Great news on the email proxy front: Dovecot is a serious contender!
I’ve been exploring the use of Dovecot as a SASL authentication server for Postfix so I’ve been reading over the documentation. Turns out that Dovecot is a really amazing system! In addition to being an IMAP(s), POP3(s) server, it can also function as a proxy for those same protocols, as well as a local delivery agent for mail transfer agents like Postfix and Exim.
Too cool, way too cool!