Can I ask what your email stack was? I'm working on a project that's a cross between TOR and bit torrent to provide free (encrypted) email to the masses that doesn't monetize data. I've been on a crash course of all things email lately.
I'm still in the process of trialling new providers. I'll be using fetchmail along with my existing procmailrc. The part I'm outsourcing is the spam filtering and availability on the MX.
1. Arch Linux.
2. OpenSmtpd.
3. SpamPD
4. DkimProxy
5. Dovecot
This replaced a Postfix stack that I was using. I miss the use of Postgrey, the greylisting daemon, and I seem to get more spam because of this.