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Track and Respond to Email Activity with Webhooks

At a glance

Webhooks allow your application to receive information about email events as they occur, and respond in a way that you define. You can configure and test webhooks via the Webhooks page in your Mailchimp Transactional app, or you can do so via the API

You could use webhooks to: 

  • Create your own custom reporting dashboard using webhook data

  • Keep your CRM in sync with events from Mailchimp Transactional 

  • Store data about your emails for longer than 30 days

For the purposes of this guide, we run a plant delivery website called Eiffel Flowers. In the course of our business, we send registered users a variety of transactional emails. 

In this guide, we’ll set up a webhook that will trigger when a particular link is clicked in one of our emails, which will allow us to then send a follow-up email advertising a relevant flower sale. We’ll walk through how to create the webhook, and then we’ll write code to handle the incoming webhook data and send a follow-up email in response.

What you’ll need

  • A paid Mailchimp Transactional account

  • A callback URL for your application that can accept HTTP POST requests

  • Your API key

Create a new webhook

First, let’s set up our webhook, which will be triggered when subscribers click a link in one of our plant emails. There are two ways to do this: using the Mailchimp Transactional app or through the API. In this guide, we’ll walk through setting it up using the app. 

To set up a webhook: 

  1. Navigate to Webhooks and click Add a Webhook at the top of the page

  2. Choose the events you want to listen for in the “Trigger on Events” section (e.g., when an email is sent, bounces, is opened, is marked as spam, is clicked)

  3. In the “Post To URL” field, input the callback URL your application will use to accept the incoming webhook

  4. Give your webhook an optional description under Description

  5. Click Create Webhook

Note: Your webhook callback URL should be set up to accept POST requests at a minimum. When you provide the URL where you want Mailchimp to POST the event data, a HEAD request will be sent to the URL to check that it exists.

If the URL doesn't exist or returns something other than a 200 HTTP response to the HEAD request, Mailchimp will fall back and attempt a POST request. In this case, the POST request’s mandrill_events parameter will be an empty array, and the POST will be signed with a generic key (with the value test-webhook).

For a full overview of webhook formatting and configuration in Mailchimp Transactional, see the Webhooks docs.

Handling the webhook response in your app

Now that we have the webhook set up, we need to handle the webhook data on the server that the webhook URL we provided points to. 

The body of the webhook request, parsed as JSON, will look something like this:

Handling webhook response

  "mandrill_events": [
      "event": "open",
      "msg": {
        "ts": 1365109999,
        "subject": "Roses Are Red, Violets Are On Sale",
        "email": "",
        "sender": "",
        "tags": ["violets"],
        "opens": [
            "ts": 1365111111
        "clicks": [
            "ts": 1365111111,
            "url": ""
        "state": "sent",
        "metadata": {
          "user_id": 111
        "_id": "7761629",
        "_version": "123"
        # trimmed for brevity

Now that we know what the webhook data will look like for our “Message Is Clicked” event, we can work on handling it on our server. 

That code might look like this:

Handling the webhook response in your app

const express = require("express");
const bodyParser = require("body-parser");

const app = express();

// notice that the body will be of type `x-www-form-urlencoded`,
// and needs to be parsed as such
    extended: false

const TARGET_LINK_URL = "";

const generateMessage = email => ({
    "<p>We noticed you were interested in violets! How about we offer you a great deal on a dozen if you buy in the next 72 hours?</p>",
    "We noticed you were interested in violets! How about we offer you a great deal on a dozen if you buy in the next 72 hours?",
  subject: "Roses Are Red, Violets Are On Sale",
  from_email: "",
  from_name: "Daisy @ Eiffel Flowers",
  to: [
      type: "to"
});"/", (req, res) => {
  const mandrillEvents = JSON.parse(req.body.mandrill_events);
  mandrillEvents.forEach(event => {
    const {
      clicks: [url],
    } = event.msg;
    if (url === TARGET_LINK_URL) {
      // send follow-up message here using the Mailchimp Transactional API
      // for more details, see
      console.log("Send follow up email with this payload:", generateMessage(email));
    } else {
      // lets us test the webhook using the Mailchimp Transactional UI (see next section)
      console.log("webhook handled successfully!");

app.listen(3000, () => console.log("Now listening at http://localhost:3000"));

You can find a full index of detailed webhook format responses in the Webhooks docs.

Test the webhook

Now let’s run a simple test of the webhook without sending any emails. First, head back to Webhooks. Find the entry for your webhook and click the Send Test button. 

If your webhook handler is working, you should find the message webhook handled successfully! in your server logs.

To test the webhook further, you’ll need to send an email containing a link with the TARGET_LINK_URL we used above, then have the recipient click the link. At that point, our handler should trigger, sending our follow-up email.

For more information about testing and debugging webhooks, see the Webhooks docs.

Authenticating webhook requests

Finally, since our app exposes sensitive data—you know what plant-selling is like!—we’re going to want to ensure that requests are coming from Mailchimp Transactional rather than an imitator. Mailchimp signs all webhook requests, allowing you to verify that incoming requests weren’t generated by some nefarious third party. This step isn’t required, but it is recommended.

First, we need to get our webhook authentication key. When you create a webhook via the Mailchimp Transactional app, a signing key is automatically generated. You’ll need this key to generate a signature in your code, and compare that to the signature that Mailchimp sent.

If you’re using the webhooks/add method, the key will be returned in the response. You can also view and reset the key from the Webhooks page in your account. To retrieve a webhook key via the Transactional API, use webhooks/info or webhooks/list.

Next, we need to generate a signature. Mailchimp signs each webhook request using the following process:

  1. Create a string with the webhook’s URL, exactly as you entered it in Mailchimp Transactional (including any query strings, if applicable)

  2. Ensure that your string will either be interpreted literally or escape any characters that will be interpreted (i.e., the slashes)

  3. Append each POST variable’s key and value to the URL string, with no delimiter

  4. Hash the resulting string with HMAC-SHA1, using your webhook’s authentication key to generate a binary signature

  5. Base64 encode the binary signature

Mailchimp includes an X-Mandrill-Signature HTTP header with webhook POST requests; this header will contain the signature for the request. To verify a webhook request, you’ll need to generate a signature using the same method and key that Mailchimp Transactional uses and compare that to the value of the X-Mandrill-Signature header.

Note: Some HMAC implementations can generate either a binary or hexadecimal signature. Mailchimp generates a binary signature and then Base64-encodes it; using a hexadecimal signature will not work.

A function to accomplish this task may look something like:

Generate a signature

const crypto = require('crypto');

function generateSignature(webhook_key, url, params) {
    var signed_data = url;
    const param_keys = Object.keys(params);
    param_keys.forEach(function (key) {
        signed_data += key + params[key];

    hmac = crypto.createHmac('sha1', webhook_key);

    return hmac.digest('base64');

You can reset a webhook’s authentication key at any time, and Mailchimp Transactional will immediately begin using the new key to sign requests. 

To ensure that you don’t lose any webhook batches between the time you reset your key and when you update your application to start using that new key, your webhook processor should reject batches with failed signatures with a non-200 status code. Mailchimp will queue the batch and retry later, which will give you time to update your application with the new key.