"Message Complaint Received" response will occur when the recipient of your messages from Engage marks MoxiWorks as “spam” in their email client.

Our system blocklists their email and you are no longer allowed to send any communication to them from Engage. Clients can choose to unsubscribe or mark our emails as spam but it is completely up to the recipient to choose not to receive more content.

Due to email privacy laws, there is no way to undo the blocking on our end unless the client willingly marks you as a safe sender.