You're probably not overthinking it. Decent open rates with low replies is a classic sign your emails are landing but reading like marketing — and HTML formatting is a likely contributor. The data points pretty clearly toward plain text for cold outreach, though it's worth knowing where that data comes from before treating any single number as gospel.
What the Data Shows — and Who's Saying It
Disclosure: All sources cited below are vendors or commercially sponsored content with a direct interest in recommending plain-text cold email. Treat their proprietary figures as directionally useful, not independently verified research.
Response Rates
The Growth List — sponsored by Sales.co, a cold email agency — analyzed 250,000 cold emails across 50 B2B companies and found plain text achieved an 88% higher response rate than HTML (7.9% vs. 4.2% total response rate). This is a single-source, single-sponsored-page figure; no neutral third party has independently verified it. That said, the directional finding is consistent with what other practitioners report.
Puzzle Inbox — a commercial cold-email sending platform — reports that plain text gets 15–25% more replies based on its own client campaigns, with plain text averaging 3.5–5% reply rates versus HTML's 2–3.5%. Note: the 15–25% improvement figure is the mathematical range derived directly from those reply-rate bands — it's the same data point expressed two ways, not two separate studies.
Inbox Placement
The Growth List's ESP analysis (Gmail, Outlook, Yahoo) found plain text emails hit the primary inbox 73% of the time versus 34% for HTML — a 115% improvement in inbox placement. The analysis does not name the specific ESPs studied or disclose its full methodology, so this figure should be read as a single sponsored data point rather than a peer-reviewed benchmark.
Click-Through Rates
The Growth List attributes a 42% higher CTR for plain text (4.1% vs. 2.8%) to HubSpot's analysis of 2.3 million emails — not to The Growth List's own 250,000-email dataset. That distinction matters: the response-rate figures are The Growth List's proprietary data; the CTR figure originates with HubSpot.
Warmforge.ai — an email warm-up tool vendor — independently reports CTR improvement as a range of 21%–42% and open-rate drops for HTML of 23%–37% when rich elements are used. These are Warmforge's own figures and do not directly confirm The Growth List's specific 42% point — they show the upper bound of Warmforge's range happens to match, but the lower bound is meaningfully different at 21%.
Why Plain Text Works: The Mechanics
The explanation isn't complicated. HTML cold emails carry technical baggage that spam filters are trained to recognize:
- Tracking pixels — invisible images that fire on open. Email providers, especially Google, treat these as a mass-email signal because person-to-person emails don't contain them.
- HTML weight — a plain text email is a few kilobytes; a styled HTML email with images and tracking code can be 20–50KB or more. Filters compare the ratio of visible text to hidden code.
- Link redirect wrappers — HTML emails often route links through tracking domains (tracking.platform.com → yoursite.com), which look identical to phishing redirects to a spam filter.
- Rendering inconsistencies — an email that looks polished in Gmail may break in Outlook, and a broken layout signals "mass email" to the recipient before they've read a word.
Beyond deliverability, there's a perception effect. Folderly — an email deliverability software vendor — puts it plainly: for cold outreach, you want to act like a human, not a commercial brand. HTML emails trigger a mental categorization as marketing material; plain text reads like a message from a colleague.
What to Actually Do
- Strip the template. No styled headers, no branded color blocks, no image banners. Write like you're sending from your personal inbox — because that's exactly what cold email is supposed to look like.
- Kill the tracking pixel. Disable open tracking in your sending platform. Reply rate is the metric that matters for cold outreach, and measuring it doesn't require any tracking code embedded in your email.
- Keep your signature simple. A plain-text signature with your name, title, company, and a URL is enough. Heavy HTML signatures with embedded logos reintroduce some of the same deliverability signals you're trying to avoid — they can partially offset the gains from sending plain text.
- Use bullets and line breaks, not design. Plain text supports bullet points, numbered lists, and white space for readability. You don't need a template to write a scannable email.
- Test it on your own list. The vendor figures above are directionally consistent, but your list, your offer, and your ICP are specific to you. Run an A/B split for 2–3 weeks and measure reply rate, not open rate.
The One Exception
If your product is inherently visual — design work, a UI-heavy SaaS, portfolio samples — there's a limited case for including a single image in a later follow-up (email 3 or 4, not the first touch). Even then, keep the HTML minimal: one image, no tracking pixel, no styled template. And test it against a pure plain text version. Most of the time, plain text still wins.