Bun is the new Javascript runtime that everyone is talking about. It's fast, secure, and includes exciting new features like macros. Bun is an interesting fit for edge computing and serverless applications and in this post we'll explore how to send email in with Bun using SMTP and the npm nodemailer package.

What is Bun.js, and Why is it Special?

Bun.js is a modern JavaScript runtime built on the Zig programming language, designed to execute JavaScript code outside a web browser. Its distinctiveness lies in its high performance, thanks to a compiler that leverages the Zig language's efficiency, and its compatibility with various web standards, including but not limited to Fetch API, Streams, and WebSockets.

Bun.js stands out for its speed in both startup time and runtime performance, making it an ideal choice for server-side applications, command-line tools, and more.

Why We Might Use Bun.js

Bun.js is particularly appealing for applications that benefit from edge computing, speed, and serverless architectures. Its efficiency and fast execution times make it suitable for edge computing applications where latency is critical.

The runtime's speed boosts server-side applications, allowing for quicker responses and better scalability. Moreover, Bun.js's architecture and compatibility with serverless environments enable developers to build and deploy applications that are both cost-effective and highly scalable.

How Node Packages Fit into the Equation

Despite its unique runtime environment, Bun.js maintains compatibility with Node.js packages, allowing developers to leverage the vast ecosystem of Node modules. This compatibility is facilitated through Bun's package manager, which can install and manage Node packages.

Nodemailer, a popular Node.js library for sending emails, can thus be used within Bun.js applications to send emails over SMTP, combining Bun.js's performance benefits with Nodemailer's flexibility and ease of use.

Getting started

Firstly, install bun like so:

Then create a new project with:

Integrating Nodemailer in Bun.js

To use Nodemailer in Bun.js, you first need to install it using Bun's package manager. Here's a basic example of how to set up Nodemailer in a Bun.js application:

Next we import the package in our bun file:

Why We Need an SMTP Mailserver to Send an Email

SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending emails across the Internet. An SMTP mail server is essential for email transmission as it acts as a mediator for outgoing mail, ensuring that emails are correctly routed from the sender to the recipient's mail server. Utilizing an SMTP server allows for reliable delivery of emails, handling of email queues, retries in case of delivery failures, and compliance with email standards and security practices.

If you don't already have a mailserver you can use MailSlurp to create new SMTP servers for free, then connect them to Bun using Nodemailer. We'll do so in this guide. First, install MailSlurp:

Then import the client:

Now we can configure a new mailserver:

How Bun Can Connect to a Mailserver via SMTP and Deliver Mail with SMTP Relay

Connecting Bun.js to an SMTP mail server for email delivery involves using a library like Nodemailer to handle the SMTP protocol's intricacies. SMTP relay, a process where an SMTP server passes email on to another SMTP server, is crucial for delivering emails from a Bun.js application to the recipients. Here's an overview of setting up Bun.js to connect to an SMTP server and deliver mail:

  1. Setup SMTP Transport: Configure Nodemailer with the SMTP server details, including the host, port, and authentication credentials.
  2. Compose the Email: Define the email's content, including the sender, recipient, subject, and body.
  3. Send the Email: Use Nodemailer's method to initiate the email sending process. Nodemailer communicates with the configured SMTP server, which then relays the email to the recipient's mail server.
  4. Handle Responses: Implement callback functions to handle the response from the SMTP server, including success confirmations and error messages.

The combination of Bun.js and Nodemailer offers a powerful solution for sending emails efficiently and effectively in modern JavaScript applications. By leveraging Bun.js for its performance advantages and Nodemailer for its email sending capabilities, developers can build robust email delivery systems that are both fast and reliable.

Configure nodemailer with SMTP access

The next step to send an email with bun is to configure a transport layer in Nodemailer. We'll use the disposable mailserver we created in the previous step:

Now we have nodemailer setup we can use it to send an email.

Sending an Email with Bun.js and Nodemailer

To send mail with bun we use the nodemailer transport layer like so:

Here we are sending between two MailSlurp throwaway email addresses. You can use any SMTP server you have access to but using the MailSlurp API allows us to read the email we sent too.

Reading email in Bun JS

We can read the email we sent using the MailSlurp client like so:

This is really useful if we are writing tests or want to verify that our email was sent.


Bun.js represents a significant advancement in the JavaScript runtime landscape, offering unparalleled speed and efficiency for server-side applications. Bun.js offers remarkable efficiency for various applications, including Bun.js email functionalities. By integrating Node packages like Nodemailer, Bun.js applications can effortlessly send emails over SMTP, combining the runtime's performance benefits with the versatility of existing Node.js libraries.

Through understanding the necessity of SMTP servers for email delivery and utilizing Bun.js's capabilities for SMTP communication, developers can harness the full potential of this modern runtime environment for their email-related tasks, ensuring both high performance and reliable email delivery.