---- For asynchronous verification, the hub MUST consider other server response codes (3xx, 4xx, and 5xx) to mean that the subscription action was temporarily not verified. If the subscriber returns an HTTP success (2xx) but the content body does not match the hub.challenge parameter, the hub MUST consider this to be a temporary failure and retry. The hub SHOULD retry verification a reasonable number of times over the course of a longer time period (e.g., 6 hours) until a definite acknowledgement (positive or negative) is received. If a definite response still cannot be determined after this retry period, the subscription action verification MUST be abandoned, leaving the previous subscription state. ----
What HTTP status code should be considered as a "definite negative acknowledgement"?
