MJML Email Templates

When to Use This Skill

Use this skill when:

• Building transactional emails (signup, password reset, invoices, notifications)
• Creating responsive email templates that work across clients
• Setting up MJML template rendering in .NET

Related skills:

• aspire/mailpit-integration - Test emails locally with Mailpit
• testing/verify-email-snapshots - Snapshot test rendered HTML

Why MJML?

Problem: Email HTML is notoriously difficult. Each email client (Outlook, Gmail, Apple Mail) renders differently, requiring complex table-based layouts and inline styles.

Solution: MJML is a markup language that compiles to responsive, cross-client HTML:

<!-- MJML - simple and readable -->
<mj-section>
  <mj-column>
    <mj-text>Hello {{UserName}}</mj-text>
    <mj-button href="{{ActionUrl}}">Click Here</mj-button>
  </mj-column>
</mj-section>

Compiles to ~200 lines of table-based HTML with inline styles that works everywhere.

Installation

Add Mjml.Net

dotnet add package Mjml.Net

Embed Templates as Resources

In your .csproj:

<ItemGroup>
  <EmbeddedResource Include="Templates\**\*.mjml" />
</ItemGroup>

Project Structure

src/
  Infrastructure/
    MyApp.Infrastructure.Mailing/
      Templates/
        _Layout.mjml              # Shared layout (header, footer)
        UserInvitations/
          UserSignupInvitation.mjml
          InvitationExpired.mjml
        PasswordReset/
          PasswordReset.mjml
        Billing/
          PaymentReceipt.mjml
          RenewalReminder.mjml
      Mjml/
        IMjmlTemplateRenderer.cs
        MjmlTemplateRenderer.cs
        MjmlEmailMessage.cs
      Composers/
        IUserEmailComposer.cs
        UserEmailComposer.cs
      MyApp.Infrastructure.Mailing.csproj

Layout Template (_Layout.mjml)

<mjml>
  <mj-head>
    <mj-title>MyApp</mj-title>
    <mj-preview>{{PreviewText}}</mj-preview>
    <mj-attributes>
      <mj-all font-family="'Helvetica Neue', Helvetica, Arial, sans-serif" />
      <mj-text font-size="14px" color="#555555" line-height="20px" />
      <mj-section padding="20px" />
    </mj-attributes>
    <mj-style inline="inline">
      a { color: #2563eb; text-decoration: none; }
      a:hover { text-decoration: underline; }
    </mj-style>
  </mj-head>
  <mj-body background-color="#f3f4f6">
    <!-- Header -->
    <mj-section background-color="#ffffff" padding-bottom="0">
      <mj-column>
        <mj-image
          src="https://myapp.com/logo.png"
          alt="MyApp"
          width="150px"
          href="{{SiteUrl}}"
          padding="30px 25px 20px 25px" />
      </mj-column>
    </mj-section>

    <!-- Content injected here -->
    <mj-section background-color="#ffffff" padding-top="20px" padding-bottom="40px">
      <mj-column>
        {{Content}}
      </mj-column>
    </mj-section>

    <!-- Footer -->
    <mj-section background-color="#f9fafb" padding="20px 25px">
      <mj-column>
        <mj-text align="center" font-size="12px" color="#9ca3af">
          &copy; 2025 MyApp Inc. All rights reserved.
        </mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>

Content Template

<!-- UserInvitations/UserSignupInvitation.mjml -->
<!-- Wrapped in _Layout.mjml automatically -->

<mj-text font-size="16px" color="#111827" font-weight="600" padding-bottom="20px">
  You've been invited to join {{OrganizationName}}
</mj-text>

<mj-text padding-bottom="15px">
  Hi {{InviteeName}},
</mj-text>

<mj-text padding-bottom="15px">
  {{InviterName}} has invited you to join <strong>{{OrganizationName}}</strong>.
</mj-text>

<mj-text padding-bottom="25px">
  Click the button below to accept your invitation:
</mj-text>

<mj-button background-color="#2563eb" color="#ffffff" font-size="16px" href="{{InvitationLink}}">
  Accept Invitation
</mj-button>

<mj-text padding-top="25px" font-size="13px" color="#6b7280">
  This invitation expires on {{ExpirationDate}}.
</mj-text>

Template Renderer

public interface IMjmlTemplateRenderer
{
    Task<string> RenderTemplateAsync(
        string templateName,
        IReadOnlyDictionary<string, string> variables,
        CancellationToken ct = default);
}

public sealed partial class MjmlTemplateRenderer : IMjmlTemplateRenderer
{
    private readonly MjmlRenderer _mjmlRenderer = new();
    private readonly Assembly _assembly;
    private readonly string _siteUrl;

    public MjmlTemplateRenderer(IConfiguration config)
    {
        _assembly = typeof(MjmlTemplateRenderer).Assembly;
        _siteUrl = config["SiteUrl"] ?? "https://myapp.com";
    }

    public async Task<string> RenderTemplateAsync(
        string templateName,
        IReadOnlyDictionary<string, string> variables,
        CancellationToken ct = default)
    {
        // Load content template
        var contentMjml = await LoadTemplateAsync(templateName, ct);

        // Load layout and inject content
        var layoutMjml = await LoadTemplateAsync("_Layout", ct);
        var combinedMjml = layoutMjml.Replace("{{Content}}", contentMjml);

        // Merge variables (layout + template-specific)
        var allVariables = new Dictionary<string, string>
        {
            { "SiteUrl", _siteUrl }
        };
        foreach (var kvp in variables)
            allVariables[kvp.Key] = kvp.Value;

        // Substitute variables
        var processedMjml = SubstituteVariables(combinedMjml, allVariables);

        // Compile to HTML
        var result = await _mjmlRenderer.RenderAsync(processedMjml, null, ct);

        if (result.Errors.Any())
            throw new InvalidOperationException(
                $"MJML compilation failed: {string.Join(", ", result.Errors.Select(e => e.Error))}");

        return result.Html;
    }

    private async Task<string> LoadTemplateAsync(string templateName, CancellationToken ct)
    {
        var resourceName = $"MyApp.Infrastructure.Mailing.Templates.{templateName.Replace('/', '.')}.mjml";

        await using var stream = _assembly.GetManifestResourceStream(resourceName)
            ?? throw new FileNotFoundException($"Template '{templateName}' not found");

        using var reader = new StreamReader(stream);
        return await reader.