Hi!

In my oppinion cyrus imapd is the best one, it is the clear leader in performance...
I run cyrusimapd, portfix, amavisd-new(virus and spam-scanning), horde/imp(webmail) and openldap.
Dont go the database way to authenticate users, ldap is the future...

One way to go could be the kolab-server, its is the programs I have packaged to work like i.e. microsoft exchange.

Fredrik