Plain Text vs HTML Cold Email

Websites: Mailchimp Instantly

Choose the email format by what the recipient needs to read, then evaluate replies in your own audience. Plain text keeps the message visually simple. HTML can add structure, links, and images, but needs rendering and accessibility checks. A multipart message can include both representations of the same content; that is different from testing two versions with separate recipients.✓ 123

• For a short, personal note: start with a plain-text draft and make every sentence work without visual design.✓ 3
• For a message that needs visual structure: use restrained HTML, with real text and clear headings.✓ 3
• For a format comparison: assign distinct versions to comparable recipients and assess replies using the same rule.✓ 45

1. The actual choice is message design

Plain text and HTML are ways to represent the message, not two established levels of sales performance. Mailchimp describes plain-text email as lacking images, rich-text formatting, and embedded links. That makes the drafting constraint visible: if the message depends on a visual panel or button to be understood, it will need a different presentation in plain text. The limitation is also useful as an editorial test. Can the recipient understand the offer, reason for contact, and requested next step from the words alone?✓ 3

HTML gives the author headings and visual layout. Those options add production work. Mailchimp says programs render HTML differently and recommends testing in different email programs. Its HTML guidance also calls for absolute paths when embedding images or linking to files. If the design relies on an image, broken asset paths or altered rendering can change what a recipient sees. These are implementation risks to inspect, not evidence that HTML produces fewer replies.✓ 2

The table is a production checklist, not a performance ranking. A short note may be easiest to maintain as text. A message that genuinely needs structured explanation may justify HTML. Both can fail when the writing is unclear. Decide what information the recipient needs before choosing a template.

2. Multipart email is not a format experiment

RFC 2046 defines multipart/alternative as body parts that carry alternative versions of the same information. It says systems should treat those parts as interchangeable and arrange them in increasing faithfulness to the original content, with the preferred format last. A receiving agent should display the last format it supports. The mechanism gives a recipient's program a compatible representation of one message; it does not place recipients into competing test groups.✓ 1

Mailchimp explains the practical version: multipart/alternative sends both HTML and plain-text versions, and the recipient's program determines which one to display. Its guidance recommends including a plain-text alternative with an HTML message. If you use that setup, inspect both bodies. Make sure the text version preserves the core meaning and the requested action. Treat the two parts as equivalent copies, because the MIME standard describes them as versions of the same information.✓ 12

This matters when someone says they have tested plain text against HTML. Sending one multipart message does not, by itself, tell you which representation each person saw or how format changed replies. To estimate that effect, you need two assigned treatments and a consistent outcome definition. The receiver's format choice in multipart delivery and the sender's treatment assignment in an experiment answer different questions.✓ 14

3. HTML needs an accessibility pass

If HTML adds value, inspect the reading order first. Mailchimp warns that multi-column templates can resize on mobile devices or screen readers and put content out of the intended order. Read the message without its visual grid. Does the reason for contact still precede the request? Is the context near the claim it explains? A screenshot alone cannot answer that because it does not show the order experienced by every reader.✓ 3

Use headings to mark sections rather than relying only on bold text or color. Mailchimp specifically recommends HTML heading elements for information hierarchy. Keep the most critical information in text and use images to complement it. Where an image carries meaning, write concise descriptive alt text. For links, use wording that tells readers where a link goes. These checks are most valuable when the design contains several sections or a visual call to action.✓ 3

Then test the rendered message in more than one email program. Mailchimp's HTML documentation says programs render differently. Confirm the subject, sender identity, first line, headings, links, and requested action in the delivered copy. Check the plain-text alternative too, since Mailchimp says a recipient's program selects the displayed format. A visually polished draft is still unfinished until both versions convey the intended meaning.✓ 2

4. Evaluate replies without changing the whole message

Start by writing one concise message. Keep the audience, sender, subject, reason for contact, request, and follow-up schedule the same in both treatments. Change only the presentation needed to compare plain text with HTML. This is a proposed local test design, not a published benchmark. Mailchimp's A/B documentation uses a single variable type and assigns combinations at random. It also prevents a subscribed contact from receiving more than one combination in the test. Those design principles help isolate the format question even when your sending system differs.✓ 4

Before sending, define the outcome. Count human replies within one stated observation window, then separately label replies that show real interest or result in a meeting. Instantly's variant analytics distinguish sent messages, replies, positive replies, opportunities, and meetings booked. Decide which of those measures answers your business question before looking at results. A reply count alone may include refusals, questions, and other responses that have different value to the team.✓ 5

Record the assigned recipients and sent counts for both treatments. Preserve exclusions and delayed replies. If your tool optimizes or reallocates variants during a campaign, note when that happened; Instantly says its default balances variant use over the campaign's lifetime, so newly added variants may be sent until they catch up. A late variant added to a live run may therefore have a different calendar mix. Freeze the treatments for a clean comparison, or report the timing difference when interpreting the result.✓ 5

Do not turn a local difference into a general format rule. A small audience, a changed subject line, or different account segments can explain an apparent gap. Report raw counts and the definition used for a qualified reply, then repeat the comparison if the audience or message changes materially. Neither the MIME standard nor the product documentation cited here supplies a universal cold-email reply advantage for one format. Their value is in defining what was sent, what was displayed, and what the test actually measured.

5. Choose for the message in front of you

For a brief introduction that works without design, send a clear text-led note. If the material needs sections or a visual explanation, use HTML only after checking reading order, headings, image alternatives, meaningful links, and rendering. If your sending system supports multipart delivery, include an equivalent text representation so the recipient program can choose a displayable version.✓ 23

When the choice remains uncertain, test the two presentations against comparable groups and judge the result by a defined reply or meeting outcome. Preserve the complete counts and the exact versions sent. That decision can guide the next campaign to a similar audience. It is more credible than copying a broad claim about format from a different context.✓ 45

FAQ

How we researched this

We read RFC 2046, Mailchimp's HTML, accessibility, and A/B documentation, and Instantly's variant analytics guide on October 15, 2026. The five dated snapshots preserve the exact source wording behind each fact marker. These pages establish format behavior and testing options; they do not provide a controlled cold-email result that establishes a universal format winner.✓ 12345