Queues a transactional email from one of the account's confirmed senders and returns it with 201.
Each email uses one credit. Follow its status with GET /v2/transactionals/{id}.

Send an Idempotency-Key header (e.g. a UUID, up to 255 printable ASCII characters) to make retries safe:
repeating the request with the same key returns the original email, with the Idempotent-Replayed: true
header, and does not send again. Reusing a key with a different request returns 422 idempotency_key_reused.