Hi, I just had to go through and do this again for one of my e-mail accounts that was not getting messages! It was corrected and working as soon as I completed this process. Looks like this might need to be done from time to time.
"Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app. " Sign into https://www.yahoo.com/ and go to your Account security page. Click Generate app password or Manage app passwords. Select your app from the drop down menu and click Generate. Follow the instructions below the password. Click Done. Use this app password and your email address to sign in to your email app.