[exim] Exim as relay box between exchange servers.

Hi,

Im quite new to exim but really need to solve this problem using it.
I currently have a situation where I have two exchange servers, one 2003
running on a SBS server and another 5.5 on an NT box. The 5.5 will be
upgraded to 2003 shortly on a full dc (not sbs).

However, the two servers belong to two different entities but both need to
share the same domain name. So what I have is a copy of Suse Pro 9.2 on a
box with exim loaded. I have a LDAP script which generates a list of email
addresses from each of the servers and puts them into two seperate files.
What I would like is a filter of some sort written to say - if the username
exists in this file then relay the email to this exchange server.

I dont want to do LDAP queries on the fly. All I need a some sort of filter
where it just checks each file and relays mail to the appropriate exhange
server according to which file the email address exists in.

Any help will be greatly appreciated ;)

Michael