Error Creating Subdomain

Question

I'm working on an issue for a school district within Azure AD. They are using AAD Connect version 2 for staff currently. Let's say the domain is school.com and a staff's email is john.doe@school .com. I'm needing to add students who use a subdomain that would look like ******@my.school.com. That is also their UPN in the local AD. After adding them to the sync to Azure AD, the upn is changed to jr.doe@school .onmicrosoft.com in Azure.

Under domain names, I have school.com set as the primary and verified. I'm thinking the issue is I don't have my.school.com registered, but I get an unknown error. I am able to register other subdomains like students.school.com. I read that it's possible the subdomain won't register because it has been added to another tenant somewhere, but I'm at a loss of how to find that and get it removed so I can get everything under this one tenant.

Any suggestions as to how to fix my problem?

Answer 1

In general, it should tell you whether the domain is registered in another tenant, so the "unknown" error is puzzling. In any case, you can verify whether the domain is in use in any other tenant by using the method outlined here: https://www.michev.info/Blog/Post/3970/find-tenantid-by-domain-name-and-vice-versa-by-leveraging-the-graph-api
If you do get a result from the query, you can try performing a domain takeover: https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/domains-admin-takeover

Is the primary domain, school.com, federated? If so, you'll have to add the subdomain via PowerShell and the New-MsolFederatedDomain cmdlet instead: https://learn.microsoft.com/en-us/troubleshoot/azure/active-directory/new-msoldomain-cmdle-add-subdomain

Answer 2

If you want to add a subdomain name ******@my.school.com to your organization, you should first add and verify the root domain school.com. The subdomain is automatically verified by Azure AD. To see that the subdomain you added is verified, refresh the domain list in the browser.
If you have already added the school.com domain to one Azure AD organization, you can verify the subdomain ad.acme.com in a different Azure AD organization. When adding the subdomain, you are prompted to add a TXT record in the DNS hosting provider.

Check out Add subdomains of a custom domain for further details.