[Dovecot] Dovecot, postifx, virtual domains and deliver, what about luser_relay/fallback_relay?

I've read documentation (obviously not the right parts) and googled, but I can't seem how to solve this simple problem:

I have a machine running postfix/dovecot and using virtual domains, delivery happens with dovecot's LDA. I also have an Exchange machine that I have to house some of the users of a given domain.

How can I tell either postifx in general or dovecot's LDA that whatever specific user that cannot be found in the virutal setup can be pawned off to a given relay-server which will sort it out?

-A