Fix email delivery issues for error code 5.7.64 in Exchange Online

It's frustrating when you get an error after sending an email message. This topic describes what you can do if you see error code 550 5.7.64 in a non-delivery report (also known as an NDR, bounce message, delivery status notification, or DSN).

Email user icon. I got this bounce message. How do I fix it? Email admin icon I'm an email admin. How can I fix this?

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

Only an email admin in your Microsoft 365 or Office 365 organization can fix this issue. Contact your email admin and refer them to this information so they can try to resolve the issue for you.

I'm an email admin. How do I fix this?

This problem happens when you use an inbound connector to receive messages from your on-premises email environment, and something has changed in your on-premises environment that makes the inbound connector's configuration incorrect. For example:

  • The host name that's specified in the certificate on the inbound connector no longer matches the source email server.

  • IP address of the source email server no longer matches the source IP address on the inbound connector.

The Diagnostic information for administrators section in the bounce message will contain the original error message when Microsoft 365 or Office 365 tried to send the message to the external email server or service.

To fix this issue, see this topic.

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See also

Email non-delivery reports in Exchange Online