smtp4dev
smtp4dev is a free fake SMTP server for development and testing, letting teams inspect test email without sending it to real customers.
What smtp4dev is
smtp4dev is a free fake SMTP server for development and testing, letting teams inspect test email without sending it to real customers. It accepts SMTP mail and provides IMAP and POP3 access to retrieve or delete messages. Developers can inspect SMTP sessions, multipart MIME, rendered HTML, validation results, and compatibility reports; a viewport switcher simulates mobile email views. A terminal interface is also available. Users can set up multiple mailboxes and routing rules, compose or reply to messages, and relay mail with optional automatic rules. TLS/SSL supports implicit and STARTTLS modes, with self-signed certificates and optional authentication. An OpenAPI/Swagger API supports automated tests and external applications. Documented testing approaches include REST API use, SignalR notifications, Testcontainers examples, and use of the server component in .NET. Installation choices include winget, Docker/OCI images, a .NET global tool, and GitHub release downloads. The maintainer warns that Docker-published ports may expose captured email beyond the host and recommends binding to localhost when external access is not needed.
Good
- Supports SMTP, IMAP, and POP3.
- Includes HTML rendering and MIME inspection.
- Offers a terminal interface and web UI.
- API supports automated tests.
- Multiple installation options are listed.
Watch out
- Docker-published ports may expose captured emails externally.
- winget requires Windows 10 version 1809 or newer.
Who it suits
smtp4dev is intended for developers and teams testing email, including production-like testing and CI/CD pipelines. It is particularly relevant when test messages need to be inspected without reaching real customers.
Verdict
smtp4dev provides a local target for examining and automating email tests, with message inspection and API options. Configure Docker port binding carefully if captured messages should remain accessible only on the host.
The facts
Each one read from smtp4dev's own pages, with the date it was checked.
What it does
- API
- The project provides an OpenAPI/Swagger API.checked 8 Oct 2026
- API and automation
- An API is available for automated tests and external applications, with endpoints for messages, sessions, mailboxes, and server status and configuration.checked 8 Oct 2026
- Client integration
- Applications can connect by SMTP using the host and port where smtp4dev runs; the documentation gives localhost and port 25 as defaults.checked 8 Oct 2026
- Configuration
- Settings can be changed in the UI, appsettings.json, environment variables, or command-line arguments.checked 8 Oct 2026
- Deployment
- Installation options include winget, Docker/OCI images, a .NET global tool, and GitHub release downloads.checked 8 Oct 2026
- Docker security
- The maintainer warns that Docker-published ports may be accessible outside the host and recommends binding to 127.0.0.1 when external access is not needed.checked 8 Oct 2026
- Email protocols
- It supports SMTP and provides IMAP and POP3 access to retrieve and delete messages.checked 8 Oct 2026
- License
- The GitHub repository identifies the project license as BSD-3-Clause.checked 8 Oct 2026
- Mail handling
- Users can configure multiple mailboxes and routing rules, compose or reply to messages, and relay messages with optional automatic rules.checked 8 Oct 2026
- Mail protocols
- It supports SMTP, IMAP, and POP3 for receiving, retrieving, and deleting test messages.checked 8 Oct 2026
- Message inspection
- Features include SMTP session logs, multipart MIME inspection, HTML rendering, validation, and compatibility reports.checked 8 Oct 2026
- Purpose
- smtp4dev is a fake SMTP server for development and testing, letting developers test email without sending it to real customers.checked 8 Oct 2026
- Security configuration
- The Docker security guide warns that published ports may expose captured emails externally and recommends binding ports to localhost unless outside access is needed.checked 8 Oct 2026
- Security features
- It supports TLS/SSL with implicit and STARTTLS modes, automatically generated self-signed certificates, and optional authentication.checked 8 Oct 2026
- SMTP debugging
- The product provides SMTP session logging and supports scripting expressions for error simulation.checked 8 Oct 2026
- Testing integrations
- The testing guide covers programmatic use with the REST API, SignalR email notifications, Testcontainers examples for multiple programming languages, and direct use of the SMTP server component in .NET.checked 8 Oct 2026
- Testing tools
- The interface includes a viewport size switcher for simulating mobile email views and a terminal UI with full functionality.checked 8 Oct 2026
- Who it is for
- The README describes it for advanced email development and testing, production-like testing, development teams, and CI/CD pipelines.checked 8 Oct 2026
Where it runs
- Platforms
- The project offers Windows and Linux binaries and containers, plus a .NET SDK tool for platforms where the SDK is available; its README also names Mac OS X.checked 8 Oct 2026
- Platforms and distribution
- The project offers Windows and Linux binaries and containers, and a .NET SDK tool for platforms where the SDK is available; its README also names Windows, Linux, and Mac OS X.checked 8 Oct 2026
- Windows requirement
- The winget instructions specify Windows 10 version 1809 (build 17763) or newer.checked 8 Oct 2026
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