Forwarding undeliverable mail to a different server in Exim 3

Hi,

We're actually migrating from Exim to Exchange (sorry!) and as part of
the gradual migration I need to be in the situation where some users
are on the UNIX system and some on Exchange. I want to basically set
up the Exim system so that any mail that comes in and is undeliverable
on that server gets forwarded to the Exchange server.

I've looked at my Exim 3 book and on the net but can't find a clear
and concise way to configure this system, which I believe is called
smarthosting. It appears to require a new router adding to the /usr/
local/etc/exim/configure file and the daemon then restarting but I've
tried loads of things but just can't get the damned thing forwarding.
It just generates an undeliverable message and doesn't forward the
message to Exchange.

Can anyone help me?