Your email was returned to your Outlook inbox with a confusing error message. This is a Non-Delivery Report, or NDR, generated by the mail system. NDRs contain specific codes that explain why delivery failed. This guide will show you how to read these reports and apply the correct fix to get your message delivered.
Key Takeaways: Understanding and Fixing NDRs
- Enhanced Status Code (e.g., 5.1.1): This three-part number is the most precise identifier for the delivery failure reason.
- File > Info > Resend This Message: Use this to quickly retry sending after you have corrected the recipient’s address.
- Outlook’s Outbox: Check this folder for stuck messages that can cause subsequent emails to generate NDRs.
What a Non-Delivery Report Tells You
An NDR is an automated message from a mail server, not from a person. It informs the sender that their email could not be delivered to the intended recipient. The report includes technical details from the point where delivery failed, which could be your own mail server, the recipient’s server, or a server in between.
The most critical piece of information is the status code. Modern systems use an “Enhanced Status Code” formatted like 5.1.1 or 4.4.7. The first digit indicates the class of the error: a ‘4’ means a temporary failure, while a ‘5’ means a permanent failure. The subsequent digits provide more specific details about the nature of the problem, such as a bad mailbox address or a message that is too large.
Common Reasons for Email Bounces
Permanent errors, indicated by a ‘5’ code, mean the address is invalid and retrying will not work. The recipient’s email account does not exist, is misspelled, or has been deleted. Temporary errors, indicated by a ‘4’ code, suggest a transient issue like the recipient’s mailbox being full, their server being offline, or your message triggering a spam filter. These may resolve on their own after some time.
Steps to Diagnose and Fix an NDR
Follow these steps to find the error code in the NDR and apply the appropriate solution. Always open the bounced message in Outlook to see the full report details.
- Locate the Status Code
Open the NDR email in Outlook. Scroll through the body of the message to find a section with headings like “Diagnostic Information” or “Original Message Headers.” Look for a three-part number such as 5.1.1, 4.7.1, or 5.2.3. This is your primary clue. - Decode the Status Code
Identify the first number. A code starting with ‘5’ (e.g., 5.1.1) is a permanent user error. A code starting with ‘4’ (e.g., 4.4.7) is a temporary system error. Search online for the full code, like “SMTP 550 5.1.1,” to get an exact explanation from Microsoft or other technical sources. - Fix Permanent Address Errors (5.x.x codes)
For codes like 5.1.1 or 5.4.1, the recipient address is wrong. Double-check the spelling of the email address for typos. If you are sure it is correct, contact the recipient through another method to confirm their current email address. Do not repeatedly send to the bad address. - Resolve Temporary Delivery Issues (4.x.x codes)
For codes like 4.3.2 or 4.7.1, wait 10-20 minutes and try sending again using File > Info > Resend This Message. If the error persists, check if your own mailbox is near its storage limit, as this can cause sends to fail. Reduce the size of your message by removing large attachments. - Clear Your Outbox
If an earlier message is stuck in your Outbox, it can block new sends. Go to your Outlook folder list, select the Outbox folder, and delete any old, stuck messages. Then try sending your original email again.
If the NDR Code is Unclear or Fixes Don’t Work
“550 5.7.1 Client host blocked” or similar relay errors
This often means your email account or network is not authorized to send through the mail server. If you are on a corporate network, contact your IT department. If you are using a personal account from a new location, you may need to adjust your SMTP settings or use your provider’s authenticated SMTP server.
Messages bounce only when sending to certain domains
The recipient’s organization may have strict filtering. Your email might be missing proper authentication like SPF or DKIM records, making it look like spam. There is little you can do as the sender except to ask the recipient to whitelist your address or contact their mail administrator.
Outlook keeps generating NDRs for all emails
This points to a problem with your Outlook profile or account setup. Run the Outlook built-in repair tool via Windows Start > Settings > Apps > Apps & features. Select Microsoft 365 or Outlook, choose Modify, and then select Quick Repair. If that fails, create a new Outlook profile via Control Panel > Mail > Show Profiles > Add.
Common NDR Status Codes and Their Meanings
| Status Code | Error Type | Typical Cause and Action |
|---|---|---|
| 5.1.1 | Permanent | Bad destination mailbox address. Verify and correct the recipient’s email |
| 5.2.3 | Permanent | Message too large. Reduce file attachment size or use a cloud link |
| 4.4.7 | Temporary | Message expired in queue. Server delays; wait and resend later |
| 5.7.1 | Permanent | Delivery not authorized. Often a relay or spam filter block; contact IT |
| 4.3.2 | Temporary | System not accepting network traffic. Recipient server issue; retry |
You can now read an NDR code to understand why your email failed. Start by searching for the specific enhanced status code online for the most accurate fix. For persistent sending problems, use the Quick Repair tool in Windows to refresh your Outlook installation. A useful advanced tip is to enable detailed logging in Outlook via File > Options > Advanced > Enable troubleshooting logging, which can provide deeper clues for complex NDRs.