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Smtp4dev

Advanced fake SMTP server for realistic email testing

Email Testing & Sandbox Inboxes

smtp4dev provides a cross‑platform dummy SMTP/IMAP server with a rich web UI, API, TLS, authentication, multiple mailboxes, and tools for HTML validation and responsive email testing.

Overview

Overview

smtp4dev is a cross‑platform dummy SMTP and IMAP server designed for developers and QA engineers who need realistic email testing without sending real messages. It runs on any .NET 8 capable OS (Windows, Linux, macOS) and is also available as a Docker image for CI/CD pipelines.

Core Capabilities

The tool offers an advanced web interface with viewport switching to preview responsive designs, detailed SMTP session logs, and a built‑in MIME inspector. Through its OpenAPI/Swagger documented REST API you can programmatically manage messages, trigger relays, or simulate errors. Features such as TLS/SSL with auto‑generated certificates, authentication, multiple mailboxes with routing rules, and scripting make it suitable for production‑like testing scenarios.

Deployment

Install via package managers or pull the official Docker container, configure ports for SMTP, IMAP, and the web UI, and start testing immediately. The flexible configuration lets you enable or disable components to match local development or automated test environments.

Highlights

Advanced web UI with viewport switcher for responsive testing
Built‑in IMAP server and TLS/SSL with auto‑generated certificates
OpenAPI/Swagger documented REST API for message management
Multiple mailboxes, rules, and scripting for relay and error simulation

Pros

  • Comprehensive feature set surpasses many alternatives
  • Cross‑platform .NET 8 runtime works on Windows, Linux, macOS
  • Docker image enables easy CI/CD integration
  • Detailed SMTP session logs aid debugging

Considerations

  • Requires .NET runtime, may be heavier than lightweight Go tools
  • UI is web‑based; no native desktop client
  • Advanced features may have a learning curve
  • Resource usage higher when handling large volumes

Managed products teams compare with

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Looking for a hosted option? These are the services engineering teams benchmark against before choosing open source.

Fit guide

Great for

  • Developers needing realistic email server behavior in local environments
  • QA teams testing HTML email rendering and responsiveness
  • CI pipelines that require automated email verification via API
  • Teams that need multiple mailboxes and rule‑based routing

Not ideal when

  • Simple one‑off email capture where a minimal tool suffices
  • Environments without .NET support or where Docker is prohibited
  • Users seeking a native GUI without a browser interface
  • High‑throughput production email relaying

How teams use it

Responsive email design verification

Render emails in various viewport sizes to ensure layout works on mobile and desktop clients

Automated CI email tests

Use the REST API to send test messages and assert content, headers, and delivery logs in pipeline runs

Multi‑mailbox routing simulation

Define rules to direct incoming messages to specific mailboxes, mimicking complex production routing

Error handling and failover testing

Inject scripted errors and TLS failures to validate application resilience

Tech snapshot

C#92%
Vue6%
TypeScript1%
PowerShell1%
CSS1%
JavaScript1%

Tags

macosxsmtp-serverwindowslinuxsmtp4dev

Frequently asked questions

Does smtp4dev support TLS/SSL?

Yes, it offers implicit and STARTTLS modes and can auto‑generate self‑signed certificates.

Can I access received messages via IMAP?

An integrated IMAP server lets you retrieve, search, and delete messages using any standard IMAP client.

Is there a Docker image for headless use?

A ready‑to‑run Docker image is provided, suitable for CI/CD and containerized environments.

How do I automate interactions?

The OpenAPI/Swagger documented REST API allows programmatic sending, fetching, and managing messages.

What platforms are supported?

Runs on any OS where .NET 8 is available, including Windows, Linux, and macOS.

Project at a glance

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LicenseBSD-3-Clause
Repo age14 years old
Last commit2 days ago
Primary languageC#

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