This article discusses how to build an advanced mail server which sports all of the latest mail protocols, checks all incoming mail for spam, and scans all incoming and outgoing mail for viruses.
The wonderful folks over at Inter7 developed vpopmail to handle the management of virtual domains using Qmail. Unfortunately vpopmail is limited to a measly 23 million virtual hosts, of which each are limited to only 23 million users. So if you have more than 529 trillion users you may need to look elsewhere.
Other Good URLs on qmail:
Dave Sill's "Life with qmail" site ***FREE
Dave Sill's "qmail Handbook" at amazon.com
Richard Blum's "Running qmail" at amazon.com