Webhook
An HTTP callback that fires when an email event happens (delivered, opened, clicked, bounced) — for real-time data sync.
A webhook is an HTTP callback that an ESP fires when specific email events occur — delivery, open, click, bounce, complaint, unsubscribe. The webhook posts JSON to a URL you configure; your application receives it and updates state accordingly.
Webhooks are essential for any email program that integrates with downstream systems: CRM sync, lifecycle-event tracking, real-time engagement scoring, analytics pipelines. The polling alternative (calling the ESP's API on a schedule) is slower, more expensive, and rate-limit-constrained.
Production webhook implementations need to handle three concerns: idempotency (the same event may fire twice if the receiver doesn't ack quickly enough), signature verification (the receiver must verify the webhook actually came from the ESP, via HMAC signing), and ordering tolerance (events can arrive out of order). Most ESPs document these patterns; getting them wrong leads to subtle data-quality bugs that surface months later.