Vulnerability Discussion

The macOS built-in Mail.app connection to Apple's iCloud service must be disabled.

Apple's iCloud service does not provide an organization with enough control over the storage and access of data and, therefore, automated mail synchronization must be controlled by an organization approved service.

Check

Verify the macOS system is configured to disable iCloud Mail with the following command:

/usr/bin/osascript -l JavaScript << EOS
$.NSUserDefaults.alloc.initWithSuiteName('com.apple.applicationaccess')\
.objectForKey('allowCloudMail').js
EOS

If the result is not "false", this is a finding.

Fix

Configure the macOS system to disable iCloud Mail by installing the "com.apple.applicationaccess" configuration profile.