Writeback for Cloud-Managed Remote Mailboxes: Now in Public Preview

In our recent updates, we talked about the launch of the Public Preview and the General Availability of Cloud-Managed Remote Mailboxes. This is a major step towards phasing out that final Exchange Server in your organisation. The feedback from our community has been fantastic, and it continues to influence our future plans.

Today, we’ve got two exciting announcements for you:

1. Public Preview of Writeback for Cloud-Managed Remote Mailboxes is now live.
2. If you’re no longer reliant on your last Exchange Server, we have a detailed guide on decommissioning your final Exchange Server, available on Microsoft Learn.

By setting the IsExchangeCloudManaged option to True on a directory-synchronised mailbox, the Exchange-attribute Source of Authority (SOA) transitions to Exchange Online. While basic identity attributes like name and department remain in your Active Directory on-premises, the Exchange-specific attributes—such as proxy addresses and custom settings—can be managed directly in the cloud.

Previously, after the Exchange-attribute SOA had shifted to the cloud, those attributes could only be altered online and didn’t sync back to your on-premises Active Directory. This created issues for organisations that still relied on on-prem solutions because attributes like proxy addresses could become outdated. However, the introduction of Writeback resolves this problem.

With Writeback implemented, any changes you make to Exchange attributes in Exchange Online are automatically updated back to your on-premises Active Directory via Microsoft Entra Cloud Sync. This keeps your on-premises AD up-to-date and ensures the continued functionality of your line-of-business applications—even when the Exchange attribute SOA is in the cloud.

Writeback employs Microsoft Entra Cloud Sync to facilitate the transfer of data from Exchange Online to your on-premises AD. If you’re already using Microsoft Entra Connect Sync, you won’t need to uninstall it. Cloud Sync will function alongside Connect Sync, handling only the Exchange attribute writeback, ensuring that your current mailboxes, users, or synchronization configurations remain unaffected.

You can find detailed instructions on installing the Cloud Sync provisioning agent, setting up the writeback synchronisation job, and verifying that everything is working correctly in our documentation: Managing Exchange Attributes for Remote Mailboxes in Hybrid Environments.

During the Public Preview phase, Writeback supports tenants with fewer than 200,000 cloud-managed mailboxes. We plan to raise this limit by the General Availability date, currently anticipated for the end of June 2026.

To see which attributes are supported for writeback, along with those that aren’t, check out this list in our documentation: Identity, Exchange Attributes, and Writeback.

Once your mailboxes are cloud-managed, and writeback is set up (if needed), you may wonder about the next big step—how to retire that last Exchange Server.

We’ve put together a comprehensive guide that outlines the entire process: Decommission the Last Exchange Server After Transitioning SOA to the Cloud. This guide includes:

• Prerequisites – Ensure all mailboxes and public folders are with Exchange Online, all directory-synced mailboxes are cloud-managed, DNS and mail routing point at Exchange Online, and any SMTP relay dependencies have been migrated.
• Pre-removal Verification – Double-check each prerequisite right before you start the process, as things may have changed.
• Cleaning Up Hybrid Configurations – Remove the Hybrid Configuration object, any intra-organisation connectors created by HCW, hybrid connectors, organisation relationships, federation trusts and certificates, OAuth service principal credentials, and the Hybrid Agent (if using modern hybrid).
• Uninstalling the Last Exchange Server – Conduct final checks and then run Setup /m:Uninstall.
• Hybrid Cleanup in Exchange Online (After Uninstallation) – Clear out any orphaned hybrid objects from the cloud that didn’t get removed with the on-prem uninstall.

If you’ve been hesitant to remove your last Exchange Server due to unclear procedures, this document is just what you need.

If you’ve been waiting for the Writeback Public Preview to kickstart your decommissioning journey, the moment has arrived. Here’s what to do next:

• Check the updated documentation for the writeback setup guide and the complete list of attributes.
• Review the new decommissioning guide for a step-by-step uninstall procedure.
• If the 200k limit for the Writeback feature presents an obstacle for you, please fill out this form. We plan to increase the limit by General Availability, but we’d like to understand your scale to help facilitate this.
• We’d love to hear from you! Share your experiences and suggestions in the comments below. Your feedback has been crucial to our previous releases, and we want to continue that with this one.

The time has come to let go of maintaining an Exchange server “just for AD sync purposes.”

Exchange Online Management and Exchange Hybrid teams