There is nothing impossible in life :-) But in your situation it looks a real problem. You have only one interface (ppoe) and both servers are using the UTM as smart host . If there where not using it as smart host , then rote them would be really easy. Because they are using it as smart host , the UTM is sending them from the WAN DG interface. Changing the WAN address for SMTP its easy , but that change the address for all nets. Because both servers are your mail servers and you hold both domains the logic and smartest thing is to use the address that you want to use for SMTP traffic to be the MX record for both domains. In this way all your mails will work perfectly as their records will be right.
