Here is what yahoo help says: Can't send or receive Yahoo email in a third-party app
This means there's something wrong with the app. You can download the Yahoo Mail app from the app store and use that instead or do any of the following:
Apps with outdated security now require you grant them access in Yahoo Mail.
Delete Yahoo Mail from the app and re-add it. Steps for mobile:
- Android Mail - Remove and re-add your account.
- iOS Mail - Remove your account then re-add your account.
Contact your app manufacturer for additional troubleshooting options.
When I went thru the delate/add process in Apple App - it made me go thru the Access key step. I have an account key - that I had just setup a day ago - maybe that forces me down the Access Key path. Nonetheless the Yahoo help clearly says "now require you grant them access" - this simply means Access key.
Everybody to their own opinion and interpretation.