Inbound traffic over two wan links

Hello

I have two WAN links and have inbound policies for mail for redundancy I would like to route traffic back SMTP traffic out the interface it came in on.

The problem is when  both links are up at times traffic goes out through the other link instead of the on that is prefered for SMTP inbound and oubound and that causes Mimecast to deny that traffic.

in its simplest form i want to route traffic back out the interface it came in on.

Thanks and Regards