POP3 & SMTP

If I understood it, want to send emails only from primary wan

Than you have to use multipath rules under interfaces. To be able to do so you have to check both lines as "default gateways"

Let us known the situation

you are right. do you have any link for this setup.

1. Check "default Gateway" in both wan Interfaces. Now Uplink Balancing should become active.
2. In multipatha rules, just enable the Sample rules for Smtp and web, but change both "by interface" and select the desired interface. The rules should be like this: