If you get the following error when using the Office.context.mailbox.makeEwsRequestAsync() method, it is probably because the supported method is not available to this function:
The requested web method is unavailable to this caller or application
The details of the supported function are outlined in this article on MSDN. If you do not see the method you are trying to call in that table, it is not supported.
However, if you are like me and you do see your function in there, like “ExpandDL” it might be because when you recently updated to CU9 or CU10 (this method was added in CU9), you did not run setup with the “PrepareAD” switch. This last step is required in order to update the policies that allow this function to work. You can also run these two commands from the Exchange PowerShell to update your server:
NOTE: If these commands are not found, you need to first run this command: