problems adding another onmicrosoft.com subdomain

Question

I am trying to add a subdomain to my tenant that more closely matches my internal domain name. I am trying to add subdomain.onmicrosoft.com (where subdomain is my internal domain name), but it is not verifying.

I started to add it via Entra > Custom Domain Names, where i tried adding it. I went all through without problem, then when it got to the Verify section, it needed me to add a TXT record to my provider's DNS management page. Well, it's an onmicrosoft.com subdomain... Needless to say, it didn't verify.

I then tried adding a DNS zone in the Azure DNS Zones section. I added a DNS zone and added the TXT record there with the details that the Custom Domain Names page gave me. It still didn't verify.

I tried adding it via admin.microsoft.com in the Domains > DefaultDomain.onmicrosoft.com section where at the bottom there is a link that says "Add ‎onmicrosoft.com‎ domain (preview)". when i try to add it there, it says "sits.onmicrosoft.com is already in use by another organization. Please try a different domain name."

My question is therefore twofold:
1: Is there a way to look up an onmicrosoft.com subdomain and find out WHEN it was created? This will show me if indeed it is in use by someone else, or if it was created by me in the course of me trying to sort this all out.

2: Am i doing something wrong when trying to add the custom subdomain and adding the TXT record in the wrong place or something else like that?

Thanks very much indeed!

Answer 1

Hi @LukeUK-0364

Thank you for posting!

When you create a Microsoft Entra tenant it comes with the initial domain name

domainname.onmicrosoft.com it cannot change or delete, but you can purchase a Custom domain name (like Contoso.com) and add your organization's DNS name as a custom domain name by verifying the TXT records which is given by domain vendor and set it as primary. By adding your domain name, you can add usernames that are familiar to your users, such as ******@contoso.com.
You can follow the steps for how to Add your custom domain name to your tenant.

The feature Add ‎onmicrosoft.com‎ domain (preview) is feature is used in a scenario Suppose your company, "Tailspin Toys," initially set up Microsoft 365 with the fallback domain tailspintoys.onmicrosoft.com. Later, you decide to add a new domain tailspintoysmarketing.onmicrosoft.com for your marketing department. By following document, you can add this new domain and configure it for specific users or services within your organization.
For this the domain should not be registered in any other Microsoft365 Tenant or Entra Tenant.

To find the details of the domain you can go to https://lookup.icann.org/en/lookup.
Hope this helps. If this is not the case, please let me know by responding in private message.

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Answer 2

You must use the M365 Admin center for this functionality, not the Entra/Azure AD blade. If you get the error message above, or are prompted to add DNS records at any point of the process, the domain in question is already registered with another tenant, and cannot be used. You can find out which tenant said domain belongs to by leveraging the findTenantInformationByDomainName Graph API method: https://learn.microsoft.com/en-us/graph/api/tenantrelationship-findtenantinformationbydomainname?view=graph-rest-1.0&tabs=http

There are also some free tools online that allow you to "lookup" a M365 domain, if you are looking for an easier solution. Either way, you can only get some limited information, such as the tenantId or display name, but it should be sufficient to identify whether this was one of you tests.