How to send transactional email in a NodeJS app using the Mailgun API

Written by Orlando Kalossakas

Categories: For Devs

5 minute read time

Sending transactional emails nowadays is very important for establishing a healthy customer base. Not only you can get powerful re-engagement based on triggers, actions, and patterns you can also communicate important information and most importantly, automatically between your platform and a customer.

It’s highly likely that during your life as a developer you’ll have to send automated transactional emails, if you haven’t already, like:

  • Confirmation emails
  • Password reminders

…and many other kinds of notifications.

In this tutorial, we’re going to learn how to send transactional emails using the Mailgun API using a NodeJS helper library.

We will cover different scenarios:

  • Sending a single transactional email
  • Sending a newsletter to an email list
  • Adding email addresses to a list
  • Sending an invoice to a single email address

Getting started

We will assume you have installed and know how to operate within the NodeJS environment.

The first need we’re going to do is generate a package.json file in a new directory (/mailgun_nodetut in my case).

This file contains details such as the project name and required dependencies.

  "name": "mailgun-node-tutorial",
  "version": "0.0.1",
  "private": true,
  "scripts": {
    "start": "node app.js"
  "dependencies": {
    "express": "4",
    "jade": "*",


As you can see, we’re going to use expressjs for our web-app scaffolding, with jade as a templating language and finally a community-contributed library for node by bojand

In the same folder where you’ve created the package.json file create two additional folders:


You’re set – now sign in to Mailgun, it’s free if you haven’t done it yet and get your API key (first page in the control panel).

Setup a simple ExpressJS app

Save the code below as app.js in the root directory of your app (where package.json is located)

//We're using the express framework and the mailgun-js wrapper
var express = require('express');
var Mailgun = require('mailgun-js');
//init express
var app = express();

//Your api key, from Mailgun’s Control Panel
var api_key = 'MAILGUN-API-KEY';

//Your domain, from the Mailgun Control Panel
var domain = '';

//Your sending email address
var from_who = '';

//Tell express to fetch files from the /js directory
app.use(express.static(__dirname + '/js'));
//We're using the Jade templating language because it's fast and neat
app.set('view engine', 'jade')

//Do something when you're landing on the first page
app.get('/', function(req, res) {
    //render the index.jade file - input forms for humans
    res.render('index', function(err, html) {
        if (err) {
            // log any error to the console for debug
        else { 
            //no error, so send the html to the browser

// Send a message to the specified email address when you navigate to /submit/
// The index redirects here
app.get('/submit/:mail', function(req,res) {

    //We pass the api_key and domain to the wrapper, or it won't be able to identify + send emails
    var mailgun = new Mailgun({apiKey: api_key, domain: domain});

    var data = {
    //Specify email data
      from: from_who,
    //The email to contact
      to: req.params.mail,
    //Subject and text data  
      subject: 'Hello from Mailgun',
      html: 'Hello, This is not a plain-text email, I wanted to test some spicy Mailgun sauce in NodeJS! <a href="' + req.params.mail + '">Click here to add your email address to a mailing list</a>'

    //Invokes the method to send emails given the above data with the helper library
    mailgun.messages().send(data, function (err, body) {
        //If there is an error, render the error page
        if (err) {
            res.render('error', { error : err});
            console.log("got an error: ", err);
        //Else we can greet    and leave
        else {
            //Here "submitted.jade" is the view file for this landing page 
            //We pass the variable "email" from the url parameter in an object rendered by Jade
            res.render('submitted', { email : req.params.mail });


app.get('/validate/:mail', function(req,res) {
    var mailgun = new Mailgun({apiKey: api_key, domain: domain});

    var members = [
        address: req.params.mail
//For the sake of this tutorial you need to create a mailing list on and put its address below
    mailgun.lists('NAME@MAILINGLIST.COM').members().add({ members: members, subscribed: true }, function (err, body) {
      if (err) {
            res.send("Error - check console");
      else {
        res.send("Added to mailing list");


app.get('/invoice/:mail', function(req,res){
    //Which file to send? I made an empty invoice.txt file in the root directory
    //We required the path module find the full path to attach the file!
    var path = require("path");
    var fp = path.join(__dirname, 'invoice.txt');
    var mailgun = new Mailgun({apiKey: api_key, domain: domain});

    var data = {
      from: from_who,
      to: req.params.mail,
      subject: 'An invoice from your friendly hackers',
      text: 'A fake invoice should be attached, it is just an empty text file after all',
      attachment: fp

    //Sending the email with attachment
    mailgun.messages().send(data, function (error, body) {
        if (error) {
            res.render('error', {error: error});
            else {
            res.send("Attachment is on its way");
            console.log("attachment sent", fp);


How it works

This above is a simple express app that will run on your local machine on port 3030. We have defined it to use expressjs and the mailgun-js wrapper. These are pretty well-known libraries that will help us quickly set up a small website that will allow users to trigger the sending of email addresses to their address.

First things first

We’ve defined 4 endpoints.

  1. /
  2. /submit/:mail
  3. /validate/:mail
  4. /invoice/:mail

Where :mail is your valid email address.

When navigating to an endpoint, Express will send to the browser a different view.

In the background, Express will take the input provided by the browser and process it with the help of the mailgun-js library.

In the first case, we will navigate to the root that is localhost:3030/

You can see there’s an input box requesting your email address. By doing this, you send yourself a nice transactional email.

This is because expressjs takes your email address from the URL parameter and by using your API key and domain does an API call to the endpoint to send emails.

Each endpoint will invoke a different layout, I’ve added the code of basic layouts below, feel free to add and remove stuff from it.


Save the code below as index.jade within the /views directory

doctype html
        title Mailgun Transactional demo in NodeJS
        p Welcome to a Mailgun form
            label(for="mail") Email
            input(type="text" id="mail" placeholder="")
            button(value="bulk" onclick="mgform.hid=this.value") Send transactional
            button(value="list" onclick="mgform.hid=this.value") Add to list
            button(value="inv" onclick="mgform.hid=this.value") Send invoice
        script(type="text/javascript" src="main.js")

Save the code below as submitted.jade within the /views directory

doctype html
        title Mailgun Transactional
        p thanks #{email} ! 

Save the code below as error.jade within the /views directory

doctype html
        title Mailgun Transactional
        p Well that's awkward: #{error}


This code allows the browser to call a URL from the first form field with your specified email address appended to it. This way we can simply parse the email address from the URL with express’ parametered route syntax.

Save the code below as main.js within the /js directory

var vForm = document.getElementById('mgform');
var vInput = document.getElementById('mail');

vForm.onsubmit = function() {
if (this.hid == "bulk") {
    location = "/submit/" + encodeURIComponent(vInput.value);
if (this.hid == "list") {
    location = "/validate/" + encodeURIComponent(vInput.value);
if (this.hid == "inv") {
    location = "/invoice/" + encodeURIComponent(vInput.value);

return false;

Finally, create an empty invoice.txt file in your root folder. This file will be sent as an attachment.

Now run npm install (or sudo npm install in some cases depending on your installation) to download dependencies, including Express.

Once that’s done run node app.js

Navigate with your browser to localhost:3030 (or

And that’s how you get started sending transactional email on demand!

In our example, we’ve seen how you can send a transactional email automatically when the user triggers a specific action, in our specific case, submitting the form.

There are thousands of applications that you could adapt this scenario to. Sending emails can be used for:

  • Registering an account on a site and Hello message email
  • Resetting a password
  • Confirming purchases or critical actions (deleting an account)
  • Two-factor authentication with multiple email addresses

What do you use transactional emails for in your day-to-day business?

Modified on: March 13, 2019

Stay up-to-date with our blog & new email resources

We'll let you know when we add new email resources and blog posts. We promise not to spam you.