In May, we announced the Public Preview of Writeback for Cloud-Managed Remote Mailboxes. Since then, many customers have enabled this feature and shared their feedback. Today, we're excited to announce that Writeback for Cloud-Managed Remote Mailboxes is now Generally Available (GA).
Writeback is available in WW, GCCH, DoD, and 21Vianet environments and supports up to 600,000 cloud-managed mailboxes per tenant.
A quick recap
Cloud-Managed Remote Mailboxes allow organizations to transfer the Source of Authority (SOA) for a directory-synchronized mailbox's Exchange attributes to Exchange Online by setting IsExchangeCloudManaged to true.
The user identity remains synchronized from on-premises Active Directory but Exchange attributes become editable in Exchange Online. Administrators can update these properties through Exchange Online PowerShell, the Exchange admin center, or the Microsoft 365 admin center.
Writeback extends this capability by synchronizing a critical set of Exchange attribute changes from Exchange Online back to on-premises Active Directory through Microsoft Entra Cloud Sync. This helps organizations whose on-premises line-of-business applications continue to read Exchange attributes from Active Directory.
If you already use Microsoft Entra Connect Sync, you don't need to uninstall or replace it. Cloud Sync runs alongside Connect Sync:
- Connect Sync continues to synchronize your directory identities and attributes as before.
- Cloud Sync handles Exchange attribute writeback.
What's new at GA
Support for up to 600,000 cloud-managed mailboxes
During Public Preview, writeback supported tenants with fewer than 200,000 cloud-managed mailboxes. At GA, the supported scale increases to 600,000 cloud-managed mailboxes per tenant.
This expanded scale enables larger organizations to adopt cloud-managed Exchange attributes while keeping the required Exchange attribute values current in on-premises Active Directory.
Writeback for the MAIL attribute
One of the most requested feature during Public Preview was support for writing the mail attribute back to on-premises Active Directory.
Based on this feedback, GA adds the mail attribute to the supported writeback set. Changes made to WindowsEmailAddress in Exchange Online can now be written back to the corresponding mail attribute in Active Directory.
The supported writeback set now includes 24 attributes, including:
-
extensionAttribute1 through extensionAttribute15
-
msExchExtensionCustomAttribute1 through msExchExtensionCustomAttribute5
-
msExchRecipientDisplayType
-
msExchRecipientTypeDetails
-
proxyAddresses
-
mail
For the complete attribute list, see Identity, Exchange Attributes and Writeback.
Existing Public Preview configurations require one update
Exchange attribute writeback configurations created on or after August 3, 2026 will have mail writeback enabled by default. No additional action is required for newly created configurations.
Configurations created before August 3, 2026 aren't updated automatically to writeback mail attribute. If you enabled writeback during Public Preview:
- Open your Exchange Online attribute writeback configuration in the Microsoft Entra admin center.
- Select Attribute mapping and then Restore default mappings.
- After the synchronization job restarts, mail writeback will be enabled.
Detailed guidance is available in the Frequently Asked Questions section of the documentation.
Important reminder about tenant-wide SOA
Tenant-wide Exchange attribute SOA is intended for organizations that have completed mailbox migration to Exchange Online and no longer create Exchange mailboxes, mail-enabled users, or remote mailboxes on-premises.
Don't enable tenant-wide SOA while on-premises mailbox migration or recipient creation is still ongoing. Doing so can cause a newly synchronized Exchange recipient to appear in Microsoft Entra ID as an identity-only user without the MailUser required in Exchange Online. This blocks mailbox onboarding and migration.
Review the prerequisites and guidance in Cloud-based management of Exchange attributes for Remote Mailboxes in hybrid environments before enabling tenant-wide SOA.
A path towards removing the last Exchange Server
Cloud-managed Exchange attributes and writeback help organizations continue using Active Directory for identity while removing their dependency on an on-premises Exchange Server for recipient management.
When you're ready to remove the server, follow Decommission the last Exchange Server after transferring SOA to cloud. The guide covers prerequisites, hybrid cleanup, Exchange Server uninstall, and post-uninstall cleanup in Exchange Online.
Exchange-attribute SOA applies to user objects with Exchange Online mailboxes. Organizations that want to manage mail-enabled groups or mail contacts from the cloud should use Group SOA or Contact SOA transfer.
Get started
- Review Cloud-based management of Exchange attributes for Remote Mailboxes in hybrid environments.
- Follow How to enable Exchange attribute writeback.
- Review the complete writeback attribute list.
- Read the last Exchange Server decommissioning guide.
Thank you to everyone who participated in the Public Preview and shared feedback. Your input led to us adding mail attribute writeback for GA.
We look forward to hearing about your experience with the GA release.
Exchange Online Management and Exchange Hybrid teams
