Following the upgrade to Messaging Gateway (SMG) 10.7.5, connections from mail servers which were previously able to send to SMG now shows the actions "Rejected message by MTA, TLS on reception enforced" even though TLS secured delivery is not required by the SMG configuration.
Release : 10.7.5
This issue is currently under investigation but appears to be associated with client certificate validation failures.
This issue may be resolved by unchecking the "Request client certificate" checkbox in Administration > Configuration > host > SMTP > Inbound
This article will be updated once investigation of the issue is complete.