X_WEBHOOK_RECEIVER_UNAVAILABLE

an inbound webhook could not be queued, so it was not acknowledged

the verified delivery reached the enqueue and the queue refused or did not answer (the database is down, the pool is exhausted, the driver is unreachable). 503 with Retry-After on purpose: the vendor redelivers, and an ack never precedes the durable row. The inner failure is named in the cause

x doctor --json — then check DATABASE_URL on the web role; the vendor redelivers on its own once the queue answers