Plain text vs HTML email for cold outreach: what the data says

Most cold email templates look like marketing emails.

Branded headers. Company logos. Colored buttons. Social media icons in the footer. The sender spent an hour in Canva making it look polished.

It lands in spam.

The format you choose for a cold email is not a design decision. It is a deliverability decision. And the data is clear: HTML formatting signals to spam filters that your email is bulk marketing, not a personal message. Plain text signals the opposite.

Here is what the research shows and why it matters for cold outreach specifically.

What spam filters actually look at

Email service providers — Gmail, Outlook, Apple Mail — filter incoming messages based on signals that distinguish personal email from bulk marketing. HTML formatting is one of those signals.

A cold email with a branded header, an image, and a button CTA looks structurally identical to a promotional newsletter. The spam filter cannot read your intent. It reads your code. HTML with embedded images and multiple links pattern-matches to bulk marketing. Plain text pattern-matches to a personal message between two people.

This matters at scale. HTML email bounce rates run 652% higher than plain text. That is not a marginal difference — it reflects emails that never reach the inbox at all.

The open rate gap

Plain text cold emails achieve a 42% average open rate compared to 23% for HTML templates — a near 2x difference driven primarily by inbox placement.

You cannot open an email you never received. The open rate gap between plain text and HTML is mostly a deliverability gap, not a subject line gap. HTML emails that do land in the inbox perform reasonably well on opens. The problem is that a meaningful share never gets there.

Images alone drop open rates by 25%. Emails with GIFs see a 37% drop in opens compared to plain text. Each visual element you add is a spam signal stacking on top of the previous one.

The reply rate gap

Open rates are one thing. Reply rates are what you are actually optimizing for.

Plain text cold emails get 15–25% more replies than HTML emails. Some studies show a 2–3x difference in reply rate between plain text and heavily designed HTML for cold outreach specifically.

The mechanism is not just deliverability. It is also perception.

A cold email with a company logo and a "Book a Meeting" button reads as a sales campaign. The recipient knows immediately that this email was sent to many people, not written for them. That recognition kills reply intent before they read a word.

A plain text email reads as a personal message. There is no visual signal that it came from a template. The recipient processes it as 1:1 communication, which is how it should feel.

HubSpot tested plain text against HTML across multiple sends and found a 21% boost in click-through rates for plain text — and that is for marketing emails where HTML is far more appropriate than cold outreach.

What "plain text" actually means

There is a common misconception that plain text means your email has to look bad or be harder to write. It means the opposite of HTML — no images, no buttons, no branded headers, no colored fonts.

What plain text looks like in practice:

Hi Sarah,

Saw you just hired a new VP of Revenue from Salesloft.

We helped a Series B team at your stage cut their cost per booked meeting by 40% while tripling reply rates.

Worth sending over the breakdown of how they did it, or is the timing off?

— Adam

That is it. No logo. No button. No footer with your company address and an unsubscribe link. It reads like a message from a person, because that is what it should be.

The only formatting cold email allows without triggering spam filters: bold text (sparingly), line breaks between paragraphs, and a plain signature with your name and title.

When HTML is appropriate

HTML is not the wrong choice in every context. It is the wrong choice for cold outreach specifically.

For cold email, your goal is to look like a 1:1 personal message. HTML is the opposite of that.

Where HTML belongs:

• Transactional emails (receipts, password resets, shipping notifications)
• Newsletters your subscribers opted into
• Product announcement emails to existing customers
• Re-engagement campaigns to your own list

In all of these cases, the recipient expects a formatted email. They signed up for a list or made a transaction. The expectation is a polished, branded experience.

For cold outreach — emailing people who did not ask to hear from you — branded HTML reads as presumptuous. You are imposing a marketing campaign on someone who has no relationship with you. Plain text is the only format that respects that context.

The multipart MIME compromise

If you are sending at volume and want insurance, the best practice is to send emails in multipart MIME format — a technical setting that includes both a plain text version and an HTML version in the same send.

The email client renders whichever version is appropriate. Gmail shows the HTML version to users who have images enabled. Email clients that strip HTML show the plain text version. Spam filters that look for a plain text alternative find one.

This does not mean you should build elaborate HTML designs. The HTML version in a multipart MIME cold email should be minimal — the same plain text content with basic structural markup. The point is to provide the fallback, not to build a campaign template.

Most serious cold email platforms (Instantly, Smartlead, Woodpecker) send in multipart MIME by default. If you are building sequences manually or using a newer platform, verify this is set.

The practical test

Before sending any cold email template, ask three questions:

1. Could this pass as an email from a friend? If someone covered the sender name and showed you the email, would it look like a personal message or a marketing campaign?

2. Does it have images or buttons? Each one is a spam trigger. If the answer is yes, remove them.

3. Does it have a branded footer? Unsubscribe links and company addresses are required for marketing emails under CAN-SPAM. For cold email, they signal mass sending. Use a plain signature instead — name, title, phone number.

If you answer yes to question 2 or 3, the template is HTML in disguise. Rewrite it in plain text.

The underlying logic

Spam filters are adversarial. They are designed to catch bulk marketing trying to look like personal email. Every HTML element you add makes that job easier.

Plain text does not guarantee inbox placement — your sender reputation, authentication setup, and list quality matter more than format. Of those, list quality is where ai lead generation moves the needle most. But format is the one variable you can fix in five minutes. Every HTML element you remove is a signal you are not sending in bulk.

The data consistently shows plain text outperforms HTML on every metric that matters for cold outreach: inbox placement, open rate, and reply rate. The reasons are structural — format determines how spam filters classify your email and how recipients perceive your intent.

For cold outreach, the format choice is not a brand decision. It is a deliverability decision. And the right answer is plain text.