PS: If that doesn't work, or as alternative fix, open your web browser (NOT your email app), go to the Yahoo login page ( https://login.yahoo.com/config/login_verify2?.intl=uk ) and see if you can delete the emails on their website.
It isn’t the App that’s the problem. According to the Yahoo App, the 2 offending emails have been cleared. It’s the Yahoo account which is one of my 3 accounts set up on my account page in iOS settings that won’t delete, it sits there with a. Yellow dot alongside.
According to Google it is. Google only reports that error message when using the default email app on iOS devices but with email accounts with many different services.