O365 - Unknown Error adding a vanity domain

Problem:

Adding a domain in Office 365 Web UI, you add a domain name, get the verification DNS record created, click on verify and you get an "Unknown error"  You receive the same if you use Confirm-MSOLdomain.   If you use the Web UI, you will have the added pain factor that every time you attempt a verification, the DNS entry will change on you.  Especially frustrating if you are not the person in control of DNS and you repeat this process a few times before realizing the issue.

Analysis:

Obviously, something as generic as "Unknown Error" we cannot possibly state the root cause is going to be the same every single time.  Your mileage may vary here, but this is what we found, and if you can ask the right questions, you may be able to resolve this without contacting support.

For us, the root cause was that the vanity domain was already verified in another tenant.   That tenant had since been abandoned and none of the Global Admins were known.  I was able to call PSS, and they confirmed that this was indeed the case.  The next time I run into this, I will challenge the domain owner more to see if they know if they had a previous tenant they attempted and abandoned.

My Request:

This seems like something simple enough to make a friendly error like "this domain is in use, see here for instructions on gaining access to the domain"

If you want, you can even link here if getting a new KB together is difficult.  :)

Solution:

Follow the below steps for removing the domain from other tenant and adding it to your tenant.

In order to remove your domain from another Office 365 account, and to add your domain under your new tenant, follow these instructions:

1. Go to this link: http://www.powerbi.com/dashboards?WT.mc_id=PowerBI_com and enter a valid e-mail address associated with the domain suffix "company.com".
2. You will receive an e-mail and Follow the link in the email.
3. Generate a new password so that you can login into company_beta.onmicrosoft.com Tenant, where your domain company.com is currently added and verified.
4. Go to: http://portal.microsoftonline.com and log in with the newly created credentials.
5. Go to Office 365 settings-> Select "Become Admin" on the left. Follow the steps to verify that this user owns domain "company.com"
6. Add the DNS verification record as provided
7. Once DNS is verified, you will become the admin of that Unknown Tenant.
8. Now you can remove the domain from company_beta.onmicrosoft.com Tenant.

Once the domain is removed, please log out and logon to your new Office 365 tenant account, where you want to add the vanity domain.

Please follow the instructions to add the domain in Office 365

https://support.office.com/en-us/article/Verify-your-domain-in-Office-365-6383f56d-3d09-4dcb-9b41-b5f5a5efd611