Exchange Online Adds Writeback to Sync Cloud Mailbox Changes to On-Prem Active Directory

Microsoft is taking a major step toward phasing out on-premises Exchange servers with the public preview of writeback for cloud-managed remote mailboxes. The new capability resolves a long-standing issue in hybrid environments by automatically synchronizing Exchange Online changes back to on-premises Active Directory.

In Exchange Online, cloud-managed remote mailboxes are a feature that lets organizations manage Exchange-related settings for directory-synced users directly in Exchange Online instead of on‑premises servers, while still keeping core identity information in Active Directory. In this setup, administrators can control things like email addresses, mailbox visibility, and custom attributes in the cloud, which reduces the need for an on‑prem Exchange server and simplifies hybrid environments.

According to Microsoft, when organizations shift management of Exchange mailbox attributes (like email aliases or visibility settings) to the cloud, those changes do not automatically update in the on‑premises Active Directory, which many legacy systems and business applications still rely on. This creates a mismatch between cloud and on‑prem data and causes inconsistencies and potential errors in applications that read from Active Directory. It forces organizations to keep an on‑prem Exchange server or manual processes just to keep everything aligned.

How does the new writeback feature fix hybrid Exchange inconsistencies?

This new writeback mechanism automatically synchronizes changes made in Exchange Online back to the on‑premises Active Directory. Instead of cloud updates staying isolated, tools like Microsoft Entra Cloud Sync ensure that key Exchange attributes (such as email addresses and custom fields) are continuously updated in both environments. This eliminates mismatches and keeps legacy systems that rely on AD functioning correctly without requiring manual updates or an on‑prem Exchange server.

“Writeback uses Microsoft Entra Cloud Sync as the transport from Exchange Online back to on-premises AD. If you already use Microsoft Entra Connect Sync, you do not need to uninstall or replace it. Cloud Sync runs alongside Connect Sync – Connect Sync continues to handle your directory synchronization exactly as before, and Cloud Sync only handles the Exchange attribute writeback. There is no impact on your existing mailboxes, users, or sync configuration,” the Exchange team explained.

What are the public preview limitations?

Microsoft mentioned that this new feature allows organizations to keep their on‑premises applications working smoothly by ensuring they still have access to necessary Active Directory data. It also helps to reduce reliance on on‑prem Exchange servers and support a gradual transition toward fully cloud-based Exchange management.

Microsoft notes that writeback for Cloud‑Managed Remote Mailboxes currently supports organizations with up to 200,000 cloud‑managed mailboxes and is available in public preview for commercial customers. The company also plans to raise this limit when the feature reaches general availability in June 2026.