Decommission Exchange Server in Cross-Forest AD migration (mailboxes in Exchange Online)

Hello Alex,

Thanks for your answer.

I would like to highlight, that we do not want to keep Exchange on-prem in the new environment and we would like to set up the new environment as a cloud-only (for mailboxes). The user objects would still be syncing from the local AD.

Is there a way to achieve this?

I was thinking to:

1. Install Exchange on-prem in the destination environment, to expand the schema of the new forest.
2. Once installation is done, we will then uninstall exchange server from the AD as we do not want to keep on-prem exchange.
3. Then perform the migration of user objects through ADMT.
4. Set AD Sync up in the destination forest, in staging mode to check the behavior of some of the migrated objects and their mail flow.

Once things appear good, we will plan to migrate the rest of the object from source AD to destination AD (cross-forest).

Please let me know your thoughts on this approach, and if there's anything which I can do differently.

Hello, @Ashutosh Kumar,

Thank you for your prompt and patient reply.

I have understood your requirement that you do not want to keep Exchange local in your new environment and want to set your new environment as cloud only (for mailboxes). Your approach is generally sound. I will then respond and make suggestions on the four points you mentioned.

1. Extending the architecture for New Forest is necessary to extend the architecture. At a minimum, you will need to install Exchange Management Tools to prepare for the Office 365 hybrid configuration.
2. According to Microsoft official document, at least one Exchange server should be left for management, it can be deactivated, but it is not recommended to uninstall it. It is not recommended to uninstall it, because after deleting it, if there are problems with AD properties, then managing local mailboxes will become complicated. It is time consuming and labor intensive to manage local servers, but it is good for management. If you feel that the local server is completely useless and you won't manage AD properties in the future, then you can uninstall the Exchange server from AD according to How and when to decommission your on-premises Exchange servers in a hybrid deployment | Microsoft Learn after the architecture extension is completed.
3. Migrating user objects via ADMT is critical for migrating user accounts to the new AD forest. Ensure that you migrate all necessary properties, including those related to mail and test with a small group of users before proceeding with a full migration.
4. Configuring Azure AD Connect staging mode allows you to verify the synchronization process without disrupting your production environment. This step is critical to ensure that migrated objects and their mail streams are working properly. After a successful staging mode check, you can proceed with a complete object migration and fully test the mail flow and other features.

As for adjustments I suggest you make, please consider the following points.

1. Install Exchange in the target environment to prepare the architecture and setup the hybrid configuration tools. Retain a minimal hybrid environment to ensure smooth administration and supportability.
2. Migrate users using ADMT and verify synchronization with Azure AD Connect in staging mode. Ensure full testing of migrated objects and mail flows.
3. Plan for long-term Exchange attribute management and consider retaining an Exchange server with minimal roles if necessary.

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Thank you for your support and understanding.

Best Wishes,

Alex Zhang