Aspire Mailpit Integration

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

CLAUDE.md

Audit 0.4.1 · source sha 871f73ffcae3 · 18 Oct 2026Report an issue or request a re-scan

• PublisherAaronontheweb GitHub userOwner of the repository this skill is read from
• Source folderAaronontheweb/dotnet-skills/skills/aspire-mailpit-integrationThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict871f73ffcae3A new audit runs when the source changes
• Repository statusNot archivedLast push 7 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its 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.

---
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:

```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": "[email protected]",
    "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; } = "[email protected]";
    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 = "[email protected]",
            Password = "SecurePassword123!"
        });
        response.EnsureSuccessStatusCode();

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

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

        welcomeEmail.To.Should().Contain("[email protected]");
        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

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/Aaronontheweb/dotnet-skills
git -C dotnet-skills checkout 871f73ffcae3

Works in

1. 2026-10-18
   ✓SAFE B 89 / 100

   first audit · source 871f73ffcae3

Questions about Aspire Mailpit Integration

• github.com/Aaronontheweb/dotnet-skills/tree/master/skills/aspire — the audited source at commit 871f73ffcae3 and every finding aboveObserved · 18 Oct 2026

Spotted something wrong? Report a correction · Methodology