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A webhook is an HTTP callback that delivers real-time data to your application when specific events occur in another system. Unlike traditional APIs where you poll for updates, webhooks push data to your endpoint immediately when triggered, enabling instant notifications for email events like deliveries, bounces, opens, and clicks.
Webhooks transform how you interact with email data by eliminating the need for constant polling. Instead of repeatedly querying an API to check for new events (which wastes resources and introduces delays), webhooks deliver information the instant it becomes available. This real-time capability is essential for time-sensitive operations like removing bounced addresses before your next campaign or triggering follow-up sequences based on recipient engagement. For email marketers and developers, webhooks enable sophisticated automation that was previously impossible. When someone clicks a link in your email, a webhook can instantly update your CRM, trigger a sales notification, or enroll the contact in a targeted nurture sequence. This immediate response to user behavior dramatically improves engagement and conversion rates. Webhooks also reduce infrastructure costs and complexity. Polling-based approaches require dedicated resources to continuously check for updates, even when nothing has changed. With webhooks, you only process data when events actually occur, making your system more efficient and scalable. This event-driven architecture is now standard practice for modern email infrastructure.
Webhooks operate on a simple but powerful mechanism: when a predefined event occurs, the source system sends an HTTP POST request to a URL you specify. This URL, called the webhook endpoint, receives a JSON payload containing detailed information about the event. For email systems, this means your application gets notified the moment an email bounces, is opened, or triggers any other tracked event. The process begins when you register your endpoint URL with the email service provider and select which events you want to receive. When a subscriber opens your email, for example, the ESP detects this action and immediately constructs a payload containing the event type, timestamp, recipient email, and other relevant metadata. This payload is then sent to your endpoint via HTTPS POST request. Your server must acknowledge receipt by returning an HTTP 200 status code. If the webhook fails to deliver (due to server downtime or network issues), most providers implement retry logic with exponential backoff. This ensures you eventually receive all event data even if temporary failures occur. The entire process typically completes in milliseconds, giving you near-instantaneous visibility into your email performance.
APIs require you to actively request data (pull model), while webhooks automatically send data to you when events occur (push model). With an API, you poll the server periodically asking "any new events?" With webhooks, the server tells you immediately when something happens. Webhooks are more efficient for real-time notifications, while APIs are better for on-demand data retrieval.
Implement multiple security layers: use HTTPS exclusively, validate webhook signatures using the secret key provided by your ESP, verify the source IP addresses if your provider publishes them, and implement rate limiting to prevent abuse. Most email services include a signature header (like X-Webhook-Signature) that you should verify against your secret before processing any payload.
Most email service providers implement automatic retry logic with exponential backoff. If your endpoint returns an error or times out, the provider will retry the delivery multiple times over several hours or days. However, after exhausting retries, events may be lost. To prevent data loss, ensure high availability for your endpoint and consider using a managed webhook service or message queue as a buffer.
Your endpoint should return an HTTP 200 response within 5-10 seconds to prevent timeout errors. The best practice is to acknowledge receipt immediately, then process the payload asynchronously using a background job queue. This prevents slow processing from causing webhook failures and allows your system to handle high volumes of concurrent events without bottlenecks.
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