Comment A plesant configuration.. (Score 1) 18
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.