[exim] reject recipient that doesnt exist on internal server

Im having a hard time finding how to do this.. I have an exim server here
that accepts mail and then relays on to an internal exchange server... it's
not causing any problems really, but there are some email addresses that
don't exist on the exchange server, that keep getting sent quite a lot of
mail, and i'd like for them to bounce at the exim end instead of being
forwarded on to exchange and then being bounced by exchange. it's not really
a big deal except it's throwing off my stats, showing lots of mail for users
that don't exist. I'd ideally like to have a list of users for exim to
reject when they first attempt to deliver mail. I could also do some kind of
LDAP setup to check for valid mailboxes but i'm not sure how complicated
that is or if it's worth the time to set it up. Thanks for any suggestions!

-arif