Plain-text email is a message sent as unformatted text, no HTML template, embedded images, or design elements, so it reads to both a filter and a recipient like a real one-to-one message instead of a marketing send.
Plain-text email skips the formatting entirely: no HTML template, no embedded logo, no multiple fonts or colors. It's the same visual format as a message one person typed to another, because that's exactly what cold email is trying to look like.
A message that's mostly image with little real text reads as low-effort bulk mail to a spam filter, and a long, heavily designed message starts to visually pattern-match to a newsletter or marketing template, a category filters treat differently than a one-to-one message. Attachments carry the most risk of all three, since they trigger extra scanning and pattern-match to phishing from a sender with no history yet.
None of this means HTML itself trips a switch inside a filter. The mechanism runs through pattern and behavior, not a literal formatting check, which is also why the personalization-vs-deliverability question gets answered the same way: it's not the format a filter scores directly, it's what that format signals about whether the message is a real one-to-one send or a bulk one.
Cold-outbound programs default new domains to plain text for exactly this reason: it looks the way a person's actual email looks. HTML templates, if a team uses them at all, tend to get saved for opted-in nurture sends where the relationship and the engagement history already exist.
People treat plain text as a rule a filter enforces directly, as if HTML tags themselves get penalized. The real mechanism is pattern-matching and behavior: a heavily designed message just looks more like a newsletter than a one-to-one email, and that's the category a filter is actually reacting to.
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