Security
Your new Yahoo Mail is not secure. Whenever I hit Log Out, it does not completely log me out me, because whenever I go to Yahoo Mail again, it automatically signs me in even though I do not have that option checked. Also, revert back to the old Classic Mail design. More secured. More visually appealing. Why fix something that is not broken? This new one is flawed.

Thank you for sharing your opinion on New Mail with us. We continually strive to improve and upgrade our products to make them even more useful to our members. As we move our products forward, we must, unfortunately, retire support for older versions.
Regarding this issue, please clear your cookies cache and history. Check this article for more information: https://help.yahoo.com/kb/sln27197.html. After that, make sure that the “Remember me” option and the Autofill in your browser are not checked.