Exchange Routing

I have an Exchange 5.5 server running 3 NICs: 2 internal, one with a hard connection to the Internet. I recently attempted to pull the Exchange server behind our ISA firewall and found that I was able to hit the OWA but my Exchange server was no longer forwarding mail to external recipients.

I changed the external address on the Exchange Server to a valid internal address and set the gateway to the ISA server. I also added the gateway to the other 2 NICs, effectively creating 3 internal cards. I am confident that the problem has to do with some Exchange setup, but, am unable to find info on the subject.