Your email functions UX/UI is less and less functional and more and more useless. I do not know why in the sent hisotory I can not see the message to which I replied and why I can not reply to email twice. Each time I want to reply again I am sending it to myself, I need to start a new email.
If you want people to stop using yahoo mail you can just say so, there is no need to make it more difficult for the user.