[Exim] Help with virtual domains

I have looked through the exim archives and I can't seem to find an
implementation of virtual domains that matches what I need. We are
currently using an old hacked up version of sendmail to do this and I'm
looking to use Exim as a replacement. This is what I need to accomplish:

I want to be able to do this with two dbm's. 1 local domain dbm and 1
user dbm.

Format of domain file would be:

domaina.com
domainb.com
domainc.com

* And of course use exim_dbmbuild to actually create the dbm's

Format of user file would be:

user1@???: whatever@???
user2@???: someone@???
*@domainA.com: whocares@???
user1@???: anyone@???
user2@???: boss@???
*@domainB.com: anything@???
etc...

Their would be no local mail on the machine, it is only used as a
forwarding service. The only other criteria that is important is that it
must be secure against 3rd party relaying. (of course)

Anyone have an example of this in action? Or know how I could go about
making it happen?

Thanks much,

-Rick