• Postfix
• Mailbox_management

Postfix/Mailbox_management

There's a bajillion products out there for doing this, all with various levels of integration and do-it-all-ness. Some links.

• Korreio - http://korreio.sourceforge.net/

• http://python-cyrus.sourceforge.net/

• Gross greylister - http://code.google.com/p/gross/

• MailNG - http://projects.koalabs.org/trac/mailng/

• pfxpold "extensible policy daemon" - http://sourceforge.net/projects/pfxpold/