ADMT Domain Migration and Office365

Question

Hello Folks , I have 3 local AD domain first is a.local second is b.local third is c.local.... c.local is the old domain and they are 500 users on this domain. b.local is the onprem exchange domain and a.local , b.local users use this domain for only exchange.. a.local is the New domain and I want to migrate c.local users to a.local.. Also I have Entra and Office365 syncronization.. All users mailbox located on the Office365... (b.local on onprem not functional right now).. I already did the migration users from c.local to a.local with Active Directory Migration tools (users accounts, computer accounts , groups vs vs ) All users can login and access to a.local domain and shared sources on local servers.. but on the Entra after migration Office 365 Mailbox connected to c.local on prem domain.(on prem domains and on prem immurableid related with the c.local Here are questions? How to connect or migrate users mailbox own c.local to a.local on prem without interruption on user side? All ADMT migration transferred with SID history. But on the Entra side a.local created new users not connected to Old Office365 mailbox!!

Answer 1

Hello, @Hakan Guzen,

Welcome to the Microsoft Q&A platform!

To migrate Office 365 mailboxes from c.local to a.local without user interruption, please follow the steps below:

1.Ensure the primary SMTP addresses match between the domains.

2.Verify the Active Directory Migration Tool (ADMT) has transferred all necessary attributes, including sIDHistory.

3.Update the ImmutableId of the new a.local users to match the ObjectGUID of the corresponding c.local users using the **Set-MsolUser** PowerShell cmdlet.

4.Configure Azure AD Connect to synchronize with a.local and perform a full directory synchronization to reflect these changes in Office 365.

5.After synchronization, verify that the Office 365 mailboxes are correctly linked to the a.local users. 6.Ensure UPN and Proxy Addresses are accurately set for the new domain.

7.Once confirmed, remove any remaining dependencies on the c.local domain.

Should you need more help on this, you can feel free to post back. 

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If the answer is helpful, please click on “Accept answer” as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.

Thank you for your support and understanding.

Best Wishes,

Alex Zhang

Answer 2

Thanks for reply Alex , I migrated all users with SID history. No problem on the local AD resources. But I didnt understand the Immutable ID step.. Which Immutable ID shoul I match with mailbox..? Users have c.local access mailbox on Office365.. a new IMMUTABLE ID should A.local users immutable ID or c.local IMMUTABLE ID.. many research says match with upn and a.local users immutable id?