Hello @Jonathan Duncan,
Thank you for your valuable time over the call.
As we have discussed over the call, we came to the conclusion that the custom domain ****.com is already been verified in another tenant.
As I have informed you before, if this custom domain ****.com is already verified in some other tenant, in this scenario you will have to first remove the domain from the other tenant and only then you will be able to add and verify the custom domain ****.com in new tenant.
Common verification issues
1.Make sure the domain name isn't already in use in another directory. A domain name can only be verified in one directory. If your domain name is currently verified in another directory, it can't also be verified in the new directory. To fix this duplication problem, you must delete the domain name from the old directory. For more information about deleting domain names, see Manage custom domain names.
For additional information, please refer to the below document for your reference.
Add and verify custom domain names - Microsoft Entra ID | Microsoft Learn
Since you are not aware of that another tenant and you don't have any contact persons of that another tenant and also you are not part of that another tenant where this custom domain ****.com is already been verified, there are no options to sign in to that another tenant to delete the custom domain ******.com, so you have deleted this custom domain ****.com from your tenant.
I hope this above information provided is helpful. Please feel free to reach out if you have any further questions.
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