yahoo mail app says unable to access mailbox

i added the yahoo mail app to my samsung galaxy 6, wont connect says unable to access mailbox....

It may be a problem on Yahoo's side. Try later. If still not working try your pc with an email program, not a browser.

The error message means that your email address (the "you" part of you@yahoo.com) isn't a mailbox at Yahoo. You could be spelling it wrong (case shouldn't count), or they may be having problems.