I've prototyped a Taskpane App which works OK for Sent and Inbox etc. but doesn't seem to be possible for Draft emails, but there seems no obvious reason why not, in theory? Can anyone confirm?
Of course the "item" object is slightly different for a Draft email, notably that it hasn't (yet) been assigned an EmailID ...
You probably prototyped your add-on based on MessageReadCommandSurface extension point, which can be activated for the mail read view. "Drafts" folder usually contains the mail items which were not sent. By opening such item, you will continue to compose it. To activate your add-on in the compose form you need to declare in your manifest MessageComposeCommandSurface extension point. The current list of Outlook extension points available at Extension points for Outlook, if you are curious what else can be extended in Outlook. I believe it may be helpful for you to have a look at the tutorial: Build a message compose Outlook add-in.
