When using an lb-group, incoming email cannot be received. Same goes for
other ports that are open, like an ssh port.
If you're only receiving email on a single WAN interface: (for instance pppoe0 )
Add extra routing table, having default route set to interface pppoe0. In modify rules, add modify rule when smtp source port = 25 (being return traffic for received email) , th erouting table just created is used.
If you want to receive smtp mail on both interfaces, and sent return traffic on correct interface, this involves marking packets,