In our ongoing commitment to enhance the security and control mechanisms of Exchange Web Services (EWS), we are announcing a significant change in the behavior of the EWSEnabled tenant-wide switch in...
Updated Feb 26, 2025
Version 2.0
Blog Post
We're on the MSFT 365 Apps Semi-Annual Enterprise Channel release cadence, and this broke our users shared free/busy time calendars as well as the Report Message plugin. In testing, it seems to have blocked the AccessRights of AvailabilityOnly and LimitedDetails. If users were changed to Reviewer rights, the issue disappeared. Having no knowledge of Microsoft's blackbox that is Exchange and Outlook, I can only speculate the Reviewer and higher access rights use a different method other than EWS.
I was fortunate enough to know when this failed it was at least likely something EWS related, as back in 2019 MSFT published an Exchange online best practice which recommended disabling EWS. When we did, the same thing happened. Unbeknown to us was the mere existence of the OrganizationConfig EWSEnabled flag, and that it had been changed from null to false at some point prior to November 2018.
I ran into the exact same issue during my testing - monthly enterprise channel. Shared calendars and Out-of-Office stopped working for most of the mailboxes.
Are you using the improved REST for your shared calendar updates or MAPI?
https://support.microsoft.com/en-us/office/how-to-enable-and-disable-the-outlook-calendar-sharing-updates-c3aec5d3-55ce-4cea-84b0-80aab6d8dc26