Our users are receiving 550 4.4.7 QUEUE.Expired undeliverable messages only to a single domain.
Using the Troubleshooting Assistant, the following Mail Flow issue is found:
Remote server mail.domain.com failed the mail acceptance test. MAIL FROM command: Respond = 503 5.5.1 Error: send HELO/EHLO first.
We confirmed all of the messages sent to this domain are stuck in the queue. No configuration changes have been made on the local Exchange server or on the remote mail server, which happens to be Postfix.