Outbound webhook delivery

Design the outbound webhook delivery system for a SaaS platform that notifies tens of thousands of customer endpoints of events (payment.succeeded, etc.) at ~50k events/sec. Requirements: at-least-once delivery with retries and exponential backoff, one slow or broken customer endpoint must not delay deliveries to everyone else, ordered delivery per customer where they want it, and customers must be able to verify a delivery is genuinely from us and replay missed ones.