What’s the impact of broken HTML on spam filters?
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Spam filters don't read your HTML the way a browser does. They parse it, and broken code that a browser forgives can trigger a spam signal in a mail server.
Why broken HTML matters to filters: spam and phishing emails are frequently produced by people who don't know or care about proper HTML. They're generating templates quickly, often through automated tools, and the code is often malformed. Over time, spam filters learned that malformed HTML is a correlated signal of low-quality or malicious senders. Your HTML doesn't have to be perfect to land in the inbox, but it shouldn't look like something generated carelessly either.
The issues that tend to matter most in email HTML specifically (it's different from web HTML):
Unclosed tags. An unclosed <table> or <td> is common and hurts rendering in email clients that parse strictly.
JavaScript or form elements. These don't work in email clients anyway, and their presence is a classic spam signal.
Excessive HTML comments. Sometimes used by spammers to hide text from content filters; having a lot of them in your email triggers pattern matching.
Broken image tags or missing alt text. Affects rendering and can look like an image-only email (a common spam pattern).
But the other problem: broken HTML hurts rendering, which hurts engagement. An email that looks broken in Outlook gets deleted. Low engagement signals hurt your sender reputation over time. So broken HTML creates a compounding problem.
Run your templates through an email-specific HTML validator before sending. Web validators won't catch email-specific issues. Our source analyzer tool can inspect your raw email source for common problems.
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