Forest and Exchange Online Migration, Company Merge

Question

Scenario

Company A has Single Forest multiple domain and has on premises Exchange server .Company A acquired company B recently. Company B has a single-forest, single-domain structure, and an on-premises exchange as well. Management wants to merge all the systems with company A so that all the users will have company A's mail domain and GAL and eventually wants to fully decommission company B. Company A also makes the decision to move Exchange to Office 365 in a hybrid model.

Question. Is it possible to configure an Exchange hybrid setup from company B to default tenant (tenant created for company A) and use company A mail domain, as well as reverse sync, users from office 365 to company A’s Active Directory using AADC?.

We would like to do the things as below. Please advise.

1.     Create trust between the domains.

2.     Create AADC (Multiple forests, single Azure AD tenant topology)

3.     Create Tenant for the company A with default domain as CompanyA.com.

4.     Create Exchange Hybrid configuration from Company B to Office 365 Tenant. (Described in the step 3)

5.     Reverse Sync company B users from office 365 to company A Active Directory (Not sure this is possible)

6.     Extend company A Network to Company B site and join the workstation to Company A domain.

7.     Add Company B domain as Accepted domain in Office 365.

8.     Change company B users mail domain to Company A and route migrated Company B users mail through Office 365.

9.     Create Exchange Hybrid configuration from company A to Office 365.

10.  Complete company A mail migration.

Answer 1

Yes, it is possible to configure an Exchange hybrid setup from company B to the default tenant (created for company A) and use company A's mail domain, as well as reverse sync users from Office 365 to company A's Active Directory using AAD Connect.

1. Creating a trust between the domains will allow for the sharing of resources and authentication between the two domains.
2. AAD Connect (Multiple forests, single Azure AD tenant topology) can be used to connect the two Active Directories and synchronize user information.
3. Creating a tenant for company A with the default domain as CompanyA.com will allow for the use of Office 365 services with that specific domain.
4. Creating an Exchange Hybrid configuration from company B to the Office 365 tenant will allow for the migration of email to the cloud.
5. Reverse syncing company B users from Office 365 to company A's Active Directory is possible, but it may require additional configuration and planning.
6. Extending company A's network to the company B site and joining the workstations to the company A domain will allow for the use of company A's resources and services.
7. Adding company B's domain as an accepted domain in Office 365 will allow for email to be sent and received using that domain.
8. Changing company B users' mail domain to company A and routing migrated company B users' mail through Office 365 will ensure that all email communication is using the same domain.
9. Creating an Exchange Hybrid configuration from company A to Office 365 will allow for the migration of email for company A as well.
10. Completing the company A mail migration will finalize the process of migrating all email to the cloud and decommissioning the on-premises Exchange servers.

However, it's important to consider that this process could be complex and may require additional planning and testing to ensure a smooth migration. It would be best to consult with a professional before proceeding with the migration.

Answer 2

Hi @Navas Muhammed ,

According to my research，Azure AD Connect can sync AD on-premises objects to Azure AD  , but Azure AD Connect cannot create these users in on-premises AD if the user objects are created directly in the cloud.

Regarding online users syncing to on-premises AD, I found the following guide for your reference：Sync Azure AD user to on-premises AD - ALI TAJRAN

(Note:Microsoft provides third-party contact information to help you find additional information about this topic. This contact information may change without notice. Microsoft does not guarantee the accuracy of third-party contact information.)

It can be roughly divided into the following steps:

1.     Create the same AD object as the cloud object and fill in the same details.

2.     Log on to the AADC server to force synchronization.

3.     Check the Azure AD Connect Synchronization Service to verify that it has added the on-premises AD user object to Azure AD with no errors.

4.     Verify the  AD Object Synchronization status in M365 Central Administration.

5.     Make cloud mailboxes visible in your Exchange on-premises deployment.

Hope this helps!

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