enable relaying from within G-apps and specified the server's public IP address as ok for relaying
Make sure port 25 is open on your firewall to only accept requests from the ip address of the VMPro server.
To test port 25, from a cmd prompt on the VMPro server:
Type: telnet example.com 25
If Port 25 is not blocked, you will get a successful 220 response
If Port 25 is blocked, you will get a connection error or no response at all.