Email delivery failed (NDR)
Summary
This guide helps you troubleshoot and resolve: Email delivery failed (NDR). Follow the steps below to fix the issue.
Common Causes
Email Delivery Failed (NDR) - What to Do
Overview
A Non-Delivery Report (NDR) means your email couldn't be delivered to the recipient. This guide helps you understand and fix the issue.
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Understanding NDR Error Codes
NDRs typically include an error code like:
- 550: Recipient doesn't exist or is blocked
- 554: Email flagged as spam or rejected
- 550 5.7.1: Recipient's organization blocking your email
- 421: Server temporarily unavailable
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Common Causes and Solutions
1. Recipient Email Address Wrong
Error: "Mailbox unavailable" or "User unknown"
Solution:
1. Check the email address for typos
2. Ask the recipient for their correct email address
3. Look them up in your company directory
2. Recipient Mailbox Full
Error: "Mailbox full" or "Quota exceeded"
Solution:
- Notify the recipient by alternative means (Teams, phone)
- They need to delete emails or contact their IT admin
3. Email Flagged as Spam
Error: "Message rejected as spam"
Solution:
- Check your sent items for suspicious activity
- Ensure email isn't containing suspicious links or attachments
- Contact your IT admin if you're flagged by mistake
4. Large Attachment
Error: "Message too large" or attachment rejected
Solution:
- Use OneDrive/SharePoint to share large files
- Compress attachments or reduce file sizes
- Check your organization's attachment limits (usually 25MB)
5. External Recipient Blocked
Error: "5.7.1" or "Relay denied"
Solution:
- The recipient's organization may block external emails
- Use an alternative method to contact them
- Contact your IT admin to whitelist the domain
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What to Do When You Get an NDR
1. Read the error message - It usually explains the problem
2. Verify the recipient - Double-check the email address
3. Try again later - Sometimes servers are temporarily unavailable
4. Contact IT - If the issue persists
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Prevent Future NDRs
✓ Double-check email addresses before sending
✓ Keep attachments under size limits
✓ Update your contact list regularly
✓ Don't send to outdated email addresses
✓ Be cautious with external emails
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Still Having Issues?
Contact IT support with:
- The original recipient's email address
- The full NDR error message
- What you were trying to send
- When the issue started