Fix problems with Yahoo Mail in iOS Mail
Most iOS Mail app issues are caused by invalid settings or if there's something wrong with the Yahoo Mail account that you're trying to add. Whenever you have this error we always recommend you to verify if your login info works on a desktop computer. Log in to Yahoo Mail, compose an email, send it to yourself and see if arrives in 5 minutes.
Apple updates their system anytime they detect and fix an issue. Make sure you're using the latest version of the iOS software.
Both Yahoo and Apple work constantly on updating their apps and systems. Sometimes these updates may cause the settings that connect Yahoo Mail to the app to become incorrect or corrupt. Remove and add back Yahoo Mail on the iOS app to refresh the info.
If you're still unable to connect to the iOS Mail app, there might be something wrong with the device. You can still access Yahoo Mail by downloading the Yahoo Mail app.