Reject non-existant users with exchange and exim

[chardlist]

In the wiki documentation I see methods how to set up exim routers to scan
mail and then pass it along to another server (exchange in this case), but I
don't see a way to configure the exchange server and the ms server to
communicate about valid addresses so that the mta on the mailscanner server
(exim) can reject the message before mailscanner has to deal with it.  Right
now I am leaving the mailscanner server open to accept any mail to the
domain with the exchange server and it's getting a bit out of hand.

I realize this is more of an exim question, but I figure this is a good
place to start.

MS 4.50.14
Exim 4.52
Redhat 9


Thank you,
-Brendan