Question : SMTP server for internal delivery only

Hello, we have a situation where certain internal applications are not allowed to send mail externally but only to internal receipents and since the hosts or sender addresses are not fixed it does not seem possible to implement this restriction at the main Exchange mail server itself but has to be a separate SMTP.
It seems to be a rather trivial problem as I only need a service that responds to SMTP requests and forwards them to the main mailserver provided the destinations are for internal use.

I  first thought that this was feasible from the inbuilt Windows 2003 IIS6 SMTP server but I really cannot see any options for explicit forwarding to another SMTP nor any filtering capabilites on destination addresses so that seems to be a dead end.
My next step was to trying to find a freeware (it unfortunately needs to be free) SMTP gateway software but so far I have not been able to find anything that satifies the criterias to be free AND supports forwarding to another SMTP with destination filtering capabilites.

It seemed to be a rather trivial issue so are there any software/ IIS SMTP plugins that could be of assistance in this matter?
At the very worst it might be possible to set up a SMTP on a LINUX server but in that case needs a very simple implementation since my LINUX knowledge is limited to say the least.     Thanks in advance!

Answer : SMTP server for internal delivery only

That will work. Simply restrict relay on a new receive connector to a pre-defined list of IP addresses (the servers which need relay access): http://technet.microsoft.com/en-us/library/bb232021.aspx.

-Matt