I firmly believe that you very badly screwed up a well designed email client, but there is one change that I actually like...

I like having a Send button at the top of email composing. Previously, Send was only at the bottom of the compose page. For short messages, which is mostly what I write, it's much better to have Send at the top.

I think you could improve the New Yahoo Mail Basic by trashing this poorly written new email client code, going back to the Classic code base, and adding this feature to Classic Mail.