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

The author discovered that Apple's Mail app contacts iCloud servers when sending non-iCloud emails, despite having iCloud disabled for Mail in System Settings. The Mail app's processes attempt connections to iCloud's CloudKit database URLs, raising privacy concerns about why Apple needs to know when emails are being sent. The author was unable to determine what data is being queried or saved due to iCloud's certificate pinning, and has since blocked these connections with a firewall tool.