Forwarding unhandle mail to another server

[Nicolas MONNET]

Oh I have a Linux box running qmail handling part of my mail account for
one domain, and I want undelivered mail to be forwarded to an Exchange
machine. 

    LL ---- LINUX ----- NT

 Mail =====>     ----->

If an account does'nt exist on the Linux machine, it should be transferred
to the NT machine. 

Any idea?