Summary
This article explains what Non-Delivery Reports (NDRs) are, why they occur, and how to understand and resolve common email delivery errors.
Environment
Microsoft 365 (Exchange Online), USNH Email System, Outlook (Desktop, Web, Mobile)
Issue
You sent an email and received a Non-Delivery Report (NDR) indicating the message could not be delivered.
Resolution
Step 1 – Locate the error code in the NDR (example: 5.1.1 or 5.7.1).
Step 2 – Match the error code to one of the common issues listed below.
Step 3 – Apply the suggested fix.
Step 4 – Try sending the message again after correcting the issue.
Common NDR Error Messages
| Error Code / Message |
Description |
Suggested Fix |
| 5.1.1 – Email address does not exist |
The recipient email address is invalid or no longer exists. |
Verify spelling and confirm the address with the recipient. |
| 5.2.2 – Mailbox is full |
The recipient mailbox cannot accept new messages. |
Try again later or notify the recipient. |
| 5.3.4 – Message too large |
The email exceeds size limits. |
Reduce attachment size or use OneDrive. |
| 4.4.7 – Message timed out |
The sending server could not reach the destination server. |
Wait and retry sending later. |
| 5.4.1 – Routing issue |
The destination domain could not be reached. |
Verify the domain and try again. |
| 5.7.1 – Message blocked |
The message was rejected due to security or policy. |
Remove suspicious content or contact recipient/IT. |
| 5.7.26 – Authentication failure |
The message failed SPF/DKIM/DMARC checks. |
Ensure you are sending from an approved system or contact IT. |
| 5.6.0 – Invalid message content |
The message format or attachment is not supported. |
Recreate the email or remove attachments. |
| 4.2.0 – Temporary mailbox issue |
The mailbox is temporarily unavailable. |
Wait and try again later. |
| 5.0.0 – General failure |
An unspecified error occurred. |
Retry sending or contact ET&S if it persists. |
Result
After following these steps, your email should send successfully or you will have enough information to provide ET&S support for further troubleshooting.
Need additional help?
Please include the full NDR message for faster assistance.
Submit an Email Services support request with as much detail as possible, or visit the Technology Help Desk Support page to locate your local campus contact information or to submit an online technology support request. For password issues you must call or visit the Help Desk in person.
For assistance concerning site creation, content sharing, file synchronization, or other common SharePoint, OneDrive, Teams, or Office app activities, we recommend our Microsoft 365 Learning sites:
Learn more about the great tools our Microsoft 365 Learning sites offer!