Dear Kim Say
Welcome to Microsoft Community.
To resolve this issue, you can follow the instructions provided in the bounceback message:
Contact the recipient by alternative means, such as by phone, and inform them about the email delivery issue.
Ask the recipient to notify their email administrator that their domain may not be properly registered at their domain registrar.
Provide the recipient's email administrator with the error details mentioned in the bounceback message.
The recipient's email administrator will need to investigate and address the domain registration issue to ensure that the domain is correctly configured in the DNS records. Once the domain registration issue is resolved, email delivery to the recipient's domain should function properly.
If you continue to experience difficulties with email delivery after the recipient's domain registration issue has been resolved, you may want to reach out to your email service provider or IT support for further assistance.
Feel free to post back if you need further assistance.
Best Regards,
Chris.S-MSFT | Microsoft Community Support Specialist