Update 3/27/2025: We updated the "Access to public folders" section below.
This post is a part of our EWS (Exchange Web Services) to Microsoft Graph Migration Guide Series.
In 2018, we announced ...
Updated Mar 27, 2025
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For anyone thinking "Oh I don't need to worry, none of my applications use EWS" - as of right now, Outlook (original) and Teams (new) both use EWS to get calendar info. That will stop working if you disable EWS, or MS, in effect, disable it for you due to the functionality change posted about in the 365 admin centre as MC1015893 and also via this blog post The way to control EWS usage in Exchange Online is changing | Microsoft Community Hub.
With EWS disabled, Teams will stop showing calendars and give you an exclamation mark overlaid on the calendar icon, and Outlook calendars for other people (i.e. additional calendars) will show a warning diamond and "no connection" in the calendar title bar.
The same is true for organisation relationships, which also use EWS to exchange free/busy between tenants using it.