I remember not long ago, the pain of trying to find out in which Office 365 tenant your vanity domain was validated, when you bumped into the error stating that the domain was in use, while adding it to your current tenant.
This was maybe because I work with multiple tenants and I recycle my tenants quite often, for testing purposes, but I’ve also seen it with others while trying to assist them in their migration projects.
Fortunately Microsoft now is very clear in the error message you’ll see, when trying to add a domain to a tenant, that is in use. This is what you will see now:
As you can see above, it will tell you exactly in which tenant the domain is, just so you can login to it and remove it.
Now is there a catch with this? Of course!! Microsoft won’t give you such privileged information, until you enter a valid DNS record for the domain validation. You’ll see something similar to this, if the domain validation is not done properly:
So remember to add the DNS record first and click “Verify”, Microsoft will either add the domain or explain exactly why they can’t, which I am sure it was for a long time one of the main asks of Office 365 admins and consultants.