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EWS API throws error “No Mailbox with such GUID”

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Recently i was working with an ISV/application developer, who created this custom application using EWS (Exchange Web Services) API. It worked with the account with impersonation rights to access other mailboxes/calendars in Azure/Office 365/Exchange Online.

But when they tried to access a specific mailbox calendar, they see the error “No mailbox with such GUID”.

We looked at code and it’s straightforward one:

 var service = new ExchangeService();
service.Credentials = new NetworkCredential("username", "password");
service.Url = new Uri("Url");

In order to isolate the issue, i tried using EWS Editor utility as well and we can repro the issue there too. When we debugged further, we found the error is caused by an unlicensed mailbox account. By license to account, customer confirmed it started working for them.

Hope this helps!!