Glossary
Webhook
An HTTP callback your provider fires when something happens.
A webhook is a POST request a service sends to your endpoint on events — delivered, bounced, opened, clicked. Serious implementations sign payloads (HMAC) so you can verify them, retry with backoff on your 5xxs, and let you replay missed deliveries.
Treat webhook handlers as distributed-systems code: acknowledge fast, work asynchronously, dedupe on event IDs. A webhook that fires once into the void is a wish, not infrastructure.
In Calder: Calder webhooks