If you are using the Yahoo app you should stop using it now that you have set up the iPhone mail app. The settings on the Yahoo app are not the correct ones and can be causing problems by still accessing your email account.
Once you have stopped using the Yahoo app you may find that in order to get the iPhone Mail app to work that you will ultimately have to change your email password.
However before doing that you should stop the iPhone Mail app from trying to access your email account. The best way to do that is to turn your phone off. then wait at least an hour or even longer before trying the app again.
This will give the servers a chance to sync. After that time, check that you can still access and send/receive on your email account via a web browser and if it is OK then restart your phone and check it the email app is working.
If that does not work you should try changing your email password.
You should stop your iPhone and any other devices from checking for email. As has already been mentioned the easiest way of doing that is to turn them off if you still have the account on your device. If you have deleted the account and it is no longer on your device do not try and add it at this time.
Once you have done that, change your email password and check that you can access your email via webmail (on a browser).
If that works you should wait at least an hour and then add the new password to your iPhone and check it works. If it does then move onto the next device if you have one.
When setting up the email account on your device you are best to set it up manually, using "other" and ensure that you are using the correct settings as per this link.
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoin...
