You can add a policy route to make sure traffic for your email server always goes via one WAN.
Add a new firewall rule on your LAN. Put the rule above your existing rule that allows traffic out so that it catches your mail traffic.
Protocol: TCP
Source: LANnet
Destination: Your mail server IP (or an alias containing a range of IPs)
Gateway: WAN1 gateway (this is a an advanced setting).
Like you have said most email servers don't have a problem with mulitwan these days. Gmail hotmail etc, no problems.
Steve
Edit: You have already asked this question twice. Both times I advised you to use a policy route. ???