[exim] exchange by default..

Hello all,

I am allowing another departments Exchange server to relay out via our
exim server.

Their exchange server is (by default) setup to accept all messages to anyuser@???
and then bounce the names that do not exist.

They take the mails in themselves. As I do not have the 'mx' entry
for them I can not pre-filter or recipient verify what they take in.

What I am trying to figure out how to do is bounce mail from their IP
and <> , and only that combination. I still want to take other
departments email which has the <> sender.

I have already asked them to adjust their Exchange install to do this,
but they either do not know how or will not..

How would I do something like that?

We get about -/+ 2500 message a day from them.. it's not much, but
it's annoying.

Thanks in advance.