I've (finally?) taken the time to write some more sections on the content filtering howto for postfix. It is still not finished yet, but contains much of the configuration information. So, if you like to experiment, you might want to give it a try.

Furthermore, I've received a (several actually) e-mails from “Alexander Hoogerhuis” about my Postfix, Dovecot, PostgreSQL, PAM how to. He had some suggestions and improvements about the Dovecot part. I've tested some of it, and will add comments about them soon in the appropriate document.