Yes, you can do that – perhaps with a few limitations – depending on your requirements. Actually, your use case is quite common.
Search the Forum for discussions of outbound rules, e.g., this posting - Understanding and Configuring Outbound Policy .
It is easy, for example, to instruct the Balance that WAN1 should be used if it is available, but it’s OK to fail-over to WAN2 if WAN1 dies. The “priority” rule could be used and the POP/SMTP ports specified. Or one could use the destination address of the server. And, the “enforced” rule can be employed to ensure that WAN2 is not used - ever – for certain traffic.
If you can accurately specify the target (domain name), source on your LAN (IP or MAC address) or ports, for example, you can easily write a rule to control the routing (or non-routing) of the traffic.
I’m not sure if Peplink has a Balance product demo available on-line. However there IS an on-line demo available of the MAX series at https://www.peplink.com/products/max-cellular-router/max-live-demo/ . The rules are developed for the Balance and MAX devices in precisely the same way. If you go to the demo you’ll find the rules at Advanced → Outbound Policy. The Balance series has the Outbound Policy in a slightly different location but, as I said, the rules operate in the same way. The on-line demo will give you a chance to see how rules can be developed and to determine if they’ll meet your needs. Check it out! (Note: Rules are interpreted from the top down.)