Hello,
I've inherited a SBS2008 server running exchange and for various reasons, need to run it (and its four users) behind a router within our network on a different subnet.
Our main router is a beefy mikrotik job, and at present, the exchange server is hooked up behind a cheap SOHO router (TP-Link or something).
Both routers are doing NAT. I've forwarded port 25 from outside to the LAN IP of the cheap router's WAN interface, and forwarded port 25 to the exchange server IP in the cheap router.
Running a SMTP test from outside (the internet), it appears that the server is responding OK. However until I change over the MX records, I can't test it or know for sure. We use google apps in our organisation so this shouldn't affect business as usual.