MS Outlook - Non-Delivery Report Troubleshooting
this article address email failures due to server issues, routing problems and misconfigurations.
Non-Delivery Reports:
An NDR is a report of an attempted email delivery failure. The report contains diagnostic information that points to the delivery failure. If you receive a non-delivery report, contact your exchange admin. If CallTower is hosting your Office 365, open a case with support and provide a copy of the report.