There are times when we cannot access our email account using some other devices or email client platforms. The main problem arises with iOS devices as there are several complex settings to Setup Yahoo mail on iPhone for using it efficiently. These problems can be as mere as a poor connection or as big as doing complicated server settings.
While doing server settings, you also need to keep in mind that the Account Type for the server should be IMAP and not POP. As IMAP allows you to use your email on different devices while POP prohibits its usage on different email clients. IMAP also syncs your email account with the email server to save your changes and reflect them on all the devices in which your email is logged in.
For doing Yahoo Mail Server Settings on app in iPhone you need to go to the phone settings and look for the “Mail and Calendar” option, by clicking on which you will see mail server settings where you have to fill in the correct information for proper mail functioning.
If you are not able to login to your Yahoo account, then you can check if it is working on different devices properly or log in using a web browser. If it login, then you need to update your Apple Mail, older version of an application might be the problem due to which you can't sign into yahoo mail.