I've not looked into either sendmail or postfix for large numbers of users, so I can't answer that for you. However, I am quite sure that pop3/smtp can be achieved in Postfix via "virtual accounts".

I've been having the strangest problem with my mail server running a combo of dovecot and postfix on FC6.<BR><BR>All mail seems to land and deliver fine at my server for the domains it's supposed to ...