WY,
The SBG appliances do not have inside and outside interfaces, but simply eth1 and eth2
There are no inside and outside interface.
The dual interfaces are basically just for performance purposes and so that the incoming and outgoing MTAs can both exist at port 25 and not have to use seperate ports on the same interface.
Depending on the nature of the mail it will be delivered using a particular MTA and source address.
The source MTA and IP also rely on what settings have been set in the device for local domain mail, non-local domain mail and mail routing settings.
As you can see there are a few variables and predicting the source of the mail becomes quiet complicated.
The most important thing is that the interfaces can fully route to the next mailhop, whether that is the internet or an internal relay etc.
I won't even start on reverse address binding etc.
Anyone thats been down that road will know what I mean...