I am using nwebmail (
http://nwebmail.sourceforge.net/) on my server and it
works very well. It directly deals with user mailboxes and talks to
'sendmail' (which points to exim, hehe). Only thing you have to do is chmod
1777 /var/spool/mail and tell exim to be "group = mail" and use "mode =
660" in the local_delivery transport.
--Ben