Outbound mail is sent with Source address of Fatpipe WAN IP, where receiving email servers reject the connection based on incorrect PTR or not belonging to an SPF record.
Solution: Configure an outbound policy for Interface Priority for DST port 25 with NAT disabled across primary WAN.
For example: Select TCP from Protocol drop down. Source IP *,Source port *, Destination IP * Destination port 25. Edit primary WAN (Typically WAN1) and disable all NAT. This allows the sendmail servers IP to route without IP manipulation out the primary WAN. For redundancy WAN2, WAN3, etc., can be edited and a static NAT configured as the sendmail server WAN1 IP to a specific IP from WAN2 or 3.