Why does Mail app contact iCloud when sending a non-iCloud email?

I use iCloud only for testing the iCloud sync feature of my App Store apps . As I mentioned in my recent blog post Apple account email address disclosure via Mail app , I don’t have an iCloud email account. As you can see in my screenshots below, iCloud is disabled in System Settings for all uses of Mail. Nonetheless, when I send an email in Mail app, from a non-iCloud account to an non-iCloud…