Owh, deep thinking:
The old way: you could delete a post and the "receiver" would never know there had been a post. So it could be retrieved as long as it showed "unread by:".
The new way: the message page shows "unread" but you don't really need to open it any more, because you read it already.
Now I have to think over the pro's and contra's of these two ways.
You cannot undo what you wrote and regret before the message hits the (un)lucky adressed one. (contra)
I see no pro sofar.