Signature verification
Every callback carries X-IdCheck-Client (your template's id) and X-IdCheck-Callback-Secret (the secret shown once when the template's key was created). Compare the secret against your own stored copy before trusting the payload - never process a callback you can't verify.
Delivery
Sent once, when every required check on a session reaches a final state - not per individual check, and not on retries. A failed delivery is currently not retried; poll GET /sessions/{id}/data if you need to recover a missed callback.