Another way to approach this in a load-sharing (not round-robin/load-balance) fashion is to specify different NAT pools based on the destination address.
If you're using Central NAT this would mean specifying multiple Central NAT policies for example from email IP to 0.0.0.0/2 using one public, from email to 220.127.116.11/2 using another, and so on.
If you're not using Central NAT, you can do the same thing with multiple firewall policies. From email IP to 0.0.0.0/2 and select a single 1-to-1 NAT, then copy that for the other 3. Just have to make sure you don't have a policy matching from email IP to "all" above those rules.