How do bad HTML practices trigger filters?

Bad HTML practices signal either incompetence or deliberate manipulation. Filters flag both because spammers frequently use broken or deceptive HTML.

Broken tags and malformed structure suggest the sender lacks professionalism or is trying to confuse parsers. Hidden text (white on white, tiny fonts, display:none) is a classic spam technique to stuff keywords invisibly.

Embedded scripts are almost always stripped or blocked. JavaScript in email is a security risk, and its presence adds spam points. External resources loaded from suspicious domains also trigger flags.

Excessive nesting and complex table structures can resemble obfuscation attempts. While some complexity is necessary for responsive design, extreme nesting raises suspicion.

Using clean, tested HTML templates from reputable sources avoids these problems. Test rendering across clients and run content through spam checkers before sending.

Sloppy construction draws inspector scrutiny. Build your message with care and craftsmanship.