As far as I've seen it's possible with postfix, although you'll need a different authentication method for your IMAP. You may want to look into something like Courier-IMAP and google for some HowTo's.
If you're so inclined you may also want to look into qmail + couirer. There is some good linkage at your local qmail.org mirror regarding virtual users. The mailbox structure is something like /virtualdomain/name with a buch of .qmail-* alaises.
That will at least give you the ability to use the virtual users. An interface to manage them is another animal, but it is possible as I've used one for admin'ing virtual users when we were on telus.net that smelled a lot like a qmail setup.
Last edited by mcleodnine; 05-30-2003 at 12:57 PM.