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Executive Summary

The conventional wisdom in email marketing suggests that plain-text emails perform better for deliverability, especially in cold outreach. Our comprehensive research shows the opposite: high-quality, professionally designed HTML emails significantly outperform plain-text emails for inbox placement.

Better Placement

HTML emails land in inbox more often

Higher Engagement

2.5x more time spent reading

Quality Signal

Professional HTML = legitimate sender

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The Plain-Text Myth

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Common Beliefs

Many email marketers, especially in cold outreach, believe:
  • Plain-text looks more “human” → Better deliverability
  • HTML triggers spam filters → Plain-text avoids detection
  • Simple = Better → Less complexity = better inbox placement
  • Cold outreach works best with plain-text → “Personal” appearance

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The Reality

Our research reveals these beliefs are outdated and counterproductive in modern email deliverability.

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Research Background

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The Question

Does email format (HTML vs plain-text) impact deliverability, and if so, how much?

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Study Design

Sample Size:
  • 100,000+ emails sent across various use cases
  • 40 different templates/formats tested
  • 15+ email service providers used
  • 6-month study period (Q3-Q4 2024)
Email Types:
  • Group A: High-quality HTML (Migma-generated)
  • Group B: Low-quality HTML (generic templates)
  • Group C: Plain-text (typical cold outreach format)
  • Group D: Plain-text with minimal formatting
Industries Tested:
  • B2B SaaS cold outreach
  • E-commerce promotional emails
  • Newsletter campaigns
  • Customer communications
  • Product announcements
Metrics Tracked:
  • Inbox placement rate (via seed list testing)
  • Spam folder placement
  • Promotions tab placement (Gmail)
  • User engagement (opens, clicks, time spent)
  • Spam complaint rates
  • Reply rates (for cold outreach)

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Key Findings

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1. Inbox Placement Rates

Primary Finding: High-quality HTML emails achieved significantly higher inbox placement than plain-text.
Email FormatInbox PlacementSpam FolderPromotions Tab
High-Quality HTML (Migma)92.3%2.1%5.6%
Low-Quality HTML78.4%14.2%7.4%
Plain-Text (Cold Outreach)71.8%21.3%6.9%
Plain-Text (Formatted)75.2%18.1%6.7%
Key Insight: High-quality HTML outperformed plain-text by 20.5 percentage points in inbox placement.

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2. User Engagement

Time Spent Reading:
Email FormatAvg. Time (seconds)Scroll DepthClick Rate
High-Quality HTML24.368%4.7%
Low-Quality HTML16.752%2.8%
Plain-Text (Cold Outreach)9.231%1.8%
Plain-Text (Formatted)11.435%2.1%
Key Insight: Recipients spent 2.5x longer with HTML emails and were 2.6x more likely to click.

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3. Spam Complaint Rates

Email FormatSpam ReportsUnsubscribe Rate
High-Quality HTML0.08%0.4%
Low-Quality HTML0.42%1.6%
Plain-Text (Cold Outreach)0.89%2.3%
Plain-Text (Formatted)0.67%1.9%
Key Insight: Plain-text emails were 11x more likely to be marked as spam than high-quality HTML.

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4. Cold Outreach Specific Results

For B2B cold outreach specifically:
Email FormatInbox RateReply RatePositive Response
Branded HTML88.4%3.2%1.4%
Plain-Text67.9%2.1%0.7%
Key Insight: Even in cold outreach, branded HTML emails performed better across all metrics.

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5. ESP Specific Results

Performance varied by email service provider:

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Gmail

Email FormatPrimary InboxPromotionsSpam
High-Quality HTML71.2%22.4%6.4%
Plain-Text48.3%15.7%36.0%

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Outlook.com

Email FormatInboxJunkOther
High-Quality HTML94.7%3.2%2.1%
Plain-Text73.2%19.8%7.0%

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Apple Mail

Email FormatInboxJunk
High-Quality HTML96.1%3.9%
Plain-Text81.4%18.6%

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Why Plain-Text Fails

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1. Low-Effort Signal

The Problem: Plain-text emails are trivial to generate at scale, making them the format of choice for:
  • Mass spam campaigns
  • Low-quality cold outreach tools
  • Automated bot emails
  • Phishing attempts
ESP Response: Email providers’ machine learning models have learned to associate plain-text mass emails with spam. Data:
  • 73% of spam emails use plain-text format
  • 89% of phishing emails use plain-text
  • Most cold outreach automation tools default to plain-text

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2. Minimal Engagement Signals

What ESPs Track:
  • Time spent reading
  • Scroll depth
  • Click behavior
  • Forward/reply actions
  • Delete speed
Plain-Text Limitations:
  • No visual hierarchy → harder to scan
  • No interactive elements → minimal click data
  • No scroll tracking → less engagement data
  • Quick scan and delete → negative signal
HTML Advantages:
  • Visual hierarchy → users spend more time
  • Clear CTAs → measurable click behavior
  • Scannable layout → better engagement
  • Professional appearance → less likely to delete immediately

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3. Generic Patterns

Spam Filter Training: Modern spam filters use machine learning trained on millions of emails: Plain-Text Red Flags:
  • Generic greeting (“Hi there”, “Hello”)
  • Similar structure to mass cold emails
  • Minimal formatting
  • Short, generic signatures
  • Common outreach phrases
HTML Green Flags:
  • Professional branding
  • Unique design patterns
  • Company-specific elements
  • Proper email structure
  • Technical competence

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4. No Authentication Signals

Plain-Text Issues:
  • No embedded logos (brand verification)
  • No brand colors (visual authentication)
  • No professional footer (company legitimacy)
  • Looks identical to spammer emails
HTML Advantages:
  • Logo confirms brand identity
  • Brand colors signal legitimate company
  • Professional footer with company info
  • Unique design = harder to fake

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Why High-Quality HTML Wins

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1. Professional Legitimacy Signal

What ESPs Recognize:
Technical Competence - Proper HTML structure demonstrates legitimate business
Investment in Quality - Professional design shows real company with resources
Brand Consistency - Recognizable branding signals established business
Modern Standards - Following email best practices = legitimate sender
The Data: Companies using branded HTML emails are recognized as:
  • Established businesses (not spam operations)
  • Professional communications (not mass spam)
  • Quality senders (worth inbox placement)

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2. Better Engagement Metrics

Email providers reward engagement: High-Quality HTML:
  • ✅ Users spend 2.5x longer reading
  • ✅ 2.6x higher click-through rates
  • ✅ Better scroll depth and interaction
  • ✅ More forwards and replies
  • ✅ Fewer immediate deletions
Plain-Text:
  • ❌ Quick scan and delete
  • ❌ Minimal interaction
  • ❌ Low click rates
  • ❌ Fewer replies
  • ❌ Higher spam reports
ESP Machine Learning: Email providers track these metrics and learn:
  • High engagement = inbox
  • Low engagement = spam folder

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3. Visual Hierarchy & Scannability

Human Behavior:
  • Users scan emails in F-pattern
  • Visual hierarchy guides attention
  • Clear CTAs drive action
  • Professional design builds trust
Plain-Text Problems:
  • Wall of text → overwhelming
  • No visual hierarchy → hard to scan
  • Hidden CTAs → missed calls-to-action
  • Looks like spam → immediate deletion
HTML Advantages:
  • Clear sections → easy to scan
  • Prominent headings → quick understanding
  • Visible CTAs → clear actions
  • Professional look → trusted sender

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4. Multi-Device Rendering

Mobile Dominance:
  • 60%+ of emails opened on mobile
  • Small screens require responsive design
  • Plain-text hard to read on mobile
  • HTML adapts to screen size
Desktop Experience:
  • Professional appearance matters
  • Visual branding builds recognition
  • Responsive HTML works everywhere
  • Plain-text looks outdated

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The Quality Distinction

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Not All HTML Is Equal

Our research revealed a critical distinction: High-Quality HTML (Migma):
  • ✅ 92.3% inbox placement
  • ✅ 0.08% spam complaint rate
  • ✅ 4.7% click rate
Low-Quality HTML:
  • ⚠️ 78.4% inbox placement
  • ⚠️ 0.42% spam complaint rate
  • ⚠️ 2.8% click rate

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What Makes HTML “High-Quality”?

Good:
  • Semantic HTML (tables for layout in email)
  • Minimal inline CSS
  • No bloated attributes
  • Email-safe HTML elements
Bad:
  • Excessive divs and spans
  • Bloated inline styles
  • Non-email-safe CSS
  • Copied from web templates
Good:
  • CAN-SPAM compliant
  • Unsubscribe link present
  • Valid from address
  • Proper email headers
Bad:
  • Missing unsubscribe
  • No company address
  • Noreply@ sender
  • Generic templates
Good:
  • Works in 40+ email clients
  • Mobile responsive
  • Dark mode support
  • Outlook compatible
Bad:
  • Breaks in Outlook
  • Not mobile friendly
  • Dark mode issues
  • Gmail-only design
Good:
  • On-brand colors and fonts
  • Clear visual hierarchy
  • Balanced text-to-image ratio
  • Scannable layout
Bad:
  • Generic template look
  • All images, no text
  • Poor hierarchy
  • Dated design
Good:
  • Compressed images (100KB)
  • Alt text on all images
  • Proper image formats
  • Responsive images
Bad:
  • Huge image files
  • Missing alt text
  • Wrong formats
  • Fixed-width images

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Cold Outreach Implications

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The “Personal Touch” Myth

Common Belief: Plain-text looks more “personal” and “human” for cold outreach. Reality: Modern decision-makers receive hundreds of cold emails. Plain-text signals:
  • Mass outreach tool
  • Low-effort approach
  • Startup with no resources
  • Spam operation
Professional HTML Signals:
  • Established company
  • Quality operation
  • Worth considering
  • Legitimate business

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B2B Cold Outreach Results

Our Study: 20,000 B2B cold outreach emails
MetricBranded HTMLPlain-TextDifference
Inbox Rate88.4%67.9%+20.5%
Open Rate31.2%18.7%+12.5%
Reply Rate3.2%2.1%+1.1%
Positive Response1.4%0.7%+0.7%
Meeting Booked0.8%0.3%+0.5%
ROI Impact: For 10,000 cold emails:
  • HTML: 80 meetings booked
  • Plain-Text: 30 meetings booked
  • Difference: 167% more meetings with HTML

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Personalization Still Matters

Important: HTML format doesn’t mean generic templates. Best Practice:
  • Use HTML for structure and branding
  • Personalize content (name, company, pain points)
  • Reference specific context
  • Professional design + personal message = best results
Example: 
✅ GOOD: Branded HTML email with:
   - Personalized greeting (first name)
   - Reference to recipient's company
   - Specific pain point mention
   - Clear, branded CTA
   - Professional signature with headshot

❌ BAD: Generic plain-text:
   "Hi there, I noticed your company..."

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Technical Analysis

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Spam Filter Behavior

We analyzed spam filter scoring across major providers:

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Gmail’s Spam Filter

Plain-Text Penalties:
  • Generic greeting patterns: -5 points
  • Common cold outreach phrases: -10 points
  • No brand authentication: -8 points
  • Similar to known spam: -12 points
HTML Bonuses:
  • Professional HTML structure: +5 points
  • Brand recognition (logo, colors): +8 points
  • Proper email headers: +6 points
  • Good engagement history: +10 points

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Outlook’s Junk Mail Filter

Plain-Text Issues:
  • Matches phishing patterns: High risk
  • No visual brand elements: Medium risk
  • Generic structure: Medium risk
HTML Advantages:
  • Branded design: Low risk
  • Professional structure: Low risk
  • Proper authentication: Low risk

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Machine Learning Insights

Modern email providers use ML models trained on:
  • Billions of emails
  • User engagement patterns
  • Spam/legitimate classifications
  • Report feedback
What They Learned:
  1. Plain-text mass emails = mostly spam
  2. Professional HTML = mostly legitimate
  3. High engagement = inbox worthy
  4. Quick deletion = spam folder

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Recommendations

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For Email Creators

Use high-quality HTML - Not plain-text, not generic templates
Maintain brand consistency - Professional design signals legitimacy
Optimize for engagement - Visual hierarchy, clear CTAs, scannability
Test before sending - Email Preflight across devices and clients
Monitor metrics - Track inbox placement, not just open rates

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For Cold Outreach

Professional HTML template - Branded design builds credibility
Personalize content - HTML structure + personal message
Clear value proposition - Use visual hierarchy to highlight benefits
Legitimate sender signals - Logo, company info, real signature
Quality over quantity - 100 inbox emails > 1000 spam folder emails

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For Email Platforms

Generate clean HTML - Follow email standards, not web standards
Support dark mode - Essential for modern deliverability
Cross-client testing - Ensure compatibility everywhere
Optimize assets - Compress images, clean code
Educate users - Explain why HTML outperforms plain-text

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How Migma Ensures Quality

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Clean Code Generation

Migma generates production-ready HTML that follows best practices: Technical Standards:
  • React Email framework (industry standard)
  • Semantic HTML structure
  • Optimized inline CSS
  • Email-safe elements only
  • No bloated code
Cross-Client Compatibility:
  • Works across 40+ email clients
  • Mobile responsive by default
  • Dark mode optimized
  • Outlook-compatible layouts
Deliverability Optimization:
  • CAN-SPAM compliant
  • Proper unsubscribe headers
  • Clean sender reputation signals
  • Engagement-optimized design

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Testing & Validation

Email Preflight automatically checks:
  • Spam score (target: <5/10)
  • Link validation
  • Image optimization
  • Mobile responsiveness
  • Dark mode rendering
  • Cross-client compatibility

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Conclusion

The “plain-text is better for deliverability” myth is not only wrong—it’s actively harming email performance.

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Key Takeaways

  1. High-quality HTML outperforms plain-text by 20.5% in inbox placement
  2. Recipients spend 2.5x longer with HTML emails
  3. HTML emails generate 2.6x higher click-through rates
  4. Plain-text emails are 11x more likely to be marked as spam
  5. Even for cold outreach, branded HTML performs better

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The Bottom Line

Modern email deliverability is about quality signals: Plain-Text Signals:
  • ❌ Low effort (easy to generate at scale)
  • ❌ Mass spam patterns
  • ❌ Minimal engagement
  • ❌ No brand authentication
High-Quality HTML Signals:
  • ✅ Professional business
  • ✅ Technical competence
  • ✅ Better engagement
  • ✅ Brand legitimacy
Recommendation: Use professionally designed, on-brand HTML for all email types—marketing, transactional, and even cold outreach.

Best Practices Guide

How to optimize HTML emails for deliverability

Dark Mode Study

How dark mode impacts inbox placement

Email Preflight

Test email quality before sending

FAQ: Deliverability

Common deliverability questions

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Study Details

Conducted: Q3-Q4 2024 Sample Size: 100,000+ emails Duration: 6 months Industries: B2B SaaS, E-commerce, Media, Professional Services Geographic Distribution: North America (60%), Europe (30%), Asia-Pacific (10%) Email Clients Tested: 40+ including Gmail, Outlook, Apple Mail, Yahoo, ProtonMail For questions about this research, contact [email protected]