smtp4dev

Free plan APILinuxMacSelf-hostedWebWindows

What smtp4dev is

smtp4dev is a free fake SMTP server for development and testing, allowing teams to inspect email without sending messages to real customers. It accepts SMTP messages and provides IMAP and POP3 access for retrieving or deleting test mail. Its interface includes SMTP session logs, multipart MIME inspection, HTML rendering, validation, and compatibility reports, plus a viewport switcher for simulating mobile email views. A terminal interface provides the same functionality. 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; the server can generate self-signed certificates and optionally require authentication. An OpenAPI/Swagger API supports automated tests and external applications, with endpoints for messages, sessions, mailboxes, and server status and configuration. Installation options include winget, Docker/OCI images, a .NET global tool, and GitHub release downloads. The project documents use in development teams and CI/CD pipelines. Its maintainer warns that published Docker ports can expose captured email outside the host and recommends binding to localhost when outside access is unnecessary.

Who it suits

smtp4dev suits developers and teams building or testing email workflows, including production-like testing and CI/CD pipelines. It can also suit automated tests that need API access to test messages and server state.

What it does well

Free to use, with no trial requirement listed.
Supports SMTP, IMAP, and POP3.
Offers HTML rendering and compatibility reports.
API supports automated tests and external applications.
Multiple installation options include Docker and winget.

What to know first

Published Docker ports may expose captured emails externally.
Windows winget requires Windows 10 version 1809 or newer.
TLS certificates are self-signed when generated automatically.

Getting started

Open the smtp4dev website or GitHub repository
Install through winget, Docker or OCI images, the .NET global tool, or a GitHub release
Run smtp4dev on a supported platform
Connect an application by SMTP to the host and port where smtp4dev runs
Use localhost and port 25 as the documented defaults
Adjust settings in the interface, appsettings.json, environment variables, or command-line arguments

Verdict

smtp4dev provides a local testing environment with message inspection, protocol access, and API support. When deploying with Docker, follow the localhost-binding guidance if external access is not needed.

Questions

What does smtp4dev cost?

smtp4dev is free and provides a free plan.

Which mail protocols does it support?

It accepts SMTP and provides IMAP and POP3 access for retrieving or deleting test messages.

Does smtp4dev provide an API?

Yes. An OpenAPI/Swagger API supports messages, sessions, mailboxes, and server status or configuration.

Where can smtp4dev run?

Platforms listed include API, Linux, macOS, self-hosted, web, and Windows. Distribution includes binaries, containers, winget, a .NET global tool, and GitHub releases.

Is smtp4dev open source?

Its GitHub repository identifies the license as BSD-3-Clause.

Facts from smtp4dev's own pages

API: The project provides an OpenAPI/Swagger API. · github.com, 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. · github.com, 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. · github.com, 8 Oct 2026
Configuration: Settings can be changed in the UI, appsettings.json, environment variables, or command-line arguments. · github.com, 8 Oct 2026
Deployment: Installation options include winget, Docker/OCI images, a .NET global tool, and GitHub release downloads. · github.com, 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. · github.com, 8 Oct 2026
Email protocols: It supports SMTP and provides IMAP and POP3 access to retrieve and delete messages. · github.com, 8 Oct 2026
License: The GitHub repository identifies the project license as BSD-3-Clause. · github.com, 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. · github.com, 8 Oct 2026
Mail protocols: It supports SMTP, IMAP, and POP3 for receiving, retrieving, and deleting test messages. · github.com, 8 Oct 2026
Message inspection: Features include SMTP session logs, multipart MIME inspection, HTML rendering, validation, and compatibility reports. · github.com, 8 Oct 2026
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. · github.com, 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. · github.com, 8 Oct 2026
Purpose: smtp4dev is a fake SMTP server for development and testing, letting developers test email without sending it to real customers. · github.com, 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. · github.com, 8 Oct 2026
Security features: It supports TLS/SSL with implicit and STARTTLS modes, automatically generated self-signed certificates, and optional authentication. · github.com, 8 Oct 2026
SMTP debugging: The product provides SMTP session logging and supports scripting expressions for error simulation. · github.com, 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. · github.com, 8 Oct 2026
Testing tools: The interface includes a viewport size switcher for simulating mobile email views and a terminal UI with full functionality. · github.com, 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. · github.com, 8 Oct 2026
Windows requirement: The winget instructions specify Windows 10 version 1809 (build 17763) or newer. · github.com, 8 Oct 2026

Where smtp4dev stands on Freedom251

Email Testing Software#16 of 28