Mail App Contacts iCloud Even When You Send a Non-iCloud Email

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

A developer discovers that Apple's Mail app triggers iCloud connections via cloudd and nsurlsessiond every time an email is sent, even when iCloud Mail is disabled and the account is non-iCloud. The connections target gateway.icloud.com, but certificate pinning prevents inspection, leaving the purpose unknown. The author plans to block these connections with Little Snitch rules.