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.

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]

Storage limit

MailDev keeps every email by default (--max-emails 0), preserving its historical behaviour and keeping persisted mail durable across restarts. Set a positive limit to cap the store: when a new email arrives and the limit is reached, the oldest is discarded — along with its .eml file and attachments, so a persisted mail directory stays bounded as well. Any backlog left in the mail directory by earlier runs is trimmed to the same limit at startup.

maildev --max-emails 1000     # keep the newest 1000
maildev --max-emails 0        # keep everything (the default)

Leaving the limit unset keeps everything. Be aware that memory use and the mail directory then grow with the inbox; with typical messages, 10,000 emails is around 150 MB of heap, so set a limit for long-running or high-volume use.

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