Subdomain in another tenant

Question

Subdomain in another tenant

Yilia Li 0

Do the Microsoft M365 global tenant and the 21Vianet-operated tenant support binding a primary (root) domain and a subdomain separately?

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2 answers

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Answer 1

Vergil-V 16,990 Microsoft External Staff Moderator

Hi @Yilia Li

Thanks for reaching out to the Microsoft Q&A forum.

Based on my research, the first response from the Q&A Assist already covers most of your questions regarding having a subdomain in a 21Vianet‑operated tenant while the root domain exists in a global tenant.

One important requirement is that the root domain must be added and verified before you can add a subdomain. For more information, you can refer to the following documentation: Add and verify custom domain names - Microsoft Entra ID | Microsoft Learn

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To add a subdomain correctly, this action is only supported through PowerShell and Microsoft Graph. For more details, you can refer to the following documentation: Change subdomain authentication type using PowerShell and Graph - Microsoft Entra ID | Microsoft Le…

I hope this provides additional clarity.

If you have any other questions, please feel free to reach out.

Thank you for your time.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Yilia Li 0 Reputation points

2026-02-02T08:30:48.73+00:00

Hello thank you very much for your response.

May I ask a follow‑up question?

Question: If our current setup has both company.com (root domain) and cn.company.com (subdomain) bound to the Global Microsoft 365 tenant, and we now need to remove only the subdomain cn.company.com from the Global tenant and re‑bind it to the Microsoft 365 operated by 21Vianet tenant, will this action impact any existing Global Microsoft 365 users who continue to use the root domain company.com?
Vergil-V 16,990 Reputation points Microsoft External Staff Moderator

2026-02-03T04:20:00.1066667+00:00

Hi @Yilia Li
Thank you for your follow-up.

Based on my research, removing the subdomain does not affect the root domain, as these are separate identities and each is treated as its own domain namespace.

The only impact will be on any email addresses or objects that currently use the subdomain @cn.company.com. After the removal, sign-in attempts will fail and email will not be delivered until the subdomain is correctly configured in the intended tenant’s DNS.

That said, please make sure to notify the affected users about the removal, double‑check whether any aliases are using the domain, and back up all necessary data before the cutover.

I hope this helps.
Vergil-V 16,990 Reputation points Microsoft External Staff Moderator

2026-02-04T12:50:29.63+00:00

Hi @Yilia Li
I just wanted to kindly follow up to see if you’ve had a chance to review my previous message. I would greatly appreciate any updates you may be able to provide, particularly regarding the usefulness of the information I shared. Your feedback is essential to my ongoing efforts to enhance the quality of my support.

If your question or concern has been addressed, please consider marking any helpful responses as “Accepted Answer” or giving them an upvote. Doing so helps others in the community find useful guidance more easily and keeps the conversation productive for everyone.

Thank you again for your cooperation.

Answer 2

To address your questions regarding the migration of the subdomain mail.company.com from a Global Microsoft 365 tenant to an Exchange Online operated by 21Vianet tenant, here are the clarifications:

Subdomain Verification: Yes, it is supported to add and verify a subdomain (mail.company.com) in the 21Vianet tenant while the root domain (company.com) remains in the Global tenant. Subdomains can be verified independently as long as the DNS TXT records are correctly configured. This allows you to manage mailboxes under the subdomain without affecting the root domain.
User Principal Names (UPNs): If you migrate only the mailboxes that use mail.company.com to the 21Vianet tenant without modifying the users’ UPNs in the on-premises Active Directory, users will still log in using their UPNs under the root domain (company.com). This setup is feasible, but you should be aware that it may lead to complications in authentication and routing since the mailboxes are hosted in a different tenant than the UPNs. Users may experience issues if there are any dependencies on the UPN for authentication or mailbox access.
Known Limitations: There are potential limitations when the identity (UPN) remains tied to the root domain in the Global tenant while the mailbox is hosted in the 21Vianet tenant under a subdomain. These include:
- Authentication Issues: Users may face challenges logging in if the authentication process does not recognize the mailbox location.
- Routing Problems: Email routing might be affected, especially if there are policies or configurations that depend on the UPN.
- Provisioning Issues: There could be complications in managing user accounts and permissions across the two tenants.

It is essential to ensure that the architecture you plan to implement is officially supported and to test the migration process in a controlled environment to identify any issues that may arise from this configuration.

References: