macOS Mail contacts iCloud when sending mail from a non-iCloud account

Sources: primary, discussion
Summary: Jeff Johnson reproduces cloudd and nsurlsessiond reaching gateway.icloud.com on every send, with Mail and Contacts both disabled in iCloud settings. He attributes it tentatively to a com.apple.mail.recents CloudKit zone. The cause is not confirmed and Apple has not commented.
Why it matters: A network call to a vendor service that the user's settings appear to have disabled is a problem for anyone running mail under a documented data-handling policy.
Follow-up: Watch for an Apple response or a confirmed root cause.