If you haven't already tried this. I went to the AT&T Yahoo Mail website. I haven't used it in years and had forgotten the password. I didn't have any issues using the forgot password option. This will bypass the app interface on the phone. The emails should be in there even if they're not syncing with the app on the phone.
I still have a few apps I don't use sending emails there but most were switched over to Microsoft and Google email apps.