We have tried using the Graph client that you provided us earlier to update Category of Email in Outlook, but that API is giving “Object reference not set to an instance of an object” error.
Attached here is the code of our Web API which accepts an email as HttpPostedFile and converts it into MapiMessage object.
TestWebAPI.zip (30.5 KB)
Below are the sample email file used for testing,
EmailFiles.zip (46.7 KB)
You can see ItemId property is “null”:
ItemId.jpg (72.1 KB)
Can you please check the sample code we have used in Web API and guide us how we can update the category of Email in Outlook?