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 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:

csharp

// 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

json

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

Configuration Class

csharp

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

csharp

// 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

csharp

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:

bash

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

csharp

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

csharp

public class MailpitClient
{
    private readonly HttpClient _client;

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

    public async Task<List<MailpitMessage>> GetMessagesAsync()
    {
        var response = await _client.GetFromJsonAsync<MailpitResponse>("/api/v1/messages");
        return response?.Messages ?? new List<MailpitMessage>();
    }

    public async Task ClearMessagesAsync()
    {
        await _client.DeleteAsync("/api/v1/messages");
    }

    public async Task<MailpitMessage?> WaitForMessageAsync(
        Func<MailpitMessage, bool> predicate,
        TimeSpan timeout)
    {
        var deadline = DateTime.UtcNow + timeout;

        while (DateTime.UtcNow < deadline)
        {
            var messages = await GetMessagesAsync();
            var match = messages.FirstOrDefault(predicate);

            if (match != null)
                return match;

            await Task.Delay(100);
        }

        return null;
    }
}

public class MailpitResponse
{
    public List<MailpitMessage> Messages { get; set; } = new();
}

public class MailpitMessage
{
    public string Id { get; set; } = "";
    public List<string> To { get; set; } = new();
    public string Subject { get; set; } = "";
    public string HtmlBody { get; set; } = "";
}

Aspire Test Fixture

csharp

public class AspireFixture : IAsyncLifetime
{
    private DistributedApplication? _app;
    private string _mailpitUrl = "";

    public async Task InitializeAsync()
    {
        var appHost = await DistributedApplicationTestingBuilder
            .CreateAsync<Projects.MyApp_AppHost>();

        // Disable persistence for clean tests
        appHost.Configuration["MyApp:UseVolumes"] = "false";

        _app = await appHost.BuildAsync();
        await _app.StartAsync();

        // Get Mailpit URL from Aspire
        var mailpit = _app.GetContainerResource("mailpit");
        _mailpitUrl = await mailpit.GetEndpointAsync("ui");
    }

    public HttpClient CreateClient()
    {
        var api = _app!.GetProjectResource("api");
        return api.CreateHttpClient();
    }

    public string GetMailpitUrl() => _mailpitUrl;

    public async Task DisposeAsync()
    {
        if (_app != null)
            await _app.DisposeAsync();
    }
}

Common Test Patterns

Wait for Async Email

Some emails are sent asynchronously. Wait for them:

csharp

[Fact]
public async Task AsyncWorkflow_EventuallySendsEmail()
{
    await _mailpit.ClearMessagesAsync();

    // Trigger async workflow
    await _client.PostAsync("/api/workflows/start", null);

    // Wait for email (with timeout)
    var email = await _mailpit.WaitForMessageAsync(
        m => m.Subject.Contains("Workflow Complete"),
        timeout: TimeSpan.FromSeconds(10));

    email.Should().NotBeNull();
}

Verify Multiple Emails

csharp

[Fact]
public async Task BulkOperation_SendsMultipleEmails()
{
    await _mailpit.ClearMessagesAsync();

    await _client.PostAsJsonAsync("/api/invitations/bulk", new
    {
        Emails = new[] { "a@test.com", "b@test.com", "c@test.com" }
    });

    var messages = await _mailpit.WaitForMessagesAsync(
        expectedCount: 3,
        timeout: TimeSpan.FromSeconds(10));

    messages.Should().HaveCount(3);
    messages.Select(m => m.To.First())
        .Should().BeEquivalentTo("a@test.com", "b@test.com", "c@test.com");
}

Verify Email Content

csharp

[Fact]
public async Task PasswordReset_ContainsValidResetLink()
{
    await _mailpit.ClearMessagesAsync();

    await _client.PostAsJsonAsync("/api/auth/forgot-password", new
    {
        Email = "user@test.com"
    });

    var email = await _mailpit.WaitForMessageAsync(
        m => m.Subject.Contains("Password Reset"),
        timeout: TimeSpan.FromSeconds(5));

    // Extract reset link from HTML
    var resetLink = Regex.Match(email!.HtmlBody, @"href=""([^""]+/reset/[^""]+)""")
        .Groups[1].Value;

    resetLink.Should().StartWith("https://myapp.com/reset/");

    // Verify the link works
    var resetResponse = await _client.GetAsync(resetLink);
    resetResponse.StatusCode.Should().Be(HttpStatusCode.OK);
}

Production vs Development

csharp

services.AddSingleton<IEmailSender>(sp =>
{
    var settings = sp.GetRequiredService<IOptions<SmtpSettings>>().Value;
    var env = sp.GetRequiredService<IHostEnvironment>();

    if (env.IsDevelopment())
    {
        // Mailpit - no auth, no SSL
        return new SmtpEmailSender(settings);
    }
    else
    {
        // Production SMTP (SendGrid, Postmark, etc.)
        return new SmtpEmailSender(settings with
        {
            EnableSsl = true
        });
    }
});

Troubleshooting

Emails Not Appearing

Check Mailpit container is running in Aspire dashboard
Verify SMTP host/port configuration
Check for exceptions in application logs

Connection Refused

bash

# Verify Mailpit is listening
curl http://localhost:8025/api/v1/messages

Aspire Endpoint Not Resolving

csharp

// Ensure endpoint reference is correct
.WithEnvironment("Smtp__Host", mailpit.GetEndpoint("smtp").Property(EndpointProperty.Host))
.WithEnvironment("Smtp__Port", mailpit.GetEndpoint("smtp").Property(EndpointProperty.Port))

Resources

Mailpit: https://github.com/axllent/mailpit
Mailpit API: https://mailpit.axllent.org/docs/api-v1/
MailKit: https://github.com/jstedfast/MailKit
Aspire Containers: https://learn.microsoft.com/en-us/dotnet/aspire/fundamentals/container-resources

mailpit-integration

NPX Install

Tags

SKILL.md Content

Email Testing with Mailpit and .NET Aspire

When to Use This Skill

What is Mailpit?

Aspire AppHost Configuration

SMTP Configuration

appsettings.json

Configuration Class

Service Registration

Email Sender Implementation

Viewing Captured Emails

Web UI

Aspire Dashboard

Integration Testing

Test Fixture with Aspire

Mailpit API Client

Aspire Test Fixture

Common Test Patterns

Wait for Async Email

Verify Multiple Emails

Verify Email Content

Production vs Development

Troubleshooting

Emails Not Appearing

Connection Refused

Aspire Endpoint Not Resolving

Resources