How to Block sending to any domain except a specified list

Hello:

I'm currently on exchange 2003 enterprise in a LAN environment.  The entire network is completely isolated except one RDP connection to get onto a central desktop and even that is behind a firewall on my LAN.  My exact scenario is as follows:

1) I have an app that generates a lot of external email traffic but for obvious reasons my exchange server won't be able to reach the outside world (as it's for internal purposes only thus my domain is somedomain.local (that's not it but you get the point)).

2) All the email traffic will hit the exchange server as it should but then it creates a queue that needs will just get backed up.

3) I'd like to silently drop the message at the exchange server unless it's going to another domain on my LAN thus avoiding the whole trying to deliver it in vein situation.

So is it possible to only allow email destine for certain (internal) domains to go through the rest to drop out of sight at the exchange server?

Thanks for any help possible.