Outbound mail in postfix via mail security gateway

SYSTEM INFORMATION
OS type and version Debian 11
Webmin version 2.011
Virtualmin version 7.5
Postfix 3.5.13

We have an email security gateway for our Windows mail systems which handles the incoming and outgoing mail. As we are in the process of refreshing the gateway, we decided to move our postfix systems behind the gateway as well.
The setup for inbound mail for each domain was easy, but the problem I’m facing is with the outbound mail. We want the outbound mail routed through the gateway to implement data loss prevention. Setting a transport mapping of * to the mail gateway breaks the mail! Internal emails don’t arrive and if I try send an email from outside, I get the following

image

I’m raising a ticket with the gateway support but wanted to check here as well in case anyone has encountered this before or knows postfix better than me (which is very easy to do)

It works using relayhost but the relay host setting sends the outbound email of all the domains via the gateway. I will keep fiddling with the settings to work out how to set for individual domains!