SMTP source IP address

For services running on the F5 Linux host, the source address would typically be the self IP associated with the vlan containing the default route in route-domain 0.

If you don't have a tmm default route in route-domain 0, but you have a default management-route, or if you want to override tmm's default route with a static management-route, then the source address would be the IP address of the F5's management interface.