While I wholeheartedly agree with what seems the majority of comments that you make way too many changes to the email system format, I have too much experience with Yahoo to think you are going to listen to your users. (I didn't pick Yahoo, by the way, I was with AT&T.)
However, if you are going to change productively something, change the order of the display of folders.
What is the most likely folder I want next? Inbox.
Why is it fourth?
What is the least likely function I want while in the middle of doing email? Sign out.
Why is it first?
I have to tiptoe around the commands every time you change something because most of the time your changes don't make sense or at least do not work well right out of the box.
If I could transfer my email addresses like one can do with telephone numbers, I would have left Yahoo a long time ago.
Arianna Munari commented
If you can add the progressive number on top while reading an email, and let us drag the folders up and down according to our needs then it's fine, otherwise go back to the previous version!