The issue was confirmed to be resolved. Any messages that failed with the error will successfully go through if resent by the sender.
Update 07/05/2023 16:08 PM 21 days
From 11:30AM Eastern until 11:50AM Eastern outbound messages were rejected with "access denied" due to a misconfigured routing rule that internally routed messages to a sibling outbound server instead of the real recipient destination. The sibiling outbound server was being tested and was only intended to receive test messages which caused the rejections to occur when it received non test messages.
Based on reports this seems to only have affected customers on our Hosted Exchange cluster.