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Moving from MAPI to REST for Calendar Sharing
Message center notification MC1287370 (last updated 5 May 2026) is interesting from several perspectives. The notification describes an automatic upgrade for Exchange Online shared calendars to move away from the legacy MAPI-based sharing model that’s been around for years to a modern REST-based model.
No user or administrator action is required because the upgrade is service-based. The upgrade is now under way and is due to be completed for commercial tenants in late July 2026. Government tenants will see the update in June 2026.
The automatic upgrade will introduce the new sharing model across Microsoft 365. Up to now, using the new model has been optional.
A Seamless Changeover
Microsoft says that the changeover is designed to be seamless. Like the Teams switchover for private channel infrastructures, everything runs in the background. Unlike the Teams switchover, there’s no PowerShell cmdlet that you can run to find out if the update is over. Microsoft says that a “hidden shared calendar folder may be visible” if you run the Get-MailboxFolderStatistics cmdlet. A command like this will list all folders for a mailbox sorted by creation date with the most recent folders at the top:
Get-MailboxFolderStatistics -Identity James.Way@office365itpros.com | Sort-Object {$_.CreationTime -as [datetime]} -Descending | Format-Table Name, CreationTime, ItemsInfolder
MC1287370 says “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.” This doesn’t mean that Exchange Online will create a new version of the user’s own Calendar folder. Instead, it refers to the copies of shared calendars created in delegate mailboxes.
The “full sync” means the population of events for the last year in the new calendar folders. Microsoft notes that the current automatic upgrade applies for up to four shared calendars per delegate or principal (calendar sharer) and will scale up over time to handle six shared calendars. Delegates who manage more than six shared calendars are excluded from the upgrade for now and will continue to use the current model.
In the light of the information given in MC1287370, I guess you could run Get-MailboxFolderStatistics to try and identify the hidden folders created for shared calendars or check the folders for shared calendars to see if they were recently created. It’s possible that you might gain some limited insight into the upgrade with this information. However, running cmdlets without really knowing what you’re looking for is the PowerShell equivalent of looking for the proverbial needle in a haystack, so it’s probably best to wait in patient expectation for everything to finish.
Seeking Better Calendar Sharing
Microsoft says that the “upgrade will improve calendar sync reliability, performance, and consistency for delegates and users who access shared calendars.” In seeking more reliable and robust calendar sharing, Microsoft is addressing known issues that have been around for a long time.
In 2023, Microsoft published a technical community post to explain the issues with calendar sharing and what they planned to do about it (which is what’s happening now). In a nutshell, the new model (Figure 1) is simpler and leverages the service instead of depending on client transactions.
Calendar sharing might seem to be a straightforward operation, but some companies make very heavy use of the facility, with some delegates managing over twenty calendars. Throw in multiple types of Outlook client, and the situation becomes way more complex.
Into the Future
Apart from sorting out calendar sharing, moving from MAPI to REST is also part of the transition towards the new Outlook and the phase-out of Outlook classic within Microsoft 365 sometime after 2029. MAPI is the foundation of Outlook classic whereas the new Outlook and OWA are based on modern service-based APIs and already use the REST-based calendar sharing model. It obviously makes sense to standardize calendar sharing for Outlook clients around what will be used in the future instead of what’s been used in the past.
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