Free Email Signature Checker — Test Your HTML in 7 Email Clients
Paste the HTML of your current signature and we tell you what happens to it in Gmail web, Gmail Android, Gmail iOS, Outlook 365, Outlook desktop, Apple Mail and iOS Mail. It is free, we ask for no email address and we do not store your signature.
What it is for
Almost every broken signature fails for the same reasons: layout without tables, base64 images Gmail refuses to show, style sheets the client throws away, or images with no dimensions. Seeing the problem named, with its fix, saves you the cycle of emailing test messages to yourself.
The 13 rules it applies
The checker uses the same engine as SignatureForever's public MCP server. Each rule is tied to a behaviour documented in the compatibility matrix:
- Layout built with <div> instead of <table>.
- Use of display:flex or display:grid.
- A <style> block instead of inline styles.
- Background images via background-image.
- Images embedded as base64.
- Images without width and height attributes.
- Images without alternative text (alt).
- Images without display:block, which leave a gap underneath.
- Tables without border-collapse.
- Tables without mso-table-lspace and mso-table-rspace for Outlook.
- Web fonts that no major client loads.
- border-radius, which Outlook desktop ignores.
- Capitals applied with text-transform.
Why a signature breaks
It is almost never bad luck. A signature with base64 images looks fine in Outlook and arrives with broken boxes in Gmail, which does not show them. A signature laid out with display:flex falls apart in Outlook for Windows, which draws email with the Word engine and stacks the columns.
External images are the other front: they have to live at a public, fixed https address. And width matters: Outlook desktop ignores max-width and percentages, so the outer table needs a pixel width, 600 at most.
How to read the result
At the top you get a verdict per client: “breaks”, “renders, with” some warnings, or “clean”. Below, each problem has its level —breaking problem, warning or imperfection—, the clients where it fails, what happens and how to fix it.
Fix what breaks first, because that is what the other person sees as a damaged signature. Warnings and imperfections degrade without breaking anything; fix them once the signature already works in all seven clients.
This checker and the signature checker are two tools
Here the analysis runs on our server against the seven clients in the matrix: Gmail web, Gmail Android, Gmail iOS, Outlook 365, Outlook desktop, Apple Mail and iOS Mail. You see the full report without leaving your email address. The other checker runs inside your browser, evaluates a different set of clients that includes Yahoo Mail and Thunderbird, and adds checks such as SVG, dark mode or Gmail's size clipping.
Frequently asked questions
Do you store the signature I paste?
No. We check it and send back the report; we don't store your HTML or ask for your email.
Can I check a signature made with another generator?
Yes. Any signature HTML works, wherever it came from, including the one you copy from your email client.
It says “breaks” in Outlook. Now what?
Follow the fix for each problem or rebuild the signature in the editor, which exports tables and inline styles designed for Outlook.
How much does it cost?
Nothing. It's free, with no signup and no limit on checks.
Are you an AI agent? This is also available as an MCP server at signatureforever.com/mcp