The plain text vs HTML email debate has been running since HTML email became widespread in the late 1990s. In 2027, the question is not as binary as it once was — modern email best practice uses both formats simultaneously through the multipart/alternative MIME type — but the relative weight given to each format, and the specific use cases where plain text is preferable to HTML, remain genuinely consequential for deliverability and engagement. This guide documents what the research and operational data show about format choice and its effects.
Email Format Overview: Plain Text, HTML, and Multipart
Commercial email can be sent in three basic format configurations, each with different implications for how it is processed and displayed:
Plain text only (text/plain): The email body contains only UTF-8 text — no HTML markup, no images, no formatted styling. The email is displayed as raw text in any email client. Links appear as bare URLs. All email clients can render plain text correctly. Tracking pixels (which require an HTML image element) cannot be embedded in plain text email — open tracking is not possible.
HTML only (text/html): The email body contains only HTML markup. Email clients that support HTML (all major ones) render the formatted email. Clients or recipients who have HTML disabled see raw HTML markup — a poor experience. Missing the plain text alternative is increasingly flagged by spam filters as a quality signal. Some corporate email security gateways strip HTML from messages before delivery when the sender has low reputation — recipients then receive the raw HTML markup, which is unreadable.
Multipart/alternative (both): The email is sent as a MIME multipart message containing both a text/plain part and a text/html part. Email clients that support HTML display the HTML part; clients or configurations that prefer or require plain text display the text/plain part. This is the correct format for all commercial email — it provides the best experience for all recipients regardless of their email client or configuration. Spam filters explicitly look for the plain text alternative as a quality signal — its presence indicates a more professionally produced email.
Deliverability: Does Format Affect Inbox Placement?
Email format has a measurable but secondary effect on deliverability relative to authentication, sender reputation, and list quality. The primary deliverability signals (domain reputation, complaint rate, bounce rate) dwarf the format content scoring signal in ISP filtering decisions. However, format does matter at the margin — and for borderline reputation senders, the margin can determine inbox vs spam placement.
The missing plain text alternative problem: HTML-only email (no plain text alternative) scores slightly higher on spam filter content evaluation at several major corporate gateways and consumer ISP spam filters. The plain text alternative is treated as a quality signal — legitimate commercial email is expected to include it; spam and phishing email frequently does not. SpamAssassin (used by many hosting providers and corporate gateways) has a specific rule (MIME_HTML_ONLY) that adds points to the spam score for HTML-only messages.
Image-to-text ratio: The ratio of image content to text content in an HTML email affects spam filter content scoring. Emails that are predominantly images with minimal readable text — common in heavy visual design templates where the "message" is entirely in a hero image — score higher on spam filter evaluation than emails with substantial text content. The rationale: spam and phishing emails frequently use images to evade text-based content scanning. A legitimate marketing email should have readable text content that expresses its message even if images are not displayed.
Format and corporate gateway reputation evaluation: Proofpoint and Barracuda evaluate email content more aggressively for unknown or borderline-reputation senders. For these senders, the format quality signals (plain text alternative present, good text-to-image ratio, no URL shorteners, branded tracking domain) collectively contribute to whether the content evaluation tips the message toward inbox or spam. For established High-reputation senders, these signals matter less because the reputation signal dominates the content signal.
Spam Filter Scoring: How Format Affects Content Evaluation
SpamAssassin — the most widely deployed content-based spam filter, used by many hosting providers and corporate gateways — assigns points to email based on content characteristics. Several SpamAssassin rules relate directly to email format:
# Format-related SpamAssassin rules (examples): MIME_HTML_ONLY +0.1 # HTML-only email (no plain text alternative) MIME_HTML_NO_TEXT +0.7 # HTML email with very short or absent text alternative HTML_IMAGE_ONLY_04 +0.6 # HTML email with almost all images, minimal text (0-4%) HTML_IMAGE_ONLY_08 +0.6 # HTML email with 0-8% text content HTML_IMAGE_RATIO_60 +0.2 # 60%+ of content area is images HTML_SHORT_LINK_85 +0.2 # Short links dominate (85%+ of links are short) NO_RECEIVED +0.5 # No Received: header (often spam indicator) # Note: These point values change with SpamAssassin version updates
Individually, these format-related rules add 0.1-0.7 points to the spam score. SpamAssassin's default spam threshold is typically 5.0 points — format issues alone will not cause spam classification for a message that is otherwise clean. However, combined with other mild spam signals (generic subject lines, some promotional language, no DMARC record), the format contribution can tip a borderline message above the spam threshold at corporate gateways running SpamAssassin.
The practical format guidelines from spam filter scoring: always include a plain text alternative (eliminates MIME_HTML_ONLY and MIME_HTML_NO_TEXT scoring). Keep image-to-text ratio below 60% in HTML email (eliminates HTML_IMAGE_ONLY_04 through HTML_IMAGE_RATIO_60 scoring). Use full-length branded tracking domain URLs, not URL shorteners (eliminates HTML_SHORT_LINK scoring).
Engagement Differences: Open Rate, Click Rate, Reply Rate
Plain text and HTML email generate different engagement patterns that reflect fundamental differences in how recipients perceive and respond to each format:
Open rate: HTML email typically achieves higher open rates than plain text in marketing contexts because: HTML allows branded visuals and subject/preheader combinations that create more compelling inbox preview; and plain text email in the inbox looks like a personal message, which may generate lower open rates for some sender types (recipients may be confused about the sender's identity or the email's purpose). However, open rate data is further complicated by Apple MPP, which affects HTML emails disproportionately — MPP pre-loads images in HTML emails but has no equivalent effect on plain text email (which has no images to pre-load). This means plain text open rate data is more reliable (less MPP-inflated) than HTML open rate data for the same sender in 2027.
Click rate: HTML email with clear visual CTAs (buttons, prominent links, image links) typically achieves higher absolute click rates than plain text. The visual design that HTML enables — large colourful CTA buttons, product images, direct link placement — drives more clicks than the bare URLs that appear in plain text. For commercial programmes where click-through is the primary engagement objective (e-commerce, newsletter to website traffic), HTML outperforms plain text on click rate.
Reply rate: Plain text email consistently generates higher reply rates than HTML. The reason is format perception: an HTML email looks like a broadcast — a carefully designed marketing piece sent to thousands of people. A plain text email looks like a personal message — something written by a person to a recipient individually. Recipients are significantly more likely to reply to what appears to be a personal email than to what is obviously a mass-sent HTML newsletter. For programmes where replies are valuable (B2B sales outreach, survey requests, relationship-building email), plain text dramatically outperforms HTML on reply rate.
The Multipart/Alternative Standard: Why Both Are Required
The industry best practice for commercial email is multipart/alternative — including both a text/plain part and a text/html part in every email. RFC 2046 specifies the multipart/alternative MIME type, and RFC 2822 (the email message format standard) specifies that HTML email should include a plain text alternative. Most ESPs automatically generate a text/plain alternative from the HTML content when the sender does not explicitly provide one — but auto-generated plain text (stripped of HTML tags) is often a poor user experience compared to a human-written plain text version that communicates the same message effectively.
The plain text alternative should be written to be read as a complete, standalone email — not just the HTML email with tags stripped out. A marketing email whose HTML version contains a hero image with promotional text, a product grid, and a CTA button needs a plain text version that communicates: the promotion offer, the product details, and a plain URL link to the destination. A plain text version that says "Your email client does not support HTML" is not an alternative — it is a failure to serve recipients who receive the plain text version.
Plain Text for Cold Email: Why It Works
Cold email — outbound sales email sent to prospects without prior consent — consistently performs better in plain text format than HTML. The reasons:
Authenticity signalling: A cold sales email that arrives as a beautifully designed HTML newsletter immediately signals "mass marketing campaign" — which triggers the recipient's marketing filter. A plain text email that looks like it was written by a person specifically to this recipient signals personalised outreach — which receives more consideration even from recipients who ultimately do not respond.
Corporate gateway content scoring: Cold email to corporate recipients passes through aggressive Proofpoint or Mimecast gateways. Plain text email scores lower on content-based spam evaluation than HTML email because it lacks the image-based content patterns that phishing and spam frequently use.
Reply rate: Cold email's success metric is replies — a reply starts a sales conversation. Plain text cold email generates 40-60% higher reply rates than HTML cold email in controlled A/B tests, entirely because the format creates the appearance of a personal message rather than a mass campaign. The higher reply rate is the commercial reason that cold email best practice universally recommends plain text.
Open tracking trade-off: Plain text cold email cannot embed tracking pixels — the sender loses open rate visibility. For cold email, this trade-off is acceptable because: (1) reply rate is more valuable than open rate as a success metric; (2) MPP inflates HTML email open rates making them unreliable anyway; and (3) the reply rate improvement from plain text more than compensates for the loss of open rate data.
HTML for Newsletters and Marketing: When It Is Right
HTML email is the correct format for subscriber-based newsletters and marketing campaigns where: the recipient has explicitly opted in and expects visually designed email; click-through to a website is the primary conversion objective; brand visual consistency is important to the programme's goals; and the content benefits from rich formatting (product grids, image galleries, styled CTAs). The key requirements for HTML marketing email that performs well on spam filter content scoring: include a plain text alternative, maintain above 60% text content by area, use branded click-tracking domains (not URL shorteners), and ensure alt text on all images.
Making the Format Decision for Your Programme
The format decision framework: (1) Cold outbound email → plain text, always. (2) B2B relationship email where replies are valued (sales follow-ups, customer success check-ins) → plain text. (3) Transactional email (order confirmations, account alerts) → HTML with strong plain text alternative; visual design aids clarity for receipts and account information. (4) Opt-in newsletters with editorial content → HTML with well-crafted plain text alternative; the brand investment in visual design is appropriate for an established subscriber relationship. (5) E-commerce marketing campaigns → HTML with strong text-to-image ratio and plain text alternative. (6) Any email going to corporate recipients through aggressive security gateways → prefer plain text or minimal HTML with high text content.
The universal format principle: never send HTML-only email (no plain text alternative). The plain text alternative is required by best practice, favoured by spam filters, and serves recipients who prefer or require plain text. It is not a significant production overhead — write it once per template, update it when the template updates, and the spam filter scoring improvement and recipient experience improvement are permanent benefits from a modest one-time investment in writing the alternative text version.