resend-email-lint

Lint email HTML before you send it. One rule set, three interfaces:

• a library for your code,

• a CLI for CI,

• an MCP server so an AI agent can check an email before sending it.

Email clients are unforgiving. Missing alt text, relative image URLs, javascript: links, <script> tags and messages over Gmail's ~102 KB clipping limit all fail silently. This tool catches them with clear messages and line numbers.

It works with any sender. It does not call any email API.

CLI

npx resend-email-lint ./emails

examples/bad.html
  1:1  warning  Missing <!DOCTYPE html>.  (missing-doctype)
  3:3  error    Image has no alt attribute. Add alt="" if it is decorative.  (img-alt)
  3:3  error    Image src "/logo.png" is relative. Use an absolute https:// URL or a cid: reference.  (img-src-relative)
  3:3  warning  Image has no width or height attribute. Outlook ignores CSS sizes and may render it at its natural size.  (img-dimensions)
  5:3  error    Link has an empty href.  (link-href)
  6:3  error    <script> is not supported in email and will be removed.  (no-script)

4 errors, 5 warnings

Exit codes: 0 clean, 1 lint errors (or too many warnings), 2 usage or I/O error.

Read from stdin with -: cat email.html | npx resend-email-lint -

React Email: lint the rendered HTML, not the .tsx. Render first (for example with email export, or render() from @react-email/render), then point the CLI at the output.

In CI

- run: npx resend-email-lint ./out/emails --max-warnings 0

Related MCP server: mail7-mcp

Library

import { lintHtml } from "resend-email-lint";

const { diagnostics, errorCount } = lintHtml(html, { marketing: false, ignore: ["html-lang"] });
if (errorCount > 0) throw new Error(diagnostics.map((d) => d.message).join("\n"));

Example with Resend, checking before you send:

import { Resend } from "resend";
import { lintHtml } from "resend-email-lint";

const resend = new Resend(process.env.RESEND_API_KEY);
const { diagnostics } = lintHtml(html);
if (diagnostics.length > 0) throw new Error(JSON.stringify(diagnostics));
await resend.emails.send({ from, to, subject, html });

This guard fails on warnings as well as errors. Some warnings matter at send time: html-size means Gmail will clip the message and hide the footer and unsubscribe link. To block only errors plus oversized HTML, filter instead:

const blocking = diagnostics.filter((d) => d.severity === "error" || d.rule === "html-size");

lintHtml is pure and synchronous: no I/O, no network.

MCP server

Add to your MCP client config (Claude Desktop, Cursor, and so on):

{
  "mcpServers": {
    "email-lint": {
      "command": "npx",
      "args": ["-y", "-p", "resend-email-lint", "resend-email-lint-mcp"]
    }
  }
}

Tools:

• lint_email_html: takes html (and optional marketing, ignore) and returns diagnostics with rule id, severity, line and column.

• list_rules: lists every rule.

Designed for agents as users. Tool descriptions say when to call them ("before sending"). Errors name the valid rule ids so an agent can self-correct. Diagnostics are structured JSON an agent can act on. The server is also deliberately pure: it has no filesystem, network or environment access, and caps input at 2 MB, so handing it to an agent adds no new permissions.

Rules

Document-level rules only run when the input contains an <html> tag, so fragments are not flagged.

Contributing

New rules are the easiest way in. See CONTRIBUTING.md.

License

MIT