You send an email. Then what? In the old days, you'd wait and hope. Maybe check your analytics dashboard tomorrow. Maybe never know if the email was delivered, opened, or bounced into the void.
Webhooks changed this. Now, the moment something happens to your email—it's delivered, it bounces, someone opens it, someone clicks a link, someone marks it as spam—your application can know immediately and respond accordingly.
This real-time feedback loop transforms email from a fire-and-forget channel into an interactive, responsive system. But webhooks also introduce complexity: you need to handle them reliably, process them efficiently, and build systems that respond appropriately to each event type.
How email webhooks work
The concept is simple: when an event occurs, your email service makes an HTTP request to a URL you specify, containing data about the event. Your server receives this request, processes the data, and responds.
The implementation details vary by provider, but the pattern is consistent. You configure a webhook endpoint in your email service's dashboard or API. You specify which events you want to receive. When those events occur, the service sends POST requests to your endpoint with JSON payloads describing what happened.
A typical webhook payload includes: the event type (delivered, opened, clicked, bounced, etc.), a timestamp, the message ID, the recipient address, and event-specific data (like which link was clicked, or what bounce error occurred).
Your endpoint needs to respond quickly—usually within a few seconds—with a 2xx status code to acknowledge receipt. If you don't respond in time, or respond with an error, most providers will retry the webhook. After repeated failures, they may disable your endpoint.
Common webhook events
Different email services offer different event types, but most include these core events:
Delivery events tell you the email reached the recipient's mail server. This doesn't mean it reached the inbox—it could be in spam—but it wasn't rejected outright. Delivery confirmation is your baseline success metric.
Bounce events indicate delivery failure. Hard bounces mean the address is permanently invalid. Soft bounces are temporary failures. The webhook typically includes the bounce reason, which helps you diagnose and respond appropriately.
Open events fire when a recipient opens your email (technically, when they load the tracking pixel). Opens are imperfect—image blocking prevents tracking, and preview panes can trigger false opens—but they're still useful for gauging engagement.
Click events tell you when someone clicks a link in your email. You'll typically get the specific URL clicked, letting you track which content resonates and which calls-to-action convert.
Complaint events are critical. When a recipient marks your email as spam, you need to know immediately. High complaint rates damage your reputation, so you want to suppress these recipients from future sends.
Unsubscribe events notify you when someone opts out. Even if your email service handles list management, you might want to sync this data to your CRM or trigger other workflows.
Building reliable webhook handlers
Webhook reliability is crucial. If you miss events, your data becomes inconsistent. If your handler is slow, you'll get timeouts and retries. If you don't handle duplicates, you'll process events multiple times.
The first rule is to respond quickly. Don't do heavy processing in the webhook handler itself. Accept the webhook, store the raw payload in a queue, and return a 200 response. Process the payload asynchronously from the queue. This keeps your response time fast and lets you handle processing failures without losing events.
Expect and handle duplicates. Webhooks can be delivered more than once—network issues, retries, provider bugs. Your handler should be idempotent: processing the same event twice should have the same result as processing it once. Use the event ID or message ID to detect duplicates.
Verify webhook authenticity. Most providers sign their webhooks with a secret key, letting you verify the request actually came from them. Always validate signatures in production—without this, anyone could send fake events to your endpoint.
Handle failures gracefully. If your database is down, you can't process the webhook. Return a 5xx error so the provider retries later. Log failures for investigation. Set up alerts for sustained webhook processing failures.
What to do with webhook data
Collecting webhooks is pointless if you don't act on them. Here's how to turn webhook data into value:
Update your records in real-time. When an email bounces, mark that address as invalid in your database immediately. When someone unsubscribes, update their preferences. When a complaint comes in, suppress that recipient. Real-time updates prevent you from repeatedly sending to bad addresses.
Trigger workflows based on engagement. Someone clicked a link to your pricing page? Maybe trigger a sales notification. Someone opened your onboarding email? Move them to the next stage of your sequence. Webhooks enable responsive, behavior-driven communication.
Build engagement scores. Track opens and clicks over time to identify your most engaged subscribers. Send your best content to engaged users; re-engage or sunset inactive ones. Webhook data is the foundation for sophisticated segmentation.
Monitor deliverability in real-time. A sudden spike in bounces might indicate a list problem or a provider issue. Rising complaint rates are an early warning of reputation damage. Webhook data lets you catch problems before they become crises.
Feed analytics and reporting. Aggregate webhook data to understand campaign performance, track trends over time, and identify what's working. Real-time data means real-time insights.
Webhook gotchas
A few common pitfalls trip up developers new to email webhooks:
Open tracking isn't reliable. Many email clients block tracking pixels by default. Apple's Mail Privacy Protection pre-fetches pixels, generating false opens. Use open data directionally, not as ground truth.
Events can arrive out of order. A click event might arrive before the corresponding open event, or even before the delivery event. Don't assume chronological order; use timestamps and handle events independently.
Webhook volume can be massive. If you send millions of emails, you'll receive millions of webhook events. Plan your infrastructure accordingly. Consider sampling or aggregating if you don't need every individual event.
Provider-specific quirks abound. Each email service has its own webhook format, event types, retry logic, and authentication method. If you switch providers, you'll need to update your webhook handling code.
Test thoroughly before going live. Most providers offer webhook testing tools or sandbox environments. Use them. A bug in your webhook handler can cause data loss or system instability.
Frequently asked questions
How quickly do webhooks arrive after an event?
Usually within seconds to minutes. Delivery and bounce events are typically fastest. Open and click events depend on recipient behavior. Some providers batch webhooks for efficiency, adding slight delays.
What if my webhook endpoint is down?
Most providers retry failed webhooks with exponential backoff—immediately, then after minutes, then hours. After enough failures (varies by provider), they may disable your endpoint. Monitor your endpoint uptime.
Can I replay missed webhooks?
Some providers offer webhook logs or replay functionality. Others don't. Check your provider's capabilities. For critical data, consider also polling the API periodically as a backup.
Should I store raw webhook payloads?
Yes, at least temporarily. Raw payloads let you debug issues, reprocess events if your handler had bugs, and analyze data you didn't originally extract. Storage is cheap; missing data is expensive.