Thanks for the advice.
I used the two parameters below to ensure I will get success/failure emails when backing up the GW system.
Added 'smtp_bind_address' to /etc/postfix/main.cf with the IP of the OES server (different then GWIA address, which is bound to its IP). This ensures postfix will not conflict with the MTA, POA, GWIA.
And the 'relayhost' is the GWIA IP address.
smtp_bind_address = 192.168.12.161
relayhost = 192.168.12.35
This works on my (and my clients) OES/GW cluster servers & non-cluster servers.
Hope this helps others.