[exim] Sending Duplicate mail to Multiple Mail servers

> We have a client that we relay mail to and they are in the process of
> transitioning from a NT server to a Sun server however during the
transition
> they would like to run the 2 servers side by side for a month or so just
in
> case they have problems with there new Sun server. How would I relay
> duplicate mail to both these servers?

I think this has been discussed on the mailing list in the past, and
people have come up with clever schemes, but I can't remember them
offhand.

One idea that comes to mind is to use the "unseen" feature of routers to
specify delivery to one server, so that you can then use another router
for the other server. Your situation sounds straightforward enough, so I
think that could work.

Could anyone tell me how to implement the above suggestion?