@emailens/mcp

Email rendering analysis for AI assistants

MCP server for email compatibility analysis. Analyze, preview, diff, and fix emails across 21 email clients, plus capture real screenshots and create shareable links with an optional API key.

Send HTML, MJML, Maizzle or React Email. Set format and the template is compiled before analysis, so what gets checked is the HTML your readers actually receive.

Why your assistant needs this: across the 298 CSS and HTML features we track, only 6 are fully supported in every major email client (see the data). Ask Claude to check your email before you send it.

Built on @emailens/engine. Also available as a GitHub Action.

Install

npx -y @emailens/mcp

Setup

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "emailens": {
      "command": "npx",
      "args": ["-y", "@emailens/mcp"]
    }
  }
}

Claude Code

claude mcp add emailens -- npx -y @emailens/mcp

With API Key (optional, unlocks screenshots + sharing)

{
  "mcpServers": {
    "emailens": {
      "command": "npx",
      "args": ["-y", "@emailens/mcp"],
      "env": {
        "EMAILENS_API_KEY": "ek_live_..."
      }
    }
  }
}

Get your free API key at app.emailens.dev/dashboard/api.

Remote (no install)

Use the hosted endpoint: no npm or Node.js needed. API key required.

{
  "mcpServers": {
    "emailens": {
      "url": "https://app.emailens.dev/api/mcp",
      "headers": {
        "Authorization": "Bearer ek_live_..."
      }
    }
  }
}

Tools

Local Tools (no account needed)

preview_email

Full email compatibility preview: transforms HTML for 21 clients, analyzes CSS, generates scores, simulates dark mode, checks inbox preview and email size.

analyze_email

Quick CSS compatibility analysis; returns per-client scores and one finding per problem. Faster than audit_email when you only need CSS compatibility.

One finding per problem, not one per client. The engine reports per client because a score is per client, and per selector because a fix is per selector. On an ordinary newsletter border-radius arrives twelve times (two clients that drop it, six selectors that use it) with the same sentence in every copy. That was 286KB of JSON, about 73,000 tokens, for an 11KB email.

Findings group by property, severity and message, listing the clients affected, with positions merged across all of them. The same email now returns 40KB.

{
  "property": "border-radius",
  "severity": "warning",
  "clients": ["outlook-windows", "outlook-windows-legacy"],
  "message": "Does not support \"border-radius\". Round corners can be used in VML…",
  "fixType": "structural",
  "hasFix": true,
  "loc": { "line": 61, "column": 28, "offset": 2753, "length": 171 },
  "alsoAtLines": [62, 64, 75, 78, 87]
}

Fix snippets are not included; they were 93KB of that 286KB, and fix_email produces them for the issues you decide to act on. hasFix tells you one is available; fixType tells you whether the repair is markup or CSS.

Pass detail: "full" for the engine's per-client shape with snippets, and clients: ["gmail-web", "outlook-windows"] to report only what you care about: the fastest further saving, roughly halving the response for two clients. Scores stay whole-email either way: narrowing the report does not change what the email is worth elsewhere. An unknown client ID is rejected by name; an empty result would read as "this email is fine for that client".

Source positions. For HTML input, every warning carries loc (line, column, offset, length) for the first occurrence, plus alsoAtLines for any others, so an assistant can edit the exact source and knows where the rest are. audit_email positions its other findings the same way.

{
  "property": "border-radius",
  "loc": { "line": 7, "column": 8, "offset": 142, "length": 25 },
  "alsoAtLines": [12, 19]
}

Later occurrences are line numbers rather than full positions on purpose: this response is read by a model paying for every token, and a real newsletter can produce over a thousand occurrences; carrying them all in full nearly doubles the payload.

Positions are reported for html input only. JSX, MJML and Maizzle are compiled before analysis, so a line number would point into generated output rather than the file you have open: the tools omit it instead of returning one that looks authoritative.

audit_email

Comprehensive quality audit: CSS compatibility, spam scoring, link validation, accessibility, images, inbox preview, size (Gmail clipping), template variables, content overflow, visual bugs, dark-mode and mobile text contrast, design consistency, and client targeting.

The last three cover what a light desktop preview cannot show: text that disappears when a client forces dark mode or when the email's own dark block repaints a surface without re-colouring the text on it, contrast below the email's breakpoint, and colours that differ by value but not to a reader.

Fix error and warning findings. info findings are not problems: the email falls back safely there, so never remove responsive (@media) or dark-mode CSS to clear them.

fix_email

Generate a structured fix prompt for compatibility issues. Returns markdown with fix instructions that the AI can apply directly.

list_clients

List all 21 supported email clients with IDs, names, engines, and dark mode support.

diff_emails

Compare two email HTML versions; shows score changes, fixed issues, and introduced issues per client.

check_deliverability

Check email deliverability for a domain: SPF, DKIM, DMARC, MX, BIMI records with a score and actionable issues.

check_design

Whether the email still looks like its design in each client's live preview. It draws the email in a real browser twice: as the client shows it, and as you wrote it. Then it compares the two layouts element by element. Each client gets a verdict, kept (Ready), changed (Needs work) or broken (Broken), with the reasons: columns that stack, a button that moved or resized, a fill or corners lost, fonts and spacing changed. This is the verdict the Emailens email page shows. An agent can call it after an edit to confirm the fix didn't break the layout somewhere else.

{ "broken": ["outlook-windows-legacy"],
  "results": [{ "clientId": "outlook-windows-legacy", "name": "Outlook Classic",
    "verdict": "broken", "label": "Broken",
    "sentence": "Outlook Classic breaks your design.",
    "reasons": ["Columns that sit side by side in your design stack on top of each other."],
    "more": 0 }] }

It needs a browser. Add playwright-core to the server, and sharp for the pixel comparison. Then either install a local Chromium or point it at your Browserless instance:

{
  "mcpServers": {
    "emailens": {
      "command": "npx",
      "args": ["-y", "-p", "@emailens/mcp", "-p", "playwright-core", "-p", "sharp", "emailens-mcp"],
      "env": { "BROWSERLESS_URL": "wss://..." }
    }
  }
}

Without BROWSERLESS_URL, it uses a local Chromium. Install one with npx playwright-core install chromium. A few more things to know:

• It runs in light mode, against our live preview, not real inboxes.
• With sharp, it also compares screenshots of the two pages pixel by pixel, as the hosted check does. That agrees more often with a human reviewer, and each result carries diff (the share of pixels off) and aligned. Without sharp, it compares page structure only.
• On Browserless, the session has to stay open for the whole call. Raise a short TIMEOUT, or pass fewer clients.
• Scripts are off in both pages.
• The font stand-ins the previews use are downloaded once from app.emailens.dev into ~/.cache/emailens/fonts.

Hosted Tools (require EMAILENS_API_KEY)

capture_screenshots

Capture real email screenshots across 21 clients in real browsers. Screenshots are hosted on CDN.

Free plan: 30 previews/day. Sign up

share_preview

Create a shareable link. Recipients see the full analysis without an account.

Requires Dev plan ($9/mo). Share links expire after 7 days (Dev) or never (Pro).

Template formats

format accepts html (the default), mjml, maizzle and jsx (React Email). maizzle is an HTML template or a Vue single-file component. A pasted Vue file with no format is compiled as maizzle. Anything but html is compiled before analysis and also decides the syntax the fix snippets come back in.

Compiling matters more than it sounds. An email client renders the output, so that is what has to be checked: handed a raw <mjml> document, an HTML parser finds no CSS in it and reports a perfectly clean email. An answer like that is worse than an error, because an assistant will repeat it.

The compilers are not bundled. MJML alone pulls 56MB, and this server is usually started with npx, so the engine keeps them as optional peer dependencies:

npm install mjml                                              # MJML
npm install @maizzle/framework@5                             # Maizzle HTML
npm install @maizzle/framework@6                             # Maizzle Vue single-file components
npm install sucrase react @react-email/components @react-email/render  # React Email

They have to be installed where the server runs, which is not always somewhere you control. If it is not, compile the template yourself and send the resulting HTML with format: "html": the tools say so when they hit this.

Supported Email Clients (21)

Each client links to its CSS support page. Setup guides for every assistant: emailens.dev/mcp.

Releasing

Two publishes: npm, and the MCP registry. The registry listing sat five releases behind because nothing pushed server.json, so that half is a workflow now. RELEASING.md has the details and the four places the version has to agree.

Development

bun install
bun run build
bun test
bun run typecheck

License

MIT

If this saved you from an Outlook surprise, a star helps other email developers find it.