A common issue is verifying whether a domain name is already attached to a Microsoft 365/Azure AD tenant. If you don't check ahead of time, you often find that you can't add a new domain name to a tenant at a critical time during a project.
To check whether a domain name is already in Microsoft 365:
If there is a tenant ID, then it's already registered.
This often gets us to the point where no one knows for sure what tenant and who might be able to log in. It might have been created for volume licensing or a Power BI trial. There's really no way to know.
Instructions on how to get the domain name back:
