In case Yahoo asks you to use third party passwords when you are signing in from any other app, like iPhone Mail App, please follow below steps:
- open your web browser and Sign in to your yahoo mail account and go to settings.
- Click Generate app password or Manage app passwords.
- Now Select your application from the drop-down menu and click Generate password.
- Follow the instructions below the password.
- Click Done.
- Now you need to use this app password and your email address to sign in to your email app.
You can also go with signing out of your mailbox > Restart your iPhone > Login to your Yahoo email account again.