Plain text vs HTML in cold email

Plain text wins in cold email. A heavily formatted HTML message — images, buttons, banners, brand fonts — looks like a marketing broadcast to spam filters and to the person reading it, and both treat it accordingly. The closer your email looks to a one-to-one note typed by a human, the more likely it lands in the inbox and earns a reply.

Why does formatting change where your email lands?

Spam filters classify email partly by what it resembles. Filters at Google and Microsoft score the ratio of markup to visible text, count images and links, and match structural fingerprints against templates their users have already marked as spam. A message that arrives as several hundred lines of HTML wrapped around a hero image matches millions of promotional sends. A short text message with a single link matches the mail colleagues send each other every day. You want to be in the second category.

Formatting is one layer of a larger system — authentication, warm-up, volume discipline and list hygiene all sit underneath it, and I cover the full stack in Cold Email Deliverability: The Practical Guide. But formatting is the layer that costs nothing to fix and is the one most often got wrong.

Doesn't every sending tool use HTML anyway?

Technically, yes. Almost every sending platform transmits your message as HTML, or as multipart with an HTML part, even when the compose window looks bare. So "plain text" in practice means minimally formatted HTML that renders like plain text. The working rules:

• No images, logos or banners.
• No layout tables, columns or dividers.
• No custom fonts or brand colours — default type only.
• No buttons. If you need a link, write it as a linked phrase inside a sentence.
• One link in the body, at most two including the opt-out.
• A signature of name, role and company — text, not an image block.

If your email would look at home in a promotions tab, it is overbuilt.

What is the filter actually doing?

The mechanism, step by step. When your email arrives at a Google or Microsoft mailbox, the receiving server checks authentication first; when SPF, DKIM and DMARC pass, content scoring begins. When the filter sees a high markup-to-text ratio, remote images and multiple tracked links, it adds risk signals. When the structure matches a template other recipients have previously marked as spam, it adds more. And when your domain is new — say it has only just finished warm-up — the filter has little positive engagement history to offset those signals, so content is weighted more heavily. Plain formatting lowers your content risk at exactly the point where your reputation cannot yet absorb it. That is also why it pays to understand what warm-up tools do and when to stop them: a mailbox warmed to 91/100 can still be dragged down by a template that reads as broadcast.

Should you turn off open tracking?

For cold campaigns, usually yes. Open tracking works by inserting an invisible one-pixel image, which quietly converts your "plain" email into one that loads a remote image from a tracking domain — a mild spam signal on its own, and a stronger one on a fresh domain. Open rates are also commonly unreliable as a metric, because several mail clients pre-fetch images whether or not a human read anything.

We measure replies instead. A healthy campaign typically produces around a 4% positive reply rate; below 3%, we treat the campaign as broken and fix targeting or copy rather than staring at opens.

Where do links and opt-outs fit?

UK B2B cold email needs sender identification and a way to opt out — PECR requires both for corporate subscribers (this is a description of the rules, not legal advice). Keep the opt-out plain as well: a single human sentence such as "Tell me if this isn't relevant and I won't write again" typically works better than a formatted unsubscribe footer, with filters and with readers. I cover the mechanics in opt-outs in cold email: compliant without killing replies.

Formatting will not rescue a bad list, though. Wrong or dead addresses generate bounces and complaints however clean the markup is, which is why the data layer matters as much as the message — see the build log of an AI agent that builds and enriches a B2B database.

What do we actually send?

Our standard cold format: sequences of four emails over 14 days, 25–40 emails per day per inbox, each message plain-formatted, one link, text signature, plain-text opt-out. It is not glamorous. It is the format that lands, and landing is the whole game.

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