Most Ispconfig setups (Perfect server) use postfix as mta and dovecot or courier as imap/pop3 daemon and Squirrelmail as webmail client. See http://www.ispconfig.org/page/en/documentation.html for anyone else interested in setups. I am guessing the OP is already past this point.
They have a plugin for Roundcube that ingrates into ispconfig database. It as little extra work but not hard. https://github.com/w2c/ispconfig3_roundcube Same for squirrelmail
I personal use Horde as my webmail client. Pain to setup!