Update: 8/3/2026: Writeback for Cloud-Managed Remote Mailboxes: Now Generally Available | Microsoft Community Hub.
In our previous posts, we announced the Public Preview and the General Availability of Cloud-Managed Remote Mailboxes – a key step toward retiring the 'last Exchange Server' in your organization. The response from the community has been incredible, and your feedback continues to shape the roadmap.
Today, we're excited to share two new milestones in this journey:
- Writeback for Cloud-Managed Remote Mailboxes is now in Public Preview.
- For customers with no remaining dependency on their last Exchange Server, a guide for decommissioning your last Exchange Server is now published on Microsoft Learn.
Writeback for Cloud-Managed Remote Mailboxes: Public Preview
When you set IsExchangeCloudManaged to True on a directory-synchronized mailbox, the Exchange-attribute Source of Authority (SOA) transfers to Exchange Online. The SOA for identity attributes (name, department, and so on) stays on-premises in Active Directory, but the Exchange-related attributes (proxy addresses, hide-from-address-book, custom attributes, and similar) become editable in the cloud.
Until now, after transferring Exchange-attribute SOA to cloud those Exchange attributes were edited cloud-side only – they didn't flow back to on-premises AD. That gap was a problem for organizations whose on-premises line-of-business applications still read attributes like proxyAddresses, custom attributes, and similar directly from AD. Once SOA flipped to the cloud, the on-premises AD copy of these attributes would start drifting out of sync with the cloud.
Writeback closes that gap. With writeback enabled, changes made in Exchange Online to designated Exchange attributes are automatically pushed back to on-premises Active Directory through Microsoft Entra Cloud Sync. Your on-premises AD stays current, and your line-of-business applications keep working – even after the Exchange-attribute SOA has moved to the cloud.
How it works
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.
Steps to install the Cloud Sync provisioning agent, configure the writeback synchronization job, and verify the round-trip flow are all in the documentation: Cloud-based management of Exchange attributes for Remote Mailboxes in hybrid environments.
Public Preview limits and GA timeline
During Public Preview, writeback supports tenants with fewer than 200,000 cloud-managed mailboxes. We will raise this limit at General Availability, now available Writeback for Cloud-Managed Remote Mailboxes: Now Generally Available | Microsoft Community Hub.
The complete list of attributes supported for writeback – which attributes flow back to AD and which don't – is available in Identity, Exchange Attributes and Writeback.
New Documentation: Decommission the Last Exchange Server
Once your mailboxes are cloud-managed (and writeback is in place if your applications need it), the next question is the one this whole effort has been about: how do you actually retire the last Exchange Server?
We've published a new end-to-end guide that walks through exactly that: Decommission the last Exchange Server after transferring SOA to cloud.
The guide covers:
- Prerequisites – confirming all mailboxes and public folders have moved to Exchange Online, all directory-synchronized mailboxes are cloud-managed, DNS and mail routing point at Exchange Online, and you've migrated any SMTP relay dependencies.
- Pre-removal verification – re-verifying each prerequisite immediately before starting, since the environment may have drifted.
- Hybrid cleanup (while Exchange is still running) – removing the Hybrid Configuration object, HCW-created intra-organization connector, hybrid connectors, organization relationship, federation trust and certificate, OAuth service principal credentials, and the Hybrid Agent (modern hybrid only).
- Uninstall the last Exchange Server – final pre-uninstall checks and running Setup /m:Uninstall.
- Hybrid cleanup in Exchange Online (post-uninstall) – removing orphaned hybrid objects from the cloud side that the on-prem uninstall doesn't clean up.
If you've been holding off on removing your last Exchange Server because the procedure wasn't clearly documented end-to-end, this is the article you've been waiting for.
Get started
If you've been waiting for Writeback Public Preview to start your decommissioning journey, now is the time:
- Review the updated documentation for the writeback setup walkthrough and the full attribute list.
- Read the new decommissioning guide for the end-to-end uninstall procedure.
- In case the limit of 200k for Writeback feature blocks your adoption, please reach out to us through this form. As communicated, we will increasing the limit by GA timeframe, but it would be good to know what scale would unblock you.
- Share your experiences and suggestions in the comments below – your feedback shaped the previous releases, and we want it for this one too.
The era of maintaining an Exchange server "just because we sync our AD" is coming to an end.
Exchange Online Management and Exchange Hybrid teams
