Forgive me if I'm missing something obvious here..... but why do most linux server distros come with both Dovecot AND Postfix (or sendmail)? As far as I'm aware all three of them are Mail Transfer Agents, with Dovecot having a 'secondary' function of being a Mail Delivery Agent...
Is Dovecot just not a very good MTA? Or is there some other reason why you'd want to use a combination of the above instead of a single program that seemingly does everything?