Um ... postfix is a MTA whereas Dovecot is a POP3/IMAP server -- they do totally different things. But more recent releases of Dovecot allow for the user list to be stored in a MySQL database. If you want to try with Dovecot for your IMAP/POP3 needs, I heartily recommend reading www.dovecot.org
and using the most recent 0.99 version.
Can't help you with the Postfix since I'm a Sendmail user -- sorry.