Updated July 20, 2026: We have updated the timeline. Thank you for your patience.
Introduction
Microsoft is introducing an automatic upgrade for shared calendars in Outlook and Exchange Online, transitioning from the legacy MAPI-based model to the modern REST-based model. This server-side upgrade will improve calendar sync reliability, performance, and consistency for delegates and users who access shared calendars. The change is designed to be seamless and will not require any action from users or administrators.
When this will happen
General Availability (Worldwide):
Till WW it is enabled and available to all WW tenants. Now we are expanding it to higher rings:
GCC High /DoD:
- Rollout start: Late April 2026
- Rollout completion: Late July 2026
Rollout starting August 2026 (previously end of June) and completing by late September 2026 (previously late July)
Note: Rollout will be progressive and timing may vary by tenant.
How this affects your organization
Who is affected
- Microsoft 365 tenants using Exchange Online
- Users who have Editor or higher permissions on shared calendars in user or shared mailboxes
- Delegates and users who access shared calendars in Outlook
What will happen
- Shared calendars will be automatically upgraded from the legacy MAPI-based model to the modern REST-based model.
A The service will create a new calendar folder will be created in a hidden state, complete a full synchronization, and will only becomethen make the calendar visible to the user once the full sync is successfully completed and validated, ensuring a seamless rollover to the new model..- The transition is expected to appear seamless, with no interruption to normal user workflows.
- Events older than one yearwill not be
excluded fromincluded in the initial sync window. However, recurring events with instances falling within or after the one-year threshold will still be included in the sync. - Existing permissions and sharing relationships will be preserved; no re-sharing or permission reset will be required.
The upgrade process is fully automatic and requires no action from the user — noNo Outlook restart,restart, profile recreation, or any other manual steps.user action will be required in normal upgrade scenarios.- No data loss is expected as a result of this change.
The rollout will initially support auto-upgrade for upUp to four shared calendars per delegate or principal, scaling up to six over time. Those managing more than six shared calendarsper principal will be excluded from the auto-eligible for upgrade during this rollout cycle and will experience no change to their current setup.
How does it effect the Classic Outlook setting 'Turn on shared calendar improvements'?
After migration, Classic Outlook will continue to honour the existing setting. Although shared calendars areslowly be upgraded to the REST model at the service level, Classic Outlook will follow the setting and use MAPI-based model if the setting is turned off. This behaviour is limited to Classic Outlook; all other endpoints (Outlook Web, Mobile, Mac, New Outlook) will use REST.6.
What you can do to prepare
No action is required.
We recommend that administrators:
During migration, a hidden shared calendar folder may be visible to tenant adminsInform helpdesk and support teams viathat temporarily hidden shared calendars during migration are expected and do not indicate a failure.- Do not recommend deleting and re-adding shared calendars, toggling Shared Calendar Improvements, or creating new Outlook profiles during the
Get-MailboxFolderStatistics cmdlet. This is expected and has no impact on end users.migration, as temporary changes may occur while the upgrade completes. - Monitor for persistent issues, such as missing meetings after the migration window or duplicate calendar folders, and contact Microsoft support if observed.
- If a manual rollback is required after the calendar becomes visible, open a support request through standard Microsoft support channels.
Compliance considerations
No compliance considerations identified. Review as appropriate for your organization.