Please don't change the UI again.
I am grateful you gave the option to change back to the original UI. The "updated" version was plagued with issues. If writing an email in a thread, the page would jump and I would have to scroll through the whole thread to find my place. Most of the time the "Send" button was not visible. I would have to write the email on my laptop, save it as a draft, then go to my Yahoo mail mobile app to send it. The "new" Yahoo mail was hiding literally hundreds of emails. I am still going through months of emails that were not visible; some were important messages that didn't show on my desktop or mobile app. When drafting an email, the header options were often not visible. Again, thanks for giving the option to change back.
