I think you guys should offer the option to install dovecot instead of courier mail server. Dovecot is much better in terms of security, stability, server load and so on. Also the sieve protocol allows server-side filtering of emails which is really powerful. An interface in plesk to manage the server side filters rould be really nice. I did integrate dovecot and roundcube webmail with sieve filter support in under 30 mins on my plesk 10 server, so it's no problem at all, but directly integrated to plesk and the ui would be really really nice.