Fix NDR Error Code 550 5.1.1

Understanding and Resolving Common Email Delivery Issues in Exchange Online

Fix NDR Error Code "550 5.1.1" through "550 5.1.20" in Exchange Online

I got this bounce message. How do I fix it?

Here are some steps that you can try to fix the problem yourself.

Solution 1: Confirm the recipient's email address

Check for correct spelling and send the message again if you find an error in the email address.

Solution 2: Remove the recipient's email address from your Auto-Complete list

Fix your Auto-Complete list entries in Outlook:

• Manage suggested recipients in the To, Cc, and Bcc boxes with Auto-Complete
• In Outlook on the web: Start typing the recipient's name or email address in the To field until the recipient appears, then press the Delete key.

Solution 3: Confirm that the recipient isn't auto-forwarding messages

Compare the recipient's email address in the NDR with the email address in your Sent Items folder. If they don't match, contact the recipient to check for forwarding rules.

Solution 4: Verify that your account hasn't been compromised

Check your Sent Items folder for strange messages. If your account has been compromised, reset your password and notify your email admin.

Solution 5: Confirm that the NDR is related to a message that you actually sent

If your Sent folder contains only messages that you know you sent, the NDR could be backscatter (useless NDR). If so, your account hasn't been compromised.

I'm an email admin. What can I do to fix this?

If the steps above don't solve the issue for the sender, you can try the following solutions.

Solution 1: Check the Microsoft 365 admin center for configuration problems or service-wide issues

• Check the Message Center for known configuration issues.
• Go to Health > Service health to see if there's a service issue affecting the user's account.
• Run the Mailflow Troubleshooter tool to check for incorrect or stale MX records.

Solution 2: Update stale MX records

Error code 5.1.1 can be caused by problems with the MX resource record for the recipient's domain. Updates can take up to 72 hours to propagate.

Solution 3: Update forwarding rules to remove incorrect email addresses

This NDR might be caused by a forwarded recipient configured for the intended recipient. Update any forwarding rules accordingly.

Solution 4: Verify on-premises mail transfer agent configuration (if applicable)

If your organization uses a hybrid topology, ensure the mail transfer agent settings are correct.

Still need help with error code 5.1.1 to 5.1.20?

If these solutions don't resolve the issue, consider contacting support for further assistance.

For more information, see Backscatter in EOP.