[exim] Exim out front, exim and exchange internal

We have exim out front scanning mail, internally we have exim and
exchange, with an application pulling email using POP out of the exim
server. I'm looking for some suggestions as to how to get the email to
deliver to the correct server. I know I could create a text file with a
list of users, but I'm looking for something more elegant. Any
suggestions?