Your local SMTP server
made simple

01 Multiple Servers
02 Email Inspector
03 HTML Compatibility Check
04 Mobile Preview
05 Drop-in API Replacement
06 Attachment Support
Download for Mac View Features
macOS 26+ Apple Silicon Glass UI
Email details
Server list

Stop fighting your email setup

No Docker containers
No configuration files
No environment variables

Just point your app to localhost:PORT and start sending.

See every email as it arrives

Watch your emails transform from styled previews to raw SMTP data. Mailbox captures everything — headers, body, attachments.

Everything you need to test emails

A complete toolkit for email development, built natively for Apple platforms.

Multiple Servers

Run unlimited SMTP servers simultaneously on any port. Test your backend, frontend emails, and transactional notifications side by side — each server captures independently so nothing gets mixed up. Perfect for microservices, multi-tenant apps, and staging environments.

Email Inspector

View HTML renders, plain text, headers, and raw source of every captured email.

HTML Compatibility Check

New

Analyze your email HTML against 300+ CSS and HTML features across 22 email clients. See an overall compatibility score, per-property support bars, and detailed per-client breakdowns — powered by caniemail.com data.

Mobile Preview

New

Preview how your emails render on mobile devices. A simulated iPhone viewport shows exactly what your recipients see on their phones — catch responsive design issues before you send.

Drop-in API Replacement

Replace SendGrid, Postmark, Brevo, Mailtrap, Mailgun, and Resend with a single local endpoint. Point your app at Mailbox instead of the real service — all emails are captured locally. No account needed.

Webhook Simulation

Receive delivery, open, and click events locally — in each platform's exact native format. Test your webhook handlers without connecting to any third-party service.

SendGrid SendGrid
Postmark Postmark
Brevo Brevo
Mailtrap Mailtrap
Mailgun Mailgun
Resend Resend

Attachment Support

Preview and download email attachments. Images, PDFs, and any file type your app sends.

Native Experience

Built from the ground up with SwiftUI and Swift NIO — not an Electron wrapper or a Java app. Runs natively on Apple Silicon with minimal memory and CPU usage. Integrates with macOS features like menu bar access, keyboard shortcuts, system notifications, and the Glass UI design language on macOS 26. Feels like it belongs on your Mac.

Up and running in seconds

Three simple steps to start capturing emails from your application.

01

Create a Server

Pick a port number and give your server a name. Hit start.

02

Point Your App

Set your app's SMTP host to localhost and the port you chose.

03

Inspect Emails

Every email your app sends is captured and displayed instantly.

Automate your email testing

REST API

Every server exposes a REST API on port + 1. Query captured emails, verify delivery, and integrate with your CI/CD pipeline.

  • List all captured emails
  • Filter by sender, recipient, or subject
  • Access raw MIME content
  • Delete emails programmatically
Terminal
$ curl http://localhost:1026/api/emails | jq
{
  "emails": [
    {
      "id": "msg-001",
      "from": "test@app.dev",
      "to": "user@localhost",
      "subject": "Welcome!",
      "date": "2026-03-21T09:14:22Z"
    }
  ]
}
Platform APIs

Drop-in replacements for popular email services. Just change the base URL — no account, no API key validation, same request format.

  • SendGrid, Postmark, Resend, Mailgun, Brevo & Mailtrap
  • Same request format — just swap the base URL
  • No API key validation or account required
  • Webhook events in each platform's native format

Frequently asked questions

Does Mailbox actually send emails?
No. Mailbox creates local SMTP servers that capture emails without delivering them. Your emails never leave your machine.
Can I run multiple servers at once?
Yes. You can create as many servers as you need, each on a different port. This is great for testing multiple services simultaneously.
Does it work with any programming language?
Absolutely. Any application that can send emails via SMTP will work with Mailbox — Node.js, Python, Ruby, PHP, Go, Swift, Java, and more.
Is there a REST API?
Yes. Each server exposes a REST API on port + 1. You can list, search, and delete captured emails programmatically.
Do I need to configure anything?
No. Just create a server, pick a port, and start it. Point your app to localhost on that port. That's it.
Is it free?
Mailbox is available on the App Store. Check the listing for current pricing details.
What platforms does Mailbox support?
SendGrid, Postmark, Brevo, Mailtrap, Mailgun, and Resend. Each endpoint accepts the exact same request format as the real service.
Do I need an account with these services?
No. Mailbox accepts any API key. You just change your base URL to localhost.
How do webhooks work?
When you send an email through a platform API, Mailbox fires a delivery webhook to your configured URL. Opening the email in Mailbox fires an open event, clicking a link fires a click event.

Simple, transparent pricing

Start for free. Upgrade when you need more power.

Free

$0

Perfect for getting started with email testing.

  • 1 SMTP server
  • Email capture and inspection
  • HTML & plain text rendering
  • HTML compatibility check
  • Mobile email preview
  • Headers inspector
  • Attachments viewer
  • Search & export
Get Started Free

Pro Monthly

$4.99 /mo

For developers who need the full toolkit.

  • Unlimited SMTP servers
  • Drop-in API replacement (6 platforms)
  • Webhook simulation
  • Email forwarding
  • REST API access
  • Priority support
Go Pro Monthly
Best Value

Pro Yearly

$29.99 /yr

$2.49/mo

Everything in Pro Monthly with a 7-day free trial.

  • Everything in Pro Monthly
  • 7-day free trial
  • Save 50%
Start Free Trial

Pro Lifetime

$49.99 once

Pay once, own forever. Every Pro feature, no expiration.

  • Everything in Pro
  • Pay once, own forever
  • All future Pro features
Buy Lifetime

Start catching emails today

Available on the Mac App Store. Download, create a server, and start testing in under a minute.

Download on the Mac App Store
100% Native
SwiftUI
Lightweight
< 10 MB
Fully Private
localhost

Need a custom solution?

For teams that need volume licensing, custom features, or priority support.