smtp4dev - the fake smtp email server for development and testing
C#
3,997
1,839 commits
updated Oct 10, 2026
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:
If you're looking for the older v2 Windows only GUI version. Grab it here.
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 |
| Terminal UI (TUI) | ✅ Full-featured | ❌ | ❌ | ❌ | ❌ |
| 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 10 | ✅ Go | ✅ Ruby | ✅ Node.js | ✅ Java |
| Docker Support | ✅ | ✅ | ✅ | ✅ | ❌ |
--incoming-secure, --incoming-cert, --incoming-key)--incoming-user/pass, --web-user/pass)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 .
Configuring your programs to send mails to smtp4dev
⚠️ Docker Users: Please read Docker Security Guidelines to prevent accidental public exposure of your smtp4dev instance to the world!
The main interface showing received emails with sender, recipient, subject, and timestamps.
View HTML emails with rich formatting, including viewport size simulation for testing responsive email designs.
Comprehensive email client compatibility reports showing which HTML/CSS features are supported across different email clients.
Inspect the raw message source with syntax highlighting and line numbers for debugging.
View detailed SMTP session logs to debug email delivery issues and protocol interactions.
Analyze multipart emails with attachments using the built-in MIME inspector.
Full dark mode support for comfortable viewing in low-light environments.
We welcome contributions! Before submitting a pull request, please:
259 followers · starred Jun 2024
133 followers · starred Jun 2024
500 followers · starred Nov 2024
16 followers · starred Oct 2025
smtp4dev - the fake smtp email server for development and testing
C#
3,997
1,839 commits
updated Oct 10, 2026
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:
If you're looking for the older v2 Windows only GUI version. Grab it here.
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 |
| Terminal UI (TUI) | ✅ Full-featured | ❌ | ❌ | ❌ | ❌ |
| 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 10 | ✅ Go | ✅ Ruby | ✅ Node.js | ✅ Java |
| Docker Support | ✅ | ✅ | ✅ | ✅ | ❌ |
--incoming-secure, --incoming-cert, --incoming-key)--incoming-user/pass, --web-user/pass)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.
Configuring your programs to send mails to smtp4dev
⚠️ Docker Users: Please read Docker Security Guidelines to prevent accidental public exposure of your smtp4dev instance to the world!
The main interface showing received emails with sender, recipient, subject, and timestamps.
View HTML emails with rich formatting, including viewport size simulation for testing responsive email designs.
Comprehensive email client compatibility reports showing which HTML/CSS features are supported across different email clients.
Inspect the raw message source with syntax highlighting and line numbers for debugging.
View detailed SMTP session logs to debug email delivery issues and protocol interactions.
Analyze multipart emails with attachments using the built-in MIME inspector.
Full dark mode support for comfortable viewing in low-light environments.
We welcome contributions! Before submitting a pull request, please:
259 followers · starred Jun 2024
133 followers · starred Jun 2024
500 followers · starred Nov 2024
16 followers · starred Oct 2025