Emails sent via Sophos Central blocked at exchange 2016 - 550 5.7.1 Sender ID (PRA) Not Permitted

Having an issue with a client server with Sophos Central Email Gateway.

Emails from the same sender get through OK, but there are some that get blocked from the exchange and end up in the emergency inbox. 

Exchange report shows the error - [{LRT=};{LED=550 5.7.1 Sender ID (PRA) Not Permitted};{IP=}][{LRT=};{LED=550 5.7.1 Sender ID (PRA) Not Permitted};{FQDN=};{IP=}]

I've disabled the sender ID Agent on the exchange servers, but the same error keeps happening.

When emails deliver successfully, they show the source as the other mail servers (Mail1 & Mail2 etc). but when they fail, is shows the IP address of the Sophos servers.