[exim] using exim to relay to exchange

I am looking for a simple way to setup an exim server to relay to our
internal exchange servers. I have reviewed the steps to use LDAP
although with my limited knowledge of how it works I was wondering if
there was a way to setup a flat file that exim would check against
before forwarding to exchange i.e. I would add user1.lastname@???
and exim is now permitted to forward email to exchange for that user.
Yes I know this sounds basic but unfortunately we do not have a
dedicated mail admin and I am a jack of all trades here and I am looking
for a solution that is easy to manage... Thanks in advance for your
suggestions.