In an existing project, Notification Scheme includes "All Watchers" and "Current Assignee" under the "Issue Assigned" event which looks like it will throw email on any change in assignment to any watchers of that issue as well as to the new assignee. It's not working that way.
I created a new issue, it was unassigned. I am not a watcher of that issue. I had a friend assign it to me. I never got email notification.