POP3 daemon, which also works with the Maildir. I use the UCSPI-TCP package's tcpserver instead of inetd to run Qmail's SMTP and POP daemons for it's added security, configurability, and speed.

For and IMAP server I use the Courier IMAP package. It is a small, featurefull server which support Maildir. It would probably be fine for your needs as it has a very well designed authentication model which supports LDAP (among other things). Courier is faster and smaller than UW-IMAP and Cyrus.

Finally I use TWIG for Webmail. It supports IMAP mail, calendars, contact lists, newsgroups, etc. It is also based on PHP (yay!).

It is a very nice setup, and wasen't too hard to setup. Oh, I forgot to mention that the server is FreeBSD 3.4.