There isn't really a 'best place' because who receives the notification will also depend on who is following the work-item or case or whatever the current object is.
You could post a question to their user profile so that only they and their followers will receive the notification.
If this is a common use case where several people will post questions to one person, then maybe you can see a separate discussion group just for that user to answer questions, sort of in a 'forum-esque' format, kind of like 'Reddit'. As long as no one follow the group, only he/she will receive the email notifications.
Hope this helps!