GWIA in a test environment without Internet access

I have a copy of our GroupWise system (currently 2014R2) in a test lab which is in totally isolated network, no access to the outside world. I use to test upgrades etc, but I can never get the GWIA to load. I assume this is because it's trying to communicate with something outside of our network on startup and dies as soon as it can't see it. I tried setting the Relay Host to a linux box running Postfix within the test network, but that didn't seem to help. I've also tried deleting the existing GWIA and creating a new one, but that also wouldn't start.
Does anyone know what might exactly could be causing this?

Cheers, Ian