How does HTML cleanliness affect filtering?

Sloppy HTML signals low-quality sending, which correlates with spam in filter training data. Unclosed tags, deprecated elements, Microsoft Word-generated markup (notorious for bloated conditional comments), and malformed structures all raise suspicion. Legitimate senders typically use proper tools and tested templates; spammers often use whatever generates output fastest, resulting in messy code.

Specific technical issues that trigger scrutiny include excessive nested tables (beyond what email layout legitimately requires), hidden text or tiny fonts, suspicious CSS like text-indent to push content off-screen, and unusual character encoding. Filters have learned these patterns from years of spam that tried to hide content or evade text analysis. Clean, semantic, well-structured HTML is what reputable ESPs and brands produce.

Beyond filtering, clean HTML improves rendering reliability across email clients. Code that works despite being malformed in one client may break catastrophically in another. The same discipline that satisfies spam filters-valid structure, proper encoding, efficient markup-also produces emails that display correctly everywhere. Treat HTML quality as both a deliverability requirement and a professional standard-there's no scenario where sloppy code helps you.