If you are using Xink for Apple Mail you need to make sure that the iCloud Drive setting is correct.  Under Xink Preferences there is a setting called "I'm using iCloud" that you should check if and only if you are using iCloud Drive for Mail. 

The following explains how to determine if you are using iCloud Drive for Mail.

1. Open System Preferences.

2. Click "iCloud".

3. Click the "Options" button next to "iCloud Drive".

4. Scroll to "Mail".

If there is a checkmark next to "Mail", you need to choose "I'm using iCloud" in the Xink app. If you aren’t using iCloud (there’s no checkmark next to “Mail”), then make sure  "I'm using iCloud" remains unchecked within Xink. 

Please see the following screenshots.