Send a batch

Up to 1,000 personalized messages in one call. Each message succeeds or fails independently; a failed message becomes a { error: … } entry in results rather than failing the whole batch, so a bad template id in message #7 does not stop #8.

The batch answers 200 even when every message failed. results is positional — entry i is the outcome of messages[i] — and you must inspect each entry. A caller that only checks the HTTP status reports a completely failed batch as a success. Supports Idempotency-Key over the whole batch.