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How To Send Transactional Email In A NodeJS App Using The Mailgun API

Orlando Kalossakas
5 min read

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 and 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.

2 "name": "mailgun-node-tutorial",
3 "version": "0.0.1",
4 "private": true,
5 "scripts": {
6 "start": "node app.js"
7 },
8 "dependencies": {
9 "express": "4",
10 "jade": "*",
11 "mailgun-js":"0.5"
12 }

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:

views/ js/

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).

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Set up a simple ExpressJS app

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

1 //We're using the express framework and the mailgun-js wrapper
2 var express = require('express');
3 var Mailgun = require('mailgun-js');
4 //init express
5 var app = express();
7 //Your api key, from Mailgun’s Control Panel
8 var api_key = 'MAILGUN-API-KEY';
10 //Your domain, from the Mailgun Control Panel
11 var domain = 'YOUR-DOMAIN.com';
13 //Your sending email address
14 var from_who = 'your@email.com';
16 //Tell express to fetch files from the /js directory
17 app.use(express.static(__dirname + '/js'));
18 //We're using the Jade templating language because it's fast and neat
19 app.set('view engine', 'jade')
21 //Do something when you're landing on the first page
22 app.get('/', function(req, res) {
23 //render the index.jade file - input forms for humans
24 res.render('index', function(err, html) {
25 if (err) {
26 // log any error to the console for debug
27 console.log(err);
28 }
29 else {
30 //no error, so send the html to the browser
31 res.send(html)
32 };
33 });
34 });
36 // Send a message to the specified email address when you navigate to /submit/someaddr@email.com
37 // The index redirects here
38 app.get('/submit/:mail', function(req,res) {
40 //We pass the api_key and domain to the wrapper, or it won't be able to identify + send emails
41 var mailgun = new Mailgun({apiKey: api_key, domain: domain});
43 var data = {
44 //Specify email data
45 from: from_who,
46 //The email to contact
47 to: req.params.mail,
48 //Subject and text data
49 subject: 'Hello from Mailgun',
50 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>'
51 }
53 //Invokes the method to send emails given the above data with the helper library
54 mailgun.messages().send(data, function (err, body) {
55 //If there is an error, render the error page
56 if (err) {
57 res.render('error', { error : err});
58 console.log("got an error: ", err);
59 }
60 //Else we can greet and leave
61 else {
62 //Here "submitted.jade" is the view file for this landing page
63 //We pass the variable "email" from the url parameter in an object rendered by Jade
64 res.render('submitted', { email : req.params.mail });
65 console.log(body);
66 }
67 });
71 app.get('/validate/:mail', function(req,res) {
72 var mailgun = new Mailgun({apiKey: api_key, domain: domain});
74 var members = [
75 {
76 address: req.params.mail
77 }
78 ];
79 //For the sake of this tutorial you need to create a mailing list on Mailgun.com/cp/lists and put its address below
80 mailgun.lists('NAME@MAILINGLIST.COM').members().add({ members: members, subscribed: true }, function (err, body) {
81 console.log(body);
82 if (err) {
83 res.send("Error - check console");
84 }
85 else {
86 res.send("Added to mailing list");
87 }
88 });
90 })
92 app.get('/invoice/:mail', function(req,res){
93 //Which file to send? I made an empty invoice.txt file in the root directory
94 //We required the path module here..to find the full path to attach the file!
95 var path = require("path");
96 var fp = path.join(__dirname, 'invoice.txt');
97 //Settings
98 var mailgun = new Mailgun({apiKey: api_key, domain: domain});
100 var data = {
101 from: from_who,
102 to: req.params.mail,
103 subject: 'An invoice from your friendly hackers',
104 text: 'A fake invoice should be attached, it is just an empty text file after all',
105 attachment: fp
106 };
108 //Sending the email with attachment
109 mailgun.messages().send(data, function (error, body) {
110 if (error) {
111 res.render('error', {error: error});
112 }
113 else {
114 res.send("Attachment is on its way");
115 console.log("attachment sent", fp);
116 }
117 });
118 })
120 app.listen(3030);

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

1doctype html
3 head
4 title Mailgun Transactional demo in NodeJS
5 body
6 p Welcome to a Mailgun form
7 form(id="mgform")
8 label(for="mail") Email
9 input(type="text" id="mail" placeholder="Youremail@address.com")
10 button(value="bulk" onclick="mgform.hid=this.value") Send transactional
11 button(value="list" onclick="mgform.hid=this.value") Add to list
12 button(value="inv" onclick="mgform.hid=this.value") Send invoice
13 script(type="text/javascript" src="main.js")

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

1doctype html
3 head
4 title Mailgun Transactional
5 body
6 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

1var vForm = document.getElementById('mgform');
2var vInput = document.getElementById('mail');
4vForm.onsubmit = function() {
5if (this.hid == "bulk") {
6 location = "/submit/" + encodeURIComponent(vInput.value);
8if (this.hid == "list") {
9 location = "/validate/" + encodeURIComponent(vInput.value);
11if (this.hid == "inv") {
12 location = "/invoice/" + encodeURIComponent(vInput.value);
15return 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?

Last updated on May 12, 2021

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