Plain text versus HTML cold email tested on 25000 sends. The reply rate gap was not even close and the reason explains itself
plaintext_wins_deb · 2026-07-20 · 1,380 views
I know this debate never fully dies, so I ran actual numbers. 25,000 emails. Same ICP. Same subject line. Same body copy. Half went as plain text. Half went as a lightly styled HTML email with one bolded phrase and a clean single-column layout. No tracking pixels. No images. Just light formatting.
Plain text pulled 3.9 percent reply rate. HTML pulled 2.4 percent.
That is a 62 percent difference in replies on the same campaign. The gap surprised me even though I had expected plain text to win.
Why this is not actually surprising once you think about it.
HTML emails are how marketing departments send newsletters. They are how SaaS companies send product announcements. Every prospect who gets 50 cold emails a week has learned, mostly unconsciously, that formatted emails with styled text are campaigns. Plain text looks like something a real person typed and sent from their actual email client.
The prospect's pattern recognition is working against HTML before they read a single word. The formatting is the tell.
The deliverability angle.
HTML emails carry more code. More code means more surface area for spam filter analysis. Google's filters are not just looking at content. They are looking at email structure, header patterns, and code complexity. Plain text has almost no surface area to trigger on. A plain text email from a properly warmed PuzzleInbox Google Workspace inbox looks, structurally, like a real conversation between two professionals.
I ran GlockApps placement tests across both variants. Plain text averaged 91 percent inbox placement. HTML averaged 81 percent on the same sending infrastructure. The deliverability gap compounded the reply rate gap.
One exception I have found.
If you are emailing a marketing or design ICP, light HTML formatting performs closer to plain text. These buyers associate plain text with mass outreach more than they do with real correspondence. They send HTML emails themselves. Their pattern recognition is flipped. For every other ICP, plain text wins.
Drop the formatting. Write like a person. Send from Instantly or Smartlead with HTML disabled in the sequence settings. Let your copy and your subject line carry the email. The format is not helping you.