PBR also needs to be backed up with a routeing statement.
So while WAN1 may be your default route, for PBR there should be a less worthy static (floating static) to 0.0.0.0/0 (with a higher distance metric) via the blurred out gateway IP and interface(/device).
In short, PBR needs to be validated against a route, even if it is only a ' candidate' .
Of course is WAN1 fails, WAN2 may take all the load (if it is feasible for it to notice and invalidate all routes via WAN1). So be sure that you can handle this eventuality if WAN2 has prohibitions from your ISP/carrier.
The source port of your mail server won' t be port 25. Typically it' ll be starting at 1024 and randomly selected at that number of higher, as per rfc6335 (or by convention / word of mouth for how these things are supposed to be done in the computer industry when you write or access net sockets).