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

Overview

A Non‑Delivery Report (NDR) means your email couldn't be delivered to the recipient. This guide helps you understand and resolve the issue.

Understanding NDR Error Codes

NDRs typically include an error code such as:

• 550 – Recipient doesn't exist or is blocked
• 554 – Email flagged as spam or rejected
• 550 5.7.1 – Recipient's organisation blocking your email
• 421 – Server temporarily unavailable
• 550 5.1.1 – User not found
• 550 5.2.2 – Mailbox full

Common Causes and Solutions

1. Recipient email address is incorrect

Error: “Mailbox unavailable”, “User unknown”, or similar.

Solution:

1. Check the email address for typographical errors.
2. Confirm the correct address with the recipient.
3. Search the address in your organisation's directory.

2. Recipient mailbox is full

Error: “Mailbox full” or “Quota exceeded”.

Solution:

• Notify the recipient via an alternative method (Teams, phone).
• Ask them to delete messages or increase their mailbox quota.

3. Email flagged as spam

Error: “Message rejected as spam”.

Solution:

• Review your sent items for suspicious content.
• Remove any suspicious links or attachments.
• If you believe the block is in error, contact your IT admin to review SPF, DKIM and DMARC settings.

4. Large attachment

Error: “Message too large” or attachment rejected.

Solution:

• Use OneDrive or SharePoint to share large files.
• Compress attachments or reduce file size.
• Check your organisation's attachment limit (commonly 25 MB).

5. External recipient blocked

Error: “5.7.1”, “Relay denied” or similar.

Solution:

• Confirm whether the recipient’s organisation blocks external mail.
• Contact the recipient via an alternative method.
• Ask your IT admin to whitelist the external domain if appropriate.

6. DNS or authentication issues

Error: NDR mentions SPF, DKIM or DMARC failures.

Solution:

• Verify that your domain’s SPF, DKIM and DMARC records are correctly published.
• Use online tools such as MXToolbox to test the records.
• Update the records if they are missing or outdated.

7. Sending domain on a blocklist

Error: “Domain blocked” or similar wording.

Solution:

• Check common blocklists (Spamhaus, Barracuda, etc.) for your sending IP or domain.
• Request delisting if your domain appears on a list.

8. Exchange Online Protection (EOP) quarantine

Solution: Review the EOP quarantine portal for any messages held for policy reasons and release or whitelist as needed.

What to Do When You Receive an NDR

1. Read the error message – it usually indicates the cause.
2. Identify whether the NDR is permanent or temporary:
   • Permanent NDRs have codes beginning with 5.x.x (e.g., 550 5.1.1) and require you to correct the address or contact the recipient’s admin.
   • Temporary NDRs have codes beginning with 4.x.x (e.g., 421) and can be retried after a short interval.
3. Verify the recipient details – double‑check the email address and confirm it is still active.
4. Retry the message after addressing any identified issues or after a brief wait for temporary errors.
5. Escalate to IT – if the problem persists, Open a Ticket and include the full NDR text.

Prevent Future NDRs

• Double‑check email addresses before sending.
• Keep attachments under your organisation's size limits or use cloud sharing.
• Maintain an up‑to‑date contact list.
• Monitor SPF, DKIM and DMARC records regularly.
• Review blocklist status of your sending domain periodically.
• Refer to Email delivery delays for additional troubleshooting steps.
• See Deleted email recovery if you suspect the message was removed.
• For broader issues, see Email not sending or receiving.
• Need to add a new account to Outlook? Follow How to add email to Outlook.

Still Having Issues?