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/secure/sendbatch queues many messages in one call.
Each element takes the same fields as a single send. A message’s to may itself be an array, which multiplies out.

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The response confirms acceptance only

There are no per-message ids in this response, and no per-message results. messageCount is how many the batch expanded to, not how many succeeded.Per-message outcomes arrive on your errback URL as separate callbacks. A client that treats a 200 here as “all three sent” is wrong in exactly the cases worth knowing about.

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Defaults and configuration

globals is merged under every message, so a field set on an individual message wins.
Unknown keys in batch_config are ignored, not refused. A typo in errback_url costs you every per-message result, silently — the batch is accepted and the callbacks simply never arrive.This is the one place on the API where a misspelled field does not produce a 400, so check it by sending a deliberately failing message and confirming the callback lands.

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Scheduling

schedule_at takes exactly two forms: Anything else is 412 Got unknown format. A date in the past is refused too, rather than sent immediately.
Scheduled batches survive a restart only when the gateway is running with a RabbitMQ broker — and not even then while the broker connection is degraded. If durable scheduling matters to you, ask your provider which queue provider they run and whether they alert on amqp.bridge.degraded.

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Limits

Array-valued destinations multiply: 100 messages each with 200 recipients is 20 000, and refused.

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Pacing

Batches pace themselves against the router queue’s depth, pausing above a high-water mark. You do not need to throttle your own submission of a batch — but you do still need to respect your account’s messages-per-second limit on ordinary sends, which is a separate ceiling and returns 429.

Send a batch

Every field, with the refusals.

Batch callbacks

What arrives on callback_url and errback_url.