You will find this post useful if you are experiencing some problems using your Yahoo account on your iPhone. Yahoo and Apple work on improving their apps and systems constantly. The settings connecting Yahoo Mail to the iPhone app can become incorrect or corrupt sometimes due to these updates. If you can't sign into Yahoo Mail on your iPhone, remove and reinstall Yahoo Mail on the iOS app to reset the information. Also, check whether you are inputting the correct login credentials. Your mobile browser cache and cookies have recently been cleared, so you are signed out of this session and need to sign in again. Yahoo blocks access to its account from applications with outdated security. Allow access to an insecure app while you change apps.

See also - create a Yahoo account with Gmail , Yahoo small business email settings, Forward Yahoo Email to Another Email