How it works
- 1
Paste your email HTML
Copy the full HTML from your email builder, template file, or ESP editor and paste it in. Analysis runs instantly in your browser — nothing is uploaded.
- 2
Review the pre-flight report
A 0–100 score summarizes four check families: client compatibility (against caniemail data for Gmail, Outlook, Apple Mail, Yahoo), spam-trigger signals, structure/accessibility, and link problems.
- 3
Fix blockers and preview dark mode
Each finding explains what breaks and where. The preview tab shows an approximate render including a simulated dark-mode pass — the thing most senders never test.
Clean HTML is half the battle — authentication is the other half
Check your domain's SPF, DKIM, and DMARC with DNS Email Diagnostics, then get alerted when records break with Deliverability Monitoring.
🛡️ Verify zero uploads — open DevTools → Network tab
Open your browser's DevTools (F12), go to the Network tab, and use this tool. You will see zero outbound requests — all processing runs inside your browser sandbox via WebAssembly or pure JavaScript. Nothing you paste or upload is ever sent anywhere.
Use cases
Catch Outlook breakage before the campaign goes out
Flexbox, grid, border-radius, background images — Outlook on Windows ignores more CSS than any other client. The compatibility table shows exactly which of your features die there.
Sanity-check spam signals on cold sequences
Trigger phrases, ALL-CAPS ratio, URL shorteners, image-only bodies, and missing unsubscribe links all raise filter scores. Fix them before the send, not after the open-rate craters.
Audit templates you inherited
Paste a legacy template and get an instant inventory of risky CSS, tracking pixels, http:// assets, and accessibility gaps — faster than reading 800 lines of table soup.
Common mistakes
2 pitfalls to avoid
▾
Common mistakes
2 pitfalls to avoid
1Testing only in the client you use
✗ Wrong: Your Apple Mail or browser preview renders almost everything, so the template looks fine — but your recipients' Outlook renders almost none of the modern CSS you relied on.
✓ Fix: Run the compatibility check and read the table for the clients your audience actually uses; treat Outlook (Windows) as the lowest common denominator.
2Relying on div + flexbox layouts
✗ Wrong: Building email layout with divs, flexbox, or grid — modern CSS layout dies in Outlook on Windows and gets partial treatment in several webmail clients.
✓ Fix: Use tables with role="presentation" for layout. It's 2005 forever in email — tables remain the only layout method that renders everywhere.
Frequently Asked Questions
Is my email HTML uploaded anywhere?
No. All checks run in your browser with JavaScript — the HTML never leaves your device. There are no network requests during analysis.
How accurate is the client-compatibility check?
It detects which CSS and HTML features your markup actually uses and cross-references each against caniemail.com community test data for Gmail (web and Android), Outlook (Windows and Outlook.com), Apple Mail, and Yahoo Mail. It tells you what is unsupported where — it does not render screenshots in real clients the way Litmus does.
How is the spam score calculated?
Heuristics modeled on common filter signals: known trigger phrases, uppercase ratio, repeated exclamation marks, image-to-text ratio, link volume, URL shorteners, and a missing unsubscribe link. A clean score here doesn't guarantee inbox placement — sender reputation and authentication (SPF/DKIM/DMARC) matter more.
What does the dark-mode preview simulate?
A Gmail-style dark pass: dark canvas, light text, and white backgrounds dimmed. Real clients each invert differently (Outlook is more aggressive), so treat it as a smoke test — if it looks broken here, it is definitely broken somewhere.
Why does it flag relative URLs?
Email has no base URL. A link or image like /images/logo.png resolves against nothing and is broken in every client — one of the most common template mistakes when copying from a website.
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