In Messaging Gateway (SMG), message audit log (MAL) searches will show any connection failure as "Rejected message by MTA" regardless of the reason the connection failed.
Messaging Gateway does not currently include the the cause of connection failures in the Message Audit Logs other than rejections stemming from IP reputation and Global Bad Sender policies so all connection failures are logged as if the connection were rejected based on policy.
For example, when a remote mail server attempts to establish a secure TLS session with SMG but the TLS communication fails, the message audit logs show the action "Rejected message by MTA" rather than a message which more clearly describes the reason for the connection failure.
This is a known limitation of the current Message Audit Log feature and additional information regarding connection status will be included in MAL in a future SMG release.
Messaging Gateway version 10.6.4 and later include a verdict indicating the "connection class" of the message. This idenifies the connection classification bucket the message was in.