We’re excited to announce that Outlook for Mac will soon start syncing calendars via REST for Office 365 customers in the Insider Fast program.


What you need to know


We're introducing a new syncing model for sharing calendars in Outlook for Mac for Office for Mac Insider Fast.  These changes will bring improved reliability and performance of calendar sharing in Outlook for Mac based on the use of REST technology.  


In the past, the list of shared calendars was previously stored locally for each installation of Mac Outlook. As a result of this upgrade, Outlook for Mac will now use the server-roamed list of calendars.


For more information about shared calendar improvements across Outlook, please see this article.


Benefits to moving to REST


When migrated to the new calendar syncing model, customers should expect to experience the following improvements

  • Consistent sharing permissions that matches the other modern Outlook clients (Outlook mobile, Outlook on the web, Outlook for Windows)
  • The ability to share a secondary calendar without having to share a primary calendar
  • New tenant-based setting to allow sharing calendars with people inside or outside your organization
  • A quick action feature to accept calendar sharing invitations from the Inbox
  • View the same calendars in Mac as you see in all the other Outlook applications
  • Open & view any calendar, even if you only have free/busy permissions
  • Support for inline images in meetings & appointments


At this time, these improvements are rolling out to Outlook for Mac Insider Fast customers with mailboxes hosted in Office 365. These Office 365 mailboxes will begin syncing calendars via REST, rather than Exchange Web Services.


We do not have specific timelines yet for Insider Slow customers or Production, but we’ll update you again when we’re close to releasing to those customers.


What the user experience will be


When a user is migrated to REST, they will be prompted to restart Outlook. Once they restart, their calendars will start syncing via REST.


After being upgraded, the user will continue to see all previously opened primary shared calendars, as well as shared calendars that were opened or added in Outlook for web, mobile, or Windows.


In the past, the list of shared calendars was previously stored locally for each installation of Mac Outlook. As a result of this upgrade, Outlook for Mac will now use the server-roamed list of calendars.


Upgrade Experience1 mac rest.png

 Post Upgrade

2 mac rest.png


View free/busy calendars in Mac

 Screen Shot 2018-10-25 at 10.43.21 AM.png


Preparing for change


On October 17th, we will to slowly roll out these improvements to Insider Insiders customers, and the initial release will be a small percentage of Outlook for Mac users.


NOTE: This means that some users will experience the switch sooner than others. Based on user feedback & telemetry, we will continue to extend the improvements to everybody in Insider Fast.



Is Outlook.com accounts supported?

No, currently only Office 365 commercial accounts in the Insider Fast program are supported and will be migrated to REST.


Are there any functional differences between the old model on EWS and the new model on REST?

Categories do not work for old-model shared calendars. Users can upgrade their shared calendar to the new model for full category support. To learn about new vs. old models of calendar sharing, please see this article.


Is REST happening for On-prem accounts?
REST is not supported for customers with Exchange servers on premises. Users on these servers will continue to sync calendars via EWS as they do today.


Is REST applying to calendar only? Why?
REST sync is only applicable to calendars. Any inbox actions, even responding to meeting invitations, will continue to use EWS. This is to ensure that we have parity of features between EWS & REST at this time.


When I add Skype or Teams, the coordinates do not show immediately in the meeting body. Why?
With our move to REST, we no longer need to rely on the Skype for Business client to help schedule online meetings. Instead, we will rely on the Exchange Online APIs that are currently used by Outlook on the web & Outlook mobile, and Outlook for Mac will have the same experience. Meeting co-ordinates are inserted into the meeting body when the meeting is created, so you won’t see the addition until the meeting is sent.







Super Contributor

Will this let the users open SharePoint Calendars now?

Occasional Visitor

How does this change offline behavior?  How does this change when the user is on a slow network? Will the UI hang until the REST request returns?

Occasional Visitor

Will this eliminate the strain put on the local Mac Outlook client when having many or large/old shared calendars enabled?

I have a number of clients experiencing high cpu load, beach balling, sync irregularities and huge calendar caches when they have many/large/old (and thereby often large :-) calendars enabled. 

Pease please say "yes" - unless not true of course :smileyhappy:

New Contributor

It's February 28th 2019 and this has STILL not rolled out. This is pathetic.

Can someone from MS comment as to when this will be available to users?

If it's in progress, is there a way to force enable it using Powershell on the org?

The latest version of Office 365 Outlook does not work with WebEx. Are you working with WebEx to fix the issue?. 

Are their any plans to add Save to OneNote? 

Occasional Visitor

After migration, my Commercial calendar stops syncing. Now shows "Sync pending for this folder". Any toughts?

Senior Member

Is this now released with the latest 16.23 Outlook versions? Many of my internal users, have received both the interface changes and the upgrade notification message.

New Contributor

@Kamen Kanev  Only some of our users are getting the update. It is not consistent.

All our versions of Outlook for Mac are on 16.23 yet it is hit and miss as to whether they get the REST calendar upgrade along with the Teams add-on in calendar.

Incredibly frustrating as there is no way to force the update. It blows my mind that Microsoft saw fit to roll it out in this manner.

Senior Member

@anon_ .... this is **bleep**.. and how can i found out who has been updated and who has not? I have been communicating this with Office365 support team in December, but it appeared that they had no idea about this upcoming change.. If i check now the Office365 road map it says it is already launched, so really lost.... 


I am also disappointed on the small amount of information available and how unclear is everything. initially, it was said that already shared calendars will not be migrated to REST unless they are re-shared, now this part has stopped being communicated. 


We are desperately looking for this changes since we are fully mac based and work a lot with shared calendars with Edit permissions, and last 24 months it was a pain in the **bleep** for us, with huge amount of duplicating appointments and meetings + cases of disappearing appointments due to local cache syncing on Outlook for Mac.

Occasional Visitor

Is it possible to disable this syncing? Desktop Outlook performance is significantly impacted by large numbers of shared calendars so we recommend users to limit the number on the desktop and use the we client for viewing other calendars. This change has severely impacted desktop performance.