MailDev

Important

MailDev 3.0 release candidate has now been released which includes a complete re-write and re-structuring of the entire project. If you run into any issues, please install the latest v2 release.

MailDev is a simple way to test your project's generated email during development, with an easy to use web interface that runs on your machine built on top of Node.js.

MailDev is sponsored by ⭐️ Inngest.

Inngest is the durable execution platform for AI agents and workflows enabling you to ship reliable products with no infrastructure.

Install

npm install -g maildev

Docker Run

If you want to use MailDev with Docker, you can use the maildev/maildev image on Docker Hub. For a guide for usage with Docker, checkout the docs.

docker run -p 1080:1080 -p 1025:1025 maildev/maildev

Usage

Usage: maildev [options]

Options	Environment variable	Description
-s, --smtp <port>	MAILDEV_SMTP_PORT	SMTP port to catch mail (default: 1025)
-w, --web <port>	MAILDEV_WEB_PORT	Port to run the Web GUI (default: 1080)
--ip <ip address>	MAILDEV_IP	IP Address to bind SMTP service to (default: ::)
--web-ip <ip address>	MAILDEV_WEB_IP	IP Address to bind HTTP service to (default: 0.0.0.0)
--mail-directory <path>	MAILDEV_MAIL_DIRECTORY	Directory for persisting mail
--https	MAILDEV_HTTPS	Switch from http to https protocol
--https-key <file>	MAILDEV_HTTPS_KEY	The file path to the ssl private key
--https-cert <file>	MAILDEV_HTTPS_CERT	The file path to the ssl cert file
--incoming-user <user>	MAILDEV_INCOMING_USER	SMTP user for incoming mail
--incoming-pass <pass>	MAILDEV_INCOMING_PASS	SMTP password for incoming mail
--incoming-secure	MAILDEV_INCOMING_SECURE	Use SMTP SSL for incoming emails
--incoming-cert <path>	MAILDEV_INCOMING_CERT	Cert file location for incoming SSL
--incoming-key <path>	MAILDEV_INCOMING_KEY	Key file location for incoming SSL
--outgoing-host <host>	MAILDEV_OUTGOING_HOST	SMTP host for outgoing mail
--outgoing-port <port>	MAILDEV_OUTGOING_PORT	SMTP port for outgoing mail
--outgoing-user <user>	MAILDEV_OUTGOING_USER	SMTP user for outgoing mail
--outgoing-pass <password>	MAILDEV_OUTGOING_PASS	SMTP password for outgoing mail
--outgoing-secure	MAILDEV_OUTGOING_SECURE	Use SMTP SSL for outgoing mail
--auto-relay [email]	MAILDEV_AUTO_RELAY	Use auto-relay mode. Optional relay email address
--auto-relay-rules <file>	MAILDEV_AUTO_RELAY_RULES	Filter rules for auto relay mode
--web-user <user>	MAILDEV_WEB_USER	HTTP user for GUI
--web-pass <password>	MAILDEV_WEB_PASS	HTTP password for GUI
--base-pathname <path>	MAILDEV_BASE_PATHNAME	Base path for URLs
--disable-web	MAILDEV_DISABLE_WEB	Disable the use of the web interface
--hide-extensions <extensions>	MAILDEV_HIDE_EXTENSIONS	Comma separated list of SMTP extensions to NOT advertise
--mcp	MAILDEV_MCP	Enable MCP server for Claude integration
--config <file>		Path to configuration file
-v, --verbose		Enable verbose logging
--silent		Disable all output
--log-mail-contents		Log a JSON representation of each incoming mail

Configuration File

MailDev supports configuration files. Create a maildev.config.js, maildev.config.ts, or .maildevrc.json file:

// maildev.config.js
export default {
  smtp: 1025,
  web: 1080,
  verbose: true,
  mcp: true,
}

// .maildevrc.json
{
  "smtp": 1025,
  "web": 1080,
  "verbose": true
}

Configuration priority: CLI args > Environment variables > Config file > Defaults

API

MailDev can be used in your Node.js application. For more info view the API docs.

import { MailDev } from 'maildev'

const maildev = new MailDev({
  smtp: 1025,
  web: 1080,
})

const { smtp } = await maildev.start()

smtp.on('new', (email) => {
  console.log('New email:', email.subject)
})

// When done
await maildev.stop()

MailDev also has a REST API. For more info view the docs.

AI Agent Integration (MCP)

MailDev includes a Model Context Protocol (MCP) server so AI agents can work with your dev inbox. It works with any MCP client — Claude (Desktop and Code), Cursor, Codex, Windsurf, and more. Enable it with --mcp:

maildev --mcp

This exposes an MCP endpoint at /mcp that lets an agent:

• Search and retrieve emails
• Extract verification links and tokens
• Analyze email content
• Monitor email delivery

For detailed setup instructions — both transports, client config, and the available tools, resources, and prompts — see the MCP docs.

Outgoing Email

MailDev optionally supports selectively relaying emails to an outgoing SMTP server. If you configure outgoing email with the --outgoing-* options you can click "Relay" on an individual email to relay through MailDev out to a real SMTP service that will actually send the email to the recipient.

Example:

maildev --outgoing-host smtp.gmail.com \
        --outgoing-secure \
        --outgoing-user 'you@gmail.com' \
        --outgoing-pass '<pass>'

Auto Relay Mode

Enabling the auto relay mode will automatically send each email to its recipient without the need to click the "Relay" button mentioned above. The outgoing email options are required to enable this feature.

Optionally, you can specify a single email address to which MailDev will forward all emails instead of the original recipient. For example, using --auto-relay you@example.com will forward all emails to that address automatically.

Additionally, you can pass a valid json file with additional configuration for which email addresses you would like to allow or deny. The last matching rule in the array will be the rule MailDev will follow.

Example:

maildev --outgoing-host smtp.gmail.com \
        --outgoing-secure \
        --outgoing-user 'you@gmail.com' \
        --outgoing-pass '<pass>' \
        --auto-relay \
        --auto-relay-rules file.json

Rules example file:

[
  { "allow": "*" },
  { "deny": "*@test.com" },
  { "allow": "ok@test.com" },
  { "deny": "*@utah.com" },
  { "allow": "johnny@utah.com" }
]

This would allow angelo@fbi.gov, ok@test.com, johnny@utah.com, but deny bodhi@test.com.

Configure Your Project

Configure your application to send emails via port 1025 and open localhost:1080 in your browser.

Nodemailer (Node.js)

const nodemailer = require('nodemailer')

const transport = nodemailer.createTransport({
  host: 'localhost',
  port: 1025,
})

transport.sendMail({
  from: 'sender@example.com',
  to: 'recipient@example.com',
  subject: 'Test Email',
  text: 'Hello from MailDev!',
})

Django -- Add EMAIL_PORT = 1025 in your settings file [source]

Rails -- config settings:

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address: "localhost",
  port: 1025,
  enable_starttls_auto: false
}

Spring Boot -- in application.properties:

spring.mail.host=localhost
spring.mail.port=1025

Features

• Modern React-based web interface
• Toggle between HTML, plain text views and email headers
• Test responsive emails with resizable preview pane
• Receive and view email attachments
• Real-time updates via WebSocket
• Relay email to an upstream SMTP server
• MCP integration for Claude AI assistant
• Configuration file support
• Full TypeScript support

Ideas

If you're using MailDev and you have a great idea, I'd love to hear it. If you're not using MailDev because it lacks a feature, I'd love to hear that too. Add an issue to the repo here.

Contributing

Any help on MailDev would be awesome. There is plenty of room for improvement. Feel free to create a Pull Request.

MailDev v3 is a TypeScript monorepo using pnpm workspaces:

packages/
  core/     # Storage and shared types
  smtp/     # SMTP server
  api/      # REST API and WebSocket server
  ui/       # React web interface
  mcp/      # MCP server for Claude
  cli/      # CLI and orchestration

To run MailDev during development:

pnpm install
pnpm dev

To send test emails:

node scripts/send.js

To run the test suite:

pnpm test

Changelog

Thanks

MailDev is built using great open source projects including React, Fastify, Tailwind CSS, and two great projects from Andris Reinman: smtp-server and mailparser. Many thanks to Andris as his projects are the backbone of this app and to MailCatcher for the inspiration.

Additionally, thanks to all the awesome contributors to the project.

License

MIT