smtp4dev - the fake SMTP email server for development and testing.

A dummy SMTP server for Windows, Linux, Mac OS-X (and maybe elsewhere where .NET Core is available). Lets you test your application without spamming your real customers.

smtp4dev support many advanced features:

• OpenAPI/Swagger API
• IMAP access to retrieve and delete messages
• SMTP session logging
• UTF8 support
• Viewport size switcher to simulate mobile etc
• Multipart MIME inspector
• HTML compatibility report and HTML validation
• TLS/SSL with implicit and STARTTLS modes and auto self-signed cert generation
• Authentication
• Multiple mailboxes along with rules to control what message goes where
• Reply, compose and relay messages including rules to auto relay
• Scripting expressions including error simulation

If you're looking for the older v2 Windows only GUI version. Grab it here.

How smtp4dev Compares

There are several fake SMTP servers available for development and testing. Here's how smtp4dev stacks up against other popular options:

Feature	smtp4dev	MailHog¹	MailCatcher²	MailDev³	FakeSMTP⁴
Web Interface	✅ Advanced	✅ Basic	✅ Basic	✅ Basic	❌ Desktop GUI
SMTP Server	✅	✅	✅	✅	✅
IMAP Server	✅	❌	❌	❌	❌
API (REST/OpenAPI)	✅ Swagger docs	✅ Basic⁵	✅ RESTful⁶	✅ Basic⁷	❌
HTML Email Rendering	✅ Advanced	✅ Basic	✅ Basic	✅ Basic	❌
HTML Compatibility Reports	✅	❌	❌	❌	❌
Responsive Email Testing	✅ Viewport switcher	❌	❌	✅ Resizable⁸	❌
MIME Parts Inspector	✅	❌	❌	❌	❌
SMTP Session Logging	✅ Detailed	❌	❌	❌	❌
TLS/SSL Support	✅ Auto-cert generation	❌	❌	✅ Basic⁹	❌
Authentication	✅	✅ Basic¹⁰	❌	✅ Basic¹¹	❌
Multiple Mailboxes	✅ With rules	❌	❌	❌	❌
Message Relay	✅ With rules	✅ Basic¹²	❌	✅ Basic¹³	❌
Message Composition	✅	❌	❌	❌	❌
Scripting & Error Simulation	✅	✅ Limited¹⁴	❌	❌	❌
Dark Mode	✅	❌	❌	❌	❌
Cross-Platform	✅ .NET 8	✅ Go	✅ Ruby	✅ Node.js	✅ Java
Docker Support	✅	✅	✅	✅	❌

Sources

1. MailHog GitHub Repository - Features verified from official documentation
2. MailCatcher GitHub Repository - Features verified from official documentation
3. MailDev GitHub Repository - Features verified from official documentation
4. FakeSMTP GitHub Repository - Features verified from official documentation
5. MailHog API: APIv1 and APIv2 documentation
6. MailCatcher API: RESTful URLs documented in README but no formal API documentation
7. MailDev API: REST API documentation
8. MailDev responsive testing: "Test responsive emails with resizable preview pane"
9. MailDev TLS/SSL: SSL options documented (--incoming-secure, --incoming-cert, --incoming-key)
10. MailHog authentication: HTTP basic authentication for UI and API
11. MailDev authentication: Authentication options (--incoming-user/pass, --web-user/pass)
12. MailHog relay: "Release messages to real SMTP servers"
13. MailDev relay: "Ability to relay email to an upstream SMTP server" with auto-relay mode
14. MailHog scripting: Chaos Monkey for failure testing

Why Choose smtp4dev?

For Advanced Development & Testing: If you need comprehensive email testing capabilities, smtp4dev offers features not found in other tools like HTML compatibility analysis, responsive design testing, and detailed SMTP session logging.

For Production-Like Testing: With IMAP support, TLS/SSL, authentication, and multiple mailboxes, smtp4dev can simulate real email server behavior more accurately.

For Team Workflows: The advanced web interface, API documentation, and message relay capabilities make smtp4dev ideal for development teams and CI/CD pipelines.

For Modern Email Development: Responsive email testing, HTML validation, and compatibility reports help ensure your emails work across all clients and devices.

If you find smtp4dev useful, please consider supporting further development by making a donation .

Getting Started

Installation Instructions

Configuration

Configuring your programs to send mails to smtp4dev

API

📚 View Full Documentation

Security

⚠️ Docker Users: Please read Docker Security Guidelines to prevent accidental public exposure of your smtp4dev instance to the world!

Screenshots

Main Message Interface

The main interface showing received emails with sender, recipient, subject, and timestamps.

HTML Email Rendering & Viewport Simulation

View HTML emails with rich formatting, including viewport size simulation for testing responsive email designs.

HTML Compatibility Analysis

Comprehensive email client compatibility reports showing which HTML/CSS features are supported across different email clients.

Message Source & Raw Content

Inspect the raw message source with syntax highlighting and line numbers for debugging.

SMTP Session Logging

View detailed SMTP session logs to debug email delivery issues and protocol interactions.

MIME Parts Inspector

Analyze multipart emails with attachments using the built-in MIME inspector.

Dark Mode Support

Full dark mode support for comfortable viewing in low-light environments.

Contributing

We welcome contributions! Before submitting a pull request, please:

1. Read our Contributing Guide
2. Sign the Contributor License Agreement

License