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 account, Mail immediately triggers an iCloud connection, as revealed by Little Snitch. In testing, I’ve reproduced this behavior repeatedly. The cloudd process attempts to connect to the URL https://gateway.icloud.com:443/ckdatabase/api/client/query/retrieve, and the nsurlsessiond process attempts to connect to the URL https://gateway.icloud.com:443/ckdatabase/api/client/record/save, over both TCP and UDP, likely falling back to the alternate protocol when I block the first connection attempt. Unfortunately, I haven’t been able to analyze the connection details with Proxyman, because iCloud uses…