The reason why Windows 10 Mail App rejects icloud password
Apple has changed recently their security policies pushing users to use two-factor authentication or two steps sign in. You probably have enabled the two-factor authentication on your iCloud without knowing it is the main reason why you can’t get iCloud to work in Windows 10 Mail App.
What is that mean? To allow access to your iCloud emails to an application that isn’t owned by apple, you’ll need to generate an app-specific password to add your iCloud account to the Mail app or Outlook. The step by step fix to iCloud account settings are out-of-date error message are described below on a video and text, so chose the solution that works better for you.