[Exim] Intranet server

I have been asked to investigate setting up an intranet to help minimise
mail traffic. We supply mail to various clients all over the country and
have noticed that users are sending local mail (to the man in the office
next door)and that mail is sent all the way down to the mail server
processed and then sent back, causing unnecessary traffic.
What do need to set up on Exim to enable the two severs to relay mail
between each other.?
I know that changes need to be done on the other server(intranet) but I will
investigate that myself.
Thanks in advance.