Just got this from Yahoo recently on their updated security:
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Password errors and access issues
If you try to use Yahoo Mail in a third-party email client using less secure sign-in technology, you might get a "Server password has changed" or an "Authentication failed" error message. This is because those email apps use outdated security protocols and we've disabled access to them by default now.
Use a secure option to access Yahoo Mail
Don't worry! You have options to keep using your Yahoo Mail securely and without interruption:
Continue using third-party mail apps securely
Use an app password
If you use Yahoo two-step verification, Account Key or an older email app, you may need to use an app password to access Yahoo Mail. Just remember, this is not intended for permanent access and you'll want to download the Yahoo Mail app or follow the steps below for specific third-party applications.
Learn how to generate third-party app passwords.
More information on specific third-party platforms
If you prefer to continue accessing your Yahoo email on your current third-party email application, including the default email application on your mobile phone, please see details below on how to ensure it's syncing with the latest secure sign-in method.
Access mail via Apple iOSRemove your Yahoo account and re-add it to the Apple email app on your mobile device. When you re-add the account, look for the Yahoo provider logo to trigger the secure sign-in method.
Access mail via Apple Mail on Mac OS
Remove your Yahoo account and re-add it to Apple Mail on Mac. When you re-add the account, look for the Yahoo provider logo to trigger the secure sign-in method.