Since this error is happening on a server that's outside Office 365, you'll need to check a few things to fix the problem. ...

Office 365

Since this error is happening on a server that's outside Office 365, you'll need to check a few things to fix the problem.

Check the NDR for information about where the problem is happening. For example, look for a domain name like contoso.com. The recipient's email server could be causing the problem, or it could be due to a third-party service that's filtering the messages.
Check the NDR for information about how to fix the problem. The NDR should tell you what went wrong and how to fix it. For example, the sender should check the spelling of the recipient address, or try to clear the recipient's entry from the Auto-Complete List then resend the message.

If you can't fix the problem, contact the responsible party's admin. Provide them with the error and the name of the external server that reported the error. You can find this in the NDR. Unfortunately, Office 365 support is unlikely to be able to help with these kinds of externally reported errors.

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