Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Dear John,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
Based on your description, I need to double-confirm with you that why you need to add MX record in the only domain as onmicrosoft.com. Generally, when your company had bought the Microsoft 365 business subscription and assign you under the onmicrosoft.com, records will automatically set up which you do not need take any action of it.
In addition, could you please let us know how you want to add MX record?
Sincerely,
Tommy | Microsoft Community Moderator
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