Create a draft. Requires message_send or draft_write. draft_write does not send mail. in_reply_to or forward_of derives threading from that message. send_at schedules it and adds the scheduled label. Idempotency-Key and client_id replay the original draft.

Optional. 1 to 256 characters: letters, digits, and - . _ ~. The same key and the same request within 24 hours returns the stored message with HTTP 200, idempotent true, and Idempotent-Replayed: true, and does not send again. A different request is 409 idempotency_key_conflict. A second call while the first is still running is 409 idempotency_request_in_progress. An empty or invalid value is 400 invalid_idempotency_key. A failed send removes the key. When client_id is also set, client_id is checked first. Keys expire after 24 hours.