A free email HTML checker that runs entirely in your browser.

Paste your HTML or open a file and Stampcheck runs 80+ checks against real email client quirks, with live previews for Gmail, Outlook and Apple Mail on desktop and phone sizes. Every issue is sorted into "will break", "may look wrong" and "tips", points to the exact line that causes it, and explains which clients are affected.

Typical catches: Outlook ignoring flexbox and grid, buttons losing their shape, layouts wider than a phone, Gmail clipping, missing preview text, dead links, missing alt text.

Nothing you paste is uploaded. The checker is free; an optional paid fix corrects the issues and re-checks the result.