Sorry, I don't use the gmail viewer (app? website?) to access my "new" cox.net account on yahoo. I do use Thunderbird and just putting in the yahoo settings and new password doesn't work. You also have to use something called OAuth2 (instead of simply "Password") for authentication.
As another example for iPhone, manually setting the yahoo servers and password probably doesn't work, either. In that case, I had to create a new account and select the special "Yahoo!" connection. That is, for the iPhone I couldn't simply change the smtp/pop-or-imap server settings.
Somebody else might have more specific details for the gmail viewer!!
