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.
Applies to: Microsoft 365 suite
Source: mc.merill.net — data via Merill Fernando's open archive (MIT).