This probally quite a simple and fast change to make.
But when you create a virtual domain you should have to option to specify an email address as part of the domain.
If a domain has a email address, all email sent to the domain is forwarded to the email address.
If no email is specified then email for that domain is treated as normal.
Yes I know there is a HOWTO on doing that but editing the files manually is fine for me but other people (who know nothing about linux) may not be able to.
I'm halfway through writing it myself but it would be nice if it was part of the standard distribution.