Linux as a firewall and make MS Exchange 2003 as mail server?

Linux box acts as firewall and also the mail server. We got an MS Exchange Server installed in one of the computers in the network, our DC is in another box wherein it runs in Microsoft Server 2003.

Now, my question is how to configure the network wherein, we will no longer use Linux as a mail server and make it only as a firewall and like a router and make our Exchange server be a mail server? I appreciate all the help that I could get.