Field is scheduled_at in the raw HTTP body — scheduledAt is the JS SDK's camelCase mapping; using the wrong case silently sends immediately in some client setups
Emails can only be scheduled up to 30 days in advance
A canceled email cannot be un-canceled or rescheduled — create a new scheduled send instead
Idempotency keys still apply: same key with a different scheduled_at is a different payload → 409
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