What’s on your mind should be on a member user profile page. The member decides what to post ON HIS WALL. This looks is not a social “facebook” what’s on your mind but a bulletin board.
Can’t you do this already with the Private Message plugin? If you can’t, a newsletter plugin is a good idea but you’re concentrating your effort on the wrong thing, right now. By default, it should email the user he has a “private Message” and also contain an excerpt or the full message, just like how forums work.