This is the body of the email I received from Apple this morning.

“Thank you for using two-step verification to protect your Apple ID and the data you store with iCloud.
This is a reminder that starting tomorrow, 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 apps.

If you are currently 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.

To generate an app-specific password:
Sign in to My Apple ID (https://appleid.apple.com)
Go to Password & Security
Click Generate App-Specific Password”

This will be fun as I have no idea how many apps this affects.