If the Microsoft account that you need to register for the customer account is already registered for another customer account, the owner of that Microsoft account must confirm ownership. To check whether ownership validation is required, use the method described in Checking the Status of the Microsoft Account.
If ownership validation is required, request the owner of that Microsoft account to create a validation user or a validation DNS record, as described in Checking the Status of the Microsoft Account.
Notes:
1. The validation user or validation DNS record created for a Microsoft account can be removed after the successful registration of that Microsoft account.
2. The validation of Microsoft account ownership can be bypassed, as described in Bypassing the Validation of Microsoft Account Ownership.