*For this entry exists the more relevant or more recent entry (MC1289719)
check before: 2026-05-01
Product:
Exchange, Microsoft 365 suite, Outlook
Platform:
Mac, mobile, Online, US Instances, Web, World tenant
Status:
Change type:
Admin impact, Updated message, User impact
Links:
(MC1289719)
Details:
Summary:
Microsoft is auto-upgrading shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between mid-May and late July 2026. The seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year, and supports up to six calendars per user.
Details:
Updated May 6, 2026: We have updated the content. 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):
Rollout starting mid-May 2026 (previously early May) and completing by late July 2026
GCC High /DoD:
Rollout starting end of June 2026 (previously mid-May) and completing by late July 2026
Note: Rollout will be progressive and timing may vary by tenant.
Change Category:
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Scope:
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Release Phase:
Created:
2026-04-22
updated:
2026-05-07
Task Type
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Docu to Check
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MS How does it affect me
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MS Preperations
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MS Urgency
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MS workload name
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summary for non-techies**
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Direct effects for Operations**
Please, look at the most relevant linked item for details
explanation for non-techies**
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** AI generated content. This information must be reviewed before use.
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change history
| Date | Property | old | new |
| 2026-05-07 | MC Messages | Updated April 29, 2026: We have updated the content. 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): Rollout starting early May 2026 and completing by late July 2026 GCC High /DoD: Rollout starting mid-May 2026 and completing by late July 2026 Note: Rollout will be progressive and timing may vary by tenant. | Updated May 6, 2026: We have updated the content. 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): Rollout starting mid-May 2026 (previously early May) and completing by late July 2026 GCC High /DoD: Rollout starting end of June 2026 (previously mid-May) and completing by late July 2026 Note: Rollout will be progressive and timing may vary by tenant. |
| 2026-05-07 | MC Last Updated | 04/29/2026 18:38:33 | 2026-05-06T18:27:55Z |
| 2026-05-07 | MC Summary | Microsoft will auto-upgrade shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between May and July 2026. This seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year, and supports up to six shared calendars per user. | Microsoft is auto-upgrading shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between mid-May and late July 2026. The seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year, and supports up to six calendars per user. |
| 2026-04-30 | MC Messages | Updated April 22, 2026: We have updated the timeline and content. 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): Rollout starting early May 2026 and completing by late July 2026 GCC High /DoD: Rollout starting mid-May 2026 and completing by late July 2026 Note: Rollout will be progressive and timing may vary by tenant. | Updated April 29, 2026: We have updated the content. 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): Rollout starting early May 2026 and completing by late July 2026 GCC High /DoD: Rollout starting mid-May 2026 and completing by late July 2026 Note: Rollout will be progressive and timing may vary by tenant. |
| 2026-04-30 | MC How Affect | 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 new calendar folder will be created in a hidden state and will only become 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 year will be excluded from 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 - no Outlook restart, profile recreation, or any other manual steps. No data loss is expected as a result of this change. The rollout will initially support auto-upgrade for up to four shared calendars per delegate or principal, scaling up to six over time. Those managing more than six shared calendars will be excluded from the auto-upgrade and will experience no change to their current setup. | 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 new calendar folder will be created in a hidden state and will only become 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 year will be excluded from 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 - no Outlook restart, profile recreation, or any other manual steps. No data loss is expected as a result of this change. The rollout will initially support auto-upgrade for up to four shared calendars per delegate or principal, scaling up to six over time. Those managing more than six shared calendars will be excluded from the auto-upgrade 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 are 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. |
| 2026-04-30 | MC Last Updated | 04/22/2026 20:47:40 | 2026-04-29T18:38:33Z |
| 2026-04-30 | MC Summary | Microsoft will auto-upgrade shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between May and July 2026. The seamless, server-side upgrade requires no user action, preserves permissions, excludes events older than one year, and supports up to six shared calendars per user. | Microsoft will auto-upgrade shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between May and July 2026. This seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year, and supports up to six shared calendars per user. |
| 2026-04-23 | MC prepare | No action is required.
We recommend that administrators: Inform helpdesk and support teams that 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 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. | No action is required.
We recommend that administrators: During migration, a hidden shared calendar folder may be visible to tenant admins and support teams via the Get-MailboxFolderStatistics cmdlet. This is expected and has no impact on end users. 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. |
| 2026-04-23 | MC MessageTagNames | User impact, Admin impact | Updated message, User impact, Admin impact |
| 2026-04-23 | MC Summary | Microsoft will auto-upgrade shared calendars in Exchange Online from the legacy MAPI model to the modern REST model between April and July 2026. The seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year initially, and requires no Outlook restart. | Microsoft will auto-upgrade shared calendars in Outlook and Exchange Online from the legacy MAPI model to the modern REST model between May and July 2026. The seamless, server-side upgrade requires no user action, preserves permissions, excludes events older than one year, and supports up to six shared calendars per user. |
| 2026-04-23 | MC Last Updated | 04/22/2026 00:51:33 | 2026-04-22T20:47:40Z |
| 2026-04-23 | MC Messages | [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): Rollout start: Late April 2026 Rollout completion: Late July 2026 Rollout timing may vary by tenant. | Updated April 22, 2026: We have updated the timeline and content. 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): Rollout starting early May 2026 and completing by late July 2026 GCC High /DoD: Rollout starting mid-May 2026 and completing by late July 2026 Note: Rollout will be progressive and timing may vary by tenant. |
| 2026-04-23 | MC Title | Auto upgrade of shared calendars from legacy MAPI model to modern REST model | (Updated) Auto upgrade of shared calendars from legacy MAPI model to modern REST model |
| 2026-04-23 | MC How Affect | 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. The service will create a new calendar folder in a hidden state, complete a full synchronization, and then make the calendar visible. The transition is expected to appear seamless, with no interruption to normal user workflows. Events older than one year will not be included in the initial sync window. Existing permissions and sharing relationships will be preserved; no re-sharing or permission reset will be required. No Outlook restart, profile recreation, or user action will be required in normal upgrade scenarios. No data loss is expected as a result of this change. Up to four shared calendars per delegate or per principal will be eligible for upgrade during this rollout cycle and will slowly be upgraded to 6. | 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 new calendar folder will be created in a hidden state and will only become 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 year will be excluded from 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 - no Outlook restart, profile recreation, or any other manual steps. No data loss is expected as a result of this change. The rollout will initially support auto-upgrade for up to four shared calendars per delegate or principal, scaling up to six over time. Those managing more than six shared calendars will be excluded from the auto-upgrade and will experience no change to their current setup. |
Last updated 1 week ago ago
