I'm currently working on a large email system which will serives millions of mailboxes - like hotmail.com.
Except sending/receiving emails, the most important feature is spam and virus filtering. I'm using SpamBayes for spam filtering and AnomySanitizer plus ClamAV for virus filtering. They look working well.
Here's a Chinese document about how to configurate SpamBayes, AnomySanitizer and ClamAV on an exists email system.
The next problem is to make the email system effective and scalable. Anyone has suggestion?