The rendering linter for email, in your terminal

CLI tool for email compatibility analysis — preview how HTML emails render across 15 email clients (Gmail, Outlook, Apple Mail, Yahoo, Samsung, Thunderbird, HEY, Superhuman).

Across the 251 CSS and HTML features we track, only 6 are fully supported in every major client (see the data). This tool catches the other 245 before your users do.

Prefer AI? Use the MCP server — same engine, works with Claude, Cursor, and any MCP client.

Install

npm install -g @emailens/cli

Or use with npx:

npx @emailens/cli analyze email.html

Commands

emailens analyze <file>

Analyze CSS compatibility and get per-client scores.

emailens analyze email.html
emailens analyze email.html --clients gmail-web,outlook-windows
emailens analyze email.html --json
cat email.html | emailens analyze -

emailens preview <file>

Full preview pipeline: transforms, analysis, dark mode simulation, and optional screenshots.

emailens preview email.html
emailens preview email.html --dark-mode
emailens preview email.html --screenshots --out ./screenshots
emailens preview email.html --json

emailens export <file>

Export a self-contained HTML or JSON report.

emailens export email.html -o ./report
emailens export email.html --json -o ./report
emailens export email.html --dark-mode --screenshots -o ./report

emailens fix <file>

Generate AI-powered fixes for email compatibility issues. Uses @emailens/engine analysis to build a structured prompt, then calls Claude to fix structural issues (table layouts, VML, MSO conditionals) that static snippets can't handle.

Requires ANTHROPIC_API_KEY environment variable and the optional @anthropic-ai/sdk dependency.

emailens fix email.html                           # Fix and print to stdout
emailens fix email.html -o fixed.html             # Write to file
emailens fix email.html --estimate                 # Show token estimate only (no AI call)
emailens fix email.html --clients outlook-windows  # Scope to one client
emailens fix email.html --json                     # Full JSON output with metadata
emailens fix email.html --max-tokens 8000          # Limit prompt size
cat email.html | emailens fix - --format jsx       # Pipe from stdin

Flag	Alias	Description
--format	-f	Input format: html, jsx, mjml, maizzle
--clients	-c	Comma-separated client IDs to scope the fix
--output	-o	Write fixed code to file instead of stdout
--json		Output as JSON (includes token estimates and metadata)
--quiet	-q	Suppress spinners and decorations
--estimate		Only show token estimate without calling the AI
--max-tokens		Maximum input tokens for the prompt (default: 16000)

emailens lint <file|glob>

CI/CD-friendly linting with structured exit codes. Flattens all audit checks (compatibility, content hygiene, links, accessibility, images, inbox preview, size, template variables) into a unified issue list.

emailens lint email.html
emailens lint src/*.html
emailens lint email.html --json
emailens lint email.html --fail-on-warning
emailens lint email.html --skip spam,links
emailens lint email.html --max-warnings 5

Flag	Alias	Description
--format	-f	Input format: html, jsx, mjml, maizzle
--json		Output as JSON
--fail-on-warning		Exit 2 if warnings found
--skip		Comma-separated checks to skip: spam,links,accessibility,images,compatibility,inboxPreview,size,templateVariables
--max-warnings		Fail if more than n warnings

Exit codes:

• 0 — clean
• 1 — errors found
• 2 — warnings only (with --fail-on-warning or --max-warnings exceeded)

Output format:

src/emails/welcome.html
  error  outlook-windows     border-radius           Not supported in Outlook Windows
  warn   spam                caps-ratio              20%+ of words are ALL CAPS

src/emails/newsletter.html
  pass   No issues found

2 files | 1 error | 1 warning

CI / GitHub Actions

Drop this into .github/workflows/email-lint.yml to fail PRs that introduce broken email CSS, spam triggers, or accessibility regressions:

name: Email lint

on:
  pull_request:
    paths:
      - 'emails/**'
      - 'src/emails/**'

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 22
      - name: Lint emails
        run: npx -y @emailens/cli lint 'emails/**/*.{html,tsx,mjml}' --fail-on-warning

For React Email / MJML / Maizzle source files, the CLI auto-detects the format from the extension. Want full preview reports (with screenshots and shareable links) on every PR? Use the Emailens GitHub Action instead — it wraps the same engine.

emailens clients

List all 15 supported email clients.

emailens clients
emailens clients --json

Options

All file-processing commands share:

Flag	Alias	Description
--format	-f	Input format: html, jsx, mjml, maizzle
--clients	-c	Comma-separated client IDs to filter
--json		Output JSON instead of terminal table
--quiet	-q	Suppress spinners and decorations

Preview and export add:

Flag	Alias	Description
--dark-mode	-d	Include dark mode simulation
--screenshots		Capture screenshots (requires BROWSERLESS_URL)
--out	-o	Output directory

AI Fixes

The fix command requires an ANTHROPIC_API_KEY environment variable and the @anthropic-ai/sdk package:

npm install @anthropic-ai/sdk
export ANTHROPIC_API_KEY=sk-ant-...

Use --estimate to check token usage before making an API call:

emailens fix email.html --estimate
#   Input tokens:    ~4,200
#   Output tokens:   ~5,400
#   Warnings:        23 (5 structural)

Framework Support

The CLI can compile React Email (JSX/TSX), MJML, and Maizzle templates to HTML before analysis. Format is auto-detected from file extension, or specify with --format.

emailens analyze newsletter.tsx              # Auto-detected as JSX
emailens analyze template.mjml               # Auto-detected as MJML
emailens preview email.html --format maizzle # Explicit format

Framework compilers are optional peer dependencies — install only what you need:

npm install sucrase react @react-email/components @react-email/render  # For JSX
npm install mjml                                                        # For MJML
npm install @maizzle/framework                                          # For Maizzle

Screenshots

Screenshots require a Browserless instance and playwright-core:

npm install playwright-core
export BROWSERLESS_URL=ws://localhost:3000
emailens preview email.html --screenshots --out ./screenshots

Piping

Read from stdin with -:

cat email.html | emailens analyze -
echo '<html><body>Hello</body></html>' | emailens preview - --json

License

MIT

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