I'm troubleshooting an issue where email sent from a specific user is blocked every time to only a specific sender. The NDR generated (from our Exchange server) has the following message:
#550 Envelope blocked - User Entry ##
The recipient says that the sending address is not in the blocked senders list. The recipient can also receive mail sent from other addresses sent from our MTA, and other users at the recipient's company can receive mail from the sender in question.
What I'm really trying to figure out is what software in the mail flow might be generating this specific message. Web searching hasn't come up with anything yet. Thanks for your help!