/aspire-mailpit-integration

name: mailpit-integration
description: Test email sending locally using Mailpit with .NET Aspire. Captures all outgoing emails without sending them. View rendered HTML, inspect headers, and verify delivery in integration tests.
user-invocable: false

Email Testing with Mailpit and .NET Aspire

When to Use This Skill

Use this skill when:

• Testing email delivery locally without sending real emails
• Setting up email infrastructure in .NET Aspire
• Writing integration tests that verify emails are sent
• Debugging email rendering and headers

**Related skills:**

• `aspnetcore/mjml-email-templates` - MJML template authoring
• `testing/verify-email-snapshots` - Snapshot test rendered HTML
• `aspire/integration-testing` - General Aspire testing patterns

---

What is Mailpit?

[Mailpit](https://github.com/axllent/mailpit) is a lightweight email testing tool that:

• Captures all SMTP traffic without delivering emails
• Provides a web UI to view captured emails
• Exposes an API for programmatic access
• Supports HTML rendering, headers, and attachments

Perfect for development and integration testing.

---

Aspire AppHost Configuration

Add Mailpit as a container in your AppHost:

// AppHost/Program.cs
var builder = DistributedApplication.CreateBuilder(args);

// Add Mailpit for email testing
var mailpit = builder.AddContainer("mailpit", "axllent/mailpit")
    .WithHttpEndpoint(port: 8025, targetPort: 8025, name: "ui")
    .WithEndpoint(port: 1025, targetPort: 1025, name: "smtp");

// Reference in your API project
var api = builder.AddProject<Projects.MyApp_Api>("api")
    .WithReference(mailpit.GetEndpoint("smtp"))
    .WithEnvironment("Smtp__Host", mailpit.GetEndpoint("smtp"));

builder.Build().Run();

---

SMTP Configuration

appsettings.json

{
  "Smtp": {
    "Host": "localhost",
    "Port": 1025,
    "EnableSsl": false,
    "FromAddress": "noreply@myapp.com",
    "FromName": "MyApp"
  }
}

Configuration Class

public class SmtpSettings
{
    public string Host { get; set; } = "localhost";
    public int Port { get; set; } = 1025;
    public bool EnableSsl { get; set; } = false;
    public string FromAddress { get; set; } = "noreply@myapp.com";
    public string FromName { get; set; } = "MyApp";

    // Optional: For production SMTP
    public string? Username { get; set; }
    public string? Password { get; set; }
}

Service Registration

// In Program.cs or extension method
services.Configure<SmtpSettings>(configuration.GetSection("Smtp"));

services.AddSingleton<IEmailSender>(sp =>
{
    var settings = sp.GetRequiredService<IOptions<SmtpSettings>>().Value;
    return new SmtpEmailSender(settings);
});

---

Email Sender Implementation

public interface IEmailSender
{
    Task SendEmailAsync(EmailMessage message, CancellationToken ct = default);
}

public sealed class SmtpEmailSender : IEmailSender
{
    private readonly SmtpSettings _settings;

    public SmtpEmailSender(SmtpSettings settings)
    {
        _settings = settings;
    }

    public async Task SendEmailAsync(EmailMessage message, CancellationToken ct = default)
    {
        using var client = new SmtpClient();

        await client.ConnectAsync(
            _settings.Host,
            _settings.Port,
            _settings.EnableSsl ? SecureSocketOptions.StartTls : SecureSocketOptions.None,
            ct);

        if (!string.IsNullOrEmpty(_settings.Username))
        {
            await client.AuthenticateAsync(_settings.Username, _settings.Password, ct);
        }

        var mailMessage = new MimeMessage();
        mailMessage.From.Add(new MailboxAddress(_settings.FromName, _settings.FromAddress));
        mailMessage.To.Add(new MailboxAddress(message.ToName, message.To));
        mailMessage.Subject = message.Subject;

        var bodyBuilder = new BodyBuilder { HtmlBody = message.HtmlBody };
        mailMessage.Body = bodyBuilder.ToMessageBody();

        await client.SendAsync(mailMessage, ct);
        await client.DisconnectAsync(true, ct);
    }
}

Requires `MailKit` package:

dotnet add package MailKit

---

Viewing Captured Emails

Web UI

Navigate to `http://localhost:8025` to see:

• **Inbox** - All captured emails
• **HTML view** - Rendered email
• **Source view** - Raw HTML/MJML output
• **Headers** - Full email headers
• **Attachments** - Any attached files

Aspire Dashboard

The Mailpit UI endpoint appears in the Aspire dashboard under Resources.

---

Integration Testing

Test Fixture with Aspire

public class EmailIntegrationTests : IClassFixture<AspireFixture>
{
    private readonly HttpClient _client;
    private readonly MailpitClient _mailpit;

    public EmailIntegrationTests(AspireFixture fixture)
    {
        _client = fixture.CreateClient();
        _mailpit = new MailpitClient(fixture.GetMailpitUrl());
    }

    [Fact]
    public async Task SignupFlow_SendsWelcomeEmail()
    {
        // Arrange
        await _mailpit.ClearMessagesAsync();

        // Act - Trigger signup flow
        var response = await _client.PostAsJsonAsync("/api/auth/signup", new
        {
            Email = "test@example.com",
            Password = "SecurePassword123!"
        });
        response.EnsureSuccessStatusCode();

        // Assert - Verify email was sent
        var messages = await _mailpit.GetMessagesAsync();

        var welcomeEmail = messages.Should().ContainSingle()
            .Which;

        welcomeEmail.To.Should().Contain("test@example.com");
        welcomeEmail.Subject.Should().Contain("Welcome");
        welcomeEmail.HtmlBody.Should().Contain("Thank you for signing up");
    }
}

Mailpit API Client

public class MailpitClient
{
    private readonly HttpClient _client;

    public MailpitClient(string baseUrl)
    {
        _client = new HttpClient { BaseAddress = new Uri(baseUrl) };
    }

    public async Task<List<MailpitMessage