postfix relay mail if user not exist

[vanio_inn]

Hi, I have a debian server with postfix installed and an exchange server(sux but doesn't depend on my decision). So I need to configure postfix to relay mail to the exchange server when it recieves mail to not existing user. For example:
mail -> postfix ->(if no such user) redirect -> exchange
Any idea how to do this?

[scathefire]

You may want to look into a split domain setup. Its going to be difficult, because a mail server handling mail to a domain when it can't find a mailbox assumes the user does not exist.

Ideally, if the exchange server or the postfix server were a different domain, you could just handle them with aliases. you can still send mail to a particular user on a domain, but it will forward it over to the domain the exchange server is running.