Apple has sent out a support email explaining that, starting June 15, third-party apps using iCloud to store data will require app-specific passwords across the board. If you want to continue using your favorite third-party app with your iCloud account, you’ll have to enable two-factor authentication and generate individual passwords for each app.
The email was reported on originally by MacRumors.
Beginning on 15 June, app-specific passwords will be required to access your iCloud data using third-party apps such as Microsoft Outlook, Mozilla Thunderbird, or other mail, contacts and calendar services not provided by Apple.
If you are already signed in to a third-party app using your primary Apple ID password, you will be signed out automatically when this change takes effect. You will need to generate an app-specific password and sign in again.
I know this sounds like a serious inconvenience, but…