UTM 9.701: Outgoing mail when using multiple external interfaces and addresses

Hi,

we have a SG330 with 4 external WAN Interfaces. 2 Interfaces have 4 IPs configured.

To prevent us to setup all IPs in Internet DNS (MX): How can I reduce SMTP traffic to only one interface/one IP?

Mailserver uses Sophos as proxy for outgoing mail.

No button for that in Email protection.

Any hints?