Plain-Text vs. HTML Cold Email: What Actually Changes for Deliverability

"Keep cold email plain text, no images, no attachments" shows up in nearly every cold outreach guide, usually with no explanation beyond "it looks more personal." That's true, but it's not the whole reason, and treating it as a style preference rather than a set of specific mechanisms means people follow the advice halfway and wonder why it didn't help.

What HTML actually costs you

• Image-to-text ratio: a message that's mostly one large image with little surrounding text matches a pattern spam filters weight heavily, regardless of what the image shows
• Tracking pixels: an open-tracking pixel is a hidden image request to a third-party domain - often the same handful of tracking domains used across an ESP's entire customer base, so the domain's reputation is only as good as the worst sender using it that week
• Link-tracking domains: a redirect through your ESP's shared click-tracking domain carries that domain's aggregate reputation into your message, for better or worse, and you don't control which direction it's currently pointed
• Pasted-from-Word formatting: HTML copied out of Word or Outlook drags in bloated inline styles and conditional comments that inflate the message and occasionally trip content filters looking for exactly that signature

What attachments actually cost you

Any attachment on an unsolicited first message gets extra scrutiny by design - antivirus and malware scanning adds latency, and some filters treat an attachment from an unknown sender as suspicious on its own, independent of what the file contains. A clean PDF case study attached to a cold email is judged by the same suspicion as a genuinely malicious one, because the filter can't know the difference in advance. Link to a hosted version instead.

Where the advice stops being useful

None of this means plain text is magic. A plain-text email with a spam-trigger-stuffed subject line and a bare, unbranded redirect link fails for the same underlying reasons an HTML one does - the format didn't cause the problem, the content and the tracking infrastructure did. Switching a bad campaign from HTML to plain text without touching either usually moves the needle very little.

Plain text removes several specific risk factors - tracking pixels, shared redirect domains, image-heavy layouts. It doesn't remove the risk of a subject line that reads like a template, or a first line that's obviously a mail-merge.

Check a subject line or body for common spam triggers

A working default for cold outreach

• Skip open-tracking pixels on cold sends - they contribute little reliable signal at this stage and cost more in deliverability than the data is usually worth
• If click tracking matters, route it through your own sending domain rather than a shared ESP tracking domain, so the reputation is entirely yours
• One link maximum in a first-touch message
• No attachments, ever, on a cold first message - link out instead
• Minimal HTML is fine if you use it - a short message with normal formatting doesn't trigger the same signals a heavy, image-led layout does