M365 tenant to tenant migration issues with SMTP and mailbox assignment

TRaG 190 Reputation points
2026-07-10T05:05:15.98+00:00

Here is the query in detail. what we have done is done a manual tenant to tenant migration where in the domain is setup now on target tenant.

During this

  1. Some of the users and mailbox were setup properly
  2. but others are not able to and throwing error regarding the Mailbox, SMTP etc, along with that not able to change to custom domain for email address
  3. User display name shows [Moderator note: Personally Identifiable Information removed] @xxxx.com and can't change it as it comes back, its happening with some users
  4. Basic license is assigned, but mailbox preparation going on from last few days
  5. SMTP shows [Moderator note: Personally Identifiable Information removed] @xxxx.com, instead of ******@xxxx.com
  6. There were some user soft deleted and removed as well during this
  7. error something like
  8. Exchange: The user ID **@xxxx.com is already associated with the user 1[Moderator note: Personally Identifiable Information removed] (or different)
  9. We tried with all PowerShell commands to change it but its not working
  10. 2 object are pointing on one id and that is creating a confusion here
  11. I have access to both tenants as GA
  12. Need help to clean up all residual object id's so that there should not be any conflict and user id and mail box could be created with custom domain

Looking for the help here

Exchange | Other
Exchange | Other

A powerful email and collaboration platform developed by Microsoft, designed to support enterprise-level communication and productivity. Miscellaneous topics that do not fit into specific categories.

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  1. Helen Luu 2,785 Reputation points Independent Advisor
    2026-07-10T07:25:25.3+00:00

    Hello @TRaG,

    Based on the information you provided, it appears that you are experiencing issues during a tenant-to-tenant migration such as unable to update the User Principal Name (UPN), mailbox preparation taking longer than 24 hours, Exchange Online not behaving as expected during the migration process.

    Given the nature of these symptoms, the issue is likely related to a backend service or tenant configuration that cannot be fully investigated through this community forum. It would be best to work directly with Microsoft Support, as they have access to the necessary backend tools and logs to identify configuration conflicts or other underlying issues.

    As a Global Administrator (GA) of both tenant, you can create a support request directly from the Microsoft 365 Admin Center. Once the support ticket is submitted, the Microsoft Support team will be able to perform a deeper analysis of your tenant environment and investigate the issue further. You can refer to: Get support - Microsoft 365 admin | Microsoft Learn

    Please note that this is a user-to-user forum. Accessing to tenant's configuration and backend diagnostic tools. Therefore, Microsoft Support is the appropriate channel for investigating and resolving this issue.

    When creating the support ticket, make sure to disable AI Assistance in the support experience. This will allow the "Contact Support" option to appear so that your request can be routed directly to a Microsoft support engineer.

    Feel free to let me know if you have any additional questions.

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