smtp4dev Development Instructions

March 12, 2026 · View on GitHub

Always follow these instructions first and fallback to additional search and context gathering only if the information in the instructions is incomplete or found to be in error.

smtp4dev is a fake SMTP email server for development and testing, built as a .NET 10.0 web application with a Vue.js 3 frontend. It provides SMTP, IMAP, and web interfaces for testing email functionality during development.

Working Effectively

Prerequisites and Dependencies

  • .NET 10.0 SDK
  • Node.js 20+ and npm 10+ (tested with Node.js 20.19.4, npm 10.8.2)
  • Git
  • Optional: Chrome/Chromium for E2E tests

Bootstrap, Build, and Test Repository

NEVER CANCEL: All build and test commands can take significant time. Use appropriate timeouts.

  1. Restore .NET dependencies:

    dotnet restore
    • Takes approximately 40-60 seconds
    • NEVER CANCEL - Set timeout to 120+ seconds

  2. Build the application:

    # Build main web application (recommended for development)
dotnet build Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj
    • Takes approximately 3-5 seconds after initial frontend build
    • NEVER CANCEL - Set timeout to 60+ seconds
    • The Desktop project will fail on Linux (expected - Windows-only)

Run the Application

Development mode (with live reload):

dotnet run --project Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj --urls="http://localhost:5000" --smtpport=2525 --imapport=1143

Published mode:


dotnet publish Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj -c Release -o ./published

cd ./published
./Rnwood.Smtp4dev --urls="http://localhost:5000" --smtpport=2525 --imapport=1143

Key startup arguments:

  • --urls: Web interface URL (default: http://localhost:5000)
  • --smtpport: SMTP server port (default: 25, use 2525 for development)
  • --imapport: IMAP server port (default: 143, use 1143 for development)
  • --help: Full list of command-line options

The application serves:

  • Web UI at the specified URL (e.g., http://localhost:5000)
  • SMTP server on the specified port
  • IMAP server on the specified port
  • API endpoints at /api/messages, /api/sessions, etc.

Testing

Unit and Integration Tests:

# SMTP Server library tests
dotnet test smtpserver/Rnwood.SmtpServer.Tests -c Release
  • Takes approximately 20-30 seconds
  • NEVER CANCEL - Set timeout to 60+ seconds
  • Some IPv6-related test failures are expected in CI environments

Frontend Linting:

cd Rnwood.Smtp4dev/ClientApp
npm run lint
  • Takes approximately 10-15 seconds
  • Some configuration warnings are expected

Validation Scenarios

ALWAYS manually validate changes with these scenarios:

  1. Basic SMTP Functionality:

    # Send a test email via SMTP
cat > /tmp/test_email.txt << 'EOF'
HELO localhost
MAIL FROM: <test@example.com>
RCPT TO: <user@example.com>
DATA
Subject: Test Email
From: test@example.com
To: user@example.com

This is a test email to validate smtp4dev functionality.
.
QUIT
EOF

nc localhost 2525 < /tmp/test_email.txt

  2. Web UI Functionality:

    • Navigate to http://localhost:5000
    • Verify the email appears in the Messages tab
    • Click on the email to view details
    • Test the Sessions tab to see SMTP session logs

  3. API Functionality:

    # Check messages API
curl -s http://localhost:5000/api/messages | jq '.'

# Check sessions API
curl -s http://localhost:5000/api/sessions | jq '.'

Project Structure

Key Projects

  • Rnwood.Smtp4dev/ - Main web application (ASP.NET Core + Vue.js)
  • Rnwood.Smtp4dev.Tests/ - Unit and E2E tests
  • Rnwood.Smtp4dev.Desktop/ - Desktop application (Windows only)
  • smtpserver/Rnwood.SmtpServer/ - SMTP server library
  • imapserver/New.LumiSoft.Net/ - IMAP server library

Important Files

  • Rnwood.Smtp4dev/appsettings.json - Main configuration with extensive documentation
  • Rnwood.Smtp4dev/ClientApp/package.json - Frontend dependencies and scripts
  • azure-pipelines.yml - CI/CD pipeline configuration
  • Rnwood.Smtp4dev/ClientApp/vite.config.js - Frontend build configuration
  • docs/ - Documentation for users

Frontend Structure

  • Built with Vue.js 3, TypeScript, and Vite
  • Located in Rnwood.Smtp4dev/ClientApp/
  • Uses Element Plus UI components
  • Build output goes to Rnwood.Smtp4dev/wwwroot/

Development Workflow

Making Changes

  1. Always build and test first to establish a baseline
  2. Make minimal, focused changes
  3. Always run validation scenarios after changes
  4. Test both development and published modes

Full Stack Development

dotnet watch run --project Rnwood.Smtp4dev/Rnwood.Smtp4dev.csproj

Both the front end and backend will reload on changes.

Configuration

  • Default configuration in appsettings.json with extensive inline documentation
  • User configuration stored in ~/.config/smtp4dev/appsettings.json on Linux
  • Environment variables: Use ServerOptions__PropertyName format
  • Command line arguments: Use --propertyname=value format

Pull Request Requirements for AI Agents

MANDATORY: All pull requests must have titles that follow the Conventional Commits specification.

Required Format

type(optional-scope): description

Valid Types for smtp4dev

  • feat: New features (e.g., feat: add IMAP search functionality)
  • fix: Bug fixes (e.g., fix(smtp): resolve memory leak in session handling)
  • docs: Documentation changes (e.g., docs: update API documentation)
  • refactor: Code refactoring (e.g., refactor(api): simplify message endpoint logic)
  • test: Test additions/fixes (e.g., test: add unit tests for email parsing)
  • chore: Maintenance tasks (e.g., chore(deps): update npm dependencies)
  • ci: CI/CD changes (e.g., ci: add conventional commit validation)
  • build: Build system changes (e.g., build: update .NET SDK to 8.0.119)
  • perf: Performance improvements (e.g., perf: optimize message list rendering)
  • style: Code style changes (e.g., style: fix linting issues)

Examples for Common Tasks

  • Adding new functionality: feat(web): add dark mode toggle (focuses on user feature, not technical implementation)
  • Fixing bugs: fix(smtp): handle malformed email headers gracefully (describes the problem solved)
  • Updating documentation: docs(api): add examples for message endpoints (describes what users gain)
  • Dependency updates: chore(deps): update Vue.js to latest version
  • CI improvements: ci: add automated security scanning

Examples of Good vs Poor Titles

✅ Good (user-focused):

  • feat: enable email filtering by date range
  • fix: prevent email list from freezing on large messages
  • docs: add troubleshooting guide for connection issues

❌ Poor (technical-focused):

  • feat: implement DateRange component with Vue3 composition API
  • fix: refactor MessageService.GetMessages() method to use async patterns
  • docs: update API.md file with new endpoint documentation

AI Agent Guidelines

  • Focus on user impact: Describe what the change does for users from a functional perspective, not technical implementation details
  • Always use lowercase after the type: The description must start with a lowercase letter (e.g., fix: resolve issue not fix: Resolve issue)
  • Use imperative mood ("add" not "added" or "adds")
  • Keep descriptions concise but descriptive
  • Don't end with a period
  • Include scope when the change affects a specific component
  • Avoid technical jargon; focus on the functional benefit or problem solved
  • The GitHub Actions workflow will automatically validate PR titles

Common Issues and Solutions

Build Issues

  • Desktop project fails on Linux: Expected - it's Windows-only
  • NPM peer dependency warnings: Expected and harmless
  • Sass deprecation warnings: Expected with current Vite/Sass versions
  • Large chunk size warnings: Expected due to comprehensive frontend dependencies

Runtime Issues

  • Port conflicts: Use non-standard ports (2525 for SMTP, 1143 for IMAP, 5001+ for web)
  • IPv6 test failures: Expected in some CI environments
  • Database locks: Application creates SQLite database in user config directory

Testing Issues

  • E2E test complexity: Requires Chrome and specific environment setup
  • Network-dependent tests: May fail in restricted environments

Integration with CI/CD

The project uses Azure Pipelines (azure-pipelines.yml) with:

  • Multi-platform builds (Windows, Linux, macOS)
  • Docker image creation
  • NuGet package publishing
  • GitHub releases

Build matrix includes:

  • win-x64, linux-x64, linux-musl-x64, linux-arm, win-arm64
  • Desktop variants (Windows only)
  • Docker images (Linux and Windows)