Yahoo! has added another layer of account security for users today in the form of second sign-in verification. The new system monitors and verifies what devices you use to sign into your Yahoo! account to prevent unauthorized users from accessing private info, such as your email. If you decide to set up this new security feature, you'll need to ensure your iOS mail app is also authorized to access your Yahoo! account using a special one-time password.
From within your Yahoo! web account you can generate a unique password that you then need to enter into your Yahoo! account on the iOS mail app. This will sync your account and verify that no other devices can access it without your permission.