We are having a hybrid exchange setup. We are trying to read the user calendar using the Microsoft Graph API and when trying to fetch the user calendar of an on-prem exchange, Graph API returning a 401 Unauthorised error. The same works fine with an Exchange Online user calendar. The application has all necessary permissions (Calendars. Read, Calendars.ReadWrite). We are able to fetch on-prem user calendars by login into Microsoft graph explorer (https://developer.microsoft.com/en-us/graph/graph-explorer), but the same not working if we are using Graph API.
Refer to the thread.
https://docs.microsoft.com/en-us/answers/questions/558831/graph-api-on-prem-exchange-read-user-calendar-fail.html?childToView=579364#comment-579364
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