Delivery status callback payload

Messaging posts each message status change to the callback_url registered on its caller. Requests never supply a callback address. request_ref is the callback outbox task ID and Idempotency-Key: retries preserve it, different notifications use different IDs. Delivery is at least once and notifications may arrive out of order. Recipients deduplicate by request_ref. Email succeeds at submitted; hard bounce changes it to failed and globally suppresses the address. Attempt failures are internal until non-retryable or exhausted; late delivery evidence may supersede a prior final failure.

POST caller.callback_url