[Joncon]

Easiest way is to:-

1. create a host object for your external mail provider / mail gateway. Populate object with required info, IP, Subnet etc.
2. create a host object for your internal mailserver. Populate object with required info, IP, Subnet etc.
3. Create a rule through Smartdashboard - Security tab - and enter the 2 objects you created above in source and destination columns. Service will need to be set to required value (e.g. smtp in your case). Action should be set to accept, and track set to log (unless you don't want to).

htp,

Joncon