How to Relay mail??

We are running Exchange 2000 on a Windows 2000 server. Our internal users are running a an emulation package to access our Unix machine. We have a program call Vsi Fax set up on our Unix machine that allows us to send faxes via our system. We are now trying to set it up to email from our system and on the Vsi fax side its fine because we can send internally, but externally its not working. The messages are just being queued up. I thought all I needed to do was to add the Unix servers IP address in the Relay restriction tab and grant access, but still nothing!

So, what I am asking is how do I set up relay mail?

Hope this all makes sense…