Email Deliverability: How Plain Text vs HTML Affects Your Inbox Placement

By Mike Buckbee

• Published January 02, 2026

Email deliverability isn't just about hitting send. It's about making sure your message actually reaches your recipient's inbox. Whether you send plain text or HTML emails can significantly impact your success rate.

This guide covers how modern spam filters treat different email formats, why plain text emails usually have better deliverability, and when HTML is worth the risk.

Why Email Format Affects Deliverability

Modern spam filters analyze everything about your email, including its format and structure. The choice between plain text and HTML sends signals to email providers about what kind of message you're sending.

Plain text emails look like personal, one-to-one messages. HTML emails, with their images, colors, and formatting, signal that you're sending marketing content. That's not inherently bad, but it means your HTML emails face more scrutiny from spam filters.

Recent studies show plain text emails are less likely to trigger spam filters. Some reports show they can achieve approximately 42% higher open rates compared to HTML emails in certain contexts.

How Modern Spam Filters Work in 2024

Spam filtering has evolved dramatically. In 2024, email providers like Gmail, Outlook, and Apple Mail use AI-powered machine learning systems that analyze dozens of signals simultaneously.

The Shift to Machine Learning

Today's spam filters don't just look for "spammy words" anymore. They use machine learning models trained on billions of emails to detect patterns. These systems analyze sender reputation and authentication, content and structure, recipient engagement history, HTML complexity and code quality, image-to-text ratios, and link patterns and destinations.

The format of your email, plain text versus HTML, affects several of these signals.

Email Client Differences

Different email providers have different filtering approaches.

Apple Mail (56% of email consumption market share) tends to be more permissive with formatting but rewards simplicity.

Gmail uses aggressive filtering and often sorts marketing emails into the Promotions tab, where HTML emails with heavy formatting are more likely to land.

Outlook has strict filters that can flag HTML emails with broken code or suspicious formatting patterns.

Plain Text Email Deliverability Advantages

Plain text emails have a clear edge when it comes to reaching the inbox.

Simplicity Equals Fewer Red Flags

Plain text emails are inherently simple. There's no HTML code to break, no images to trigger filters, and no complex CSS that might look suspicious. This simplicity means fewer things can go wrong.

Research shows that plain text emails are significantly less likely to be flagged as spam. They load instantly, work across all devices, and email providers generally treat them as personal communication rather than bulk marketing.

When Plain Text Wins

Plain text is particularly effective for cold outreach. When you're reaching out to someone for the first time, plain text feels personal and authentic. HTML formatting immediately signals "mass email."

For B2B communication, professional emails between businesses benefit from the personal touch of plain text. Studies show plain text emails achieve 23% higher open rates in B2B contexts.

Simple confirmations, receipts, and notifications feel more authentic in plain text. If you want recipients to respond to your email, plain text dramatically increases reply rates.

Plain Text Limitations

Plain text isn't perfect for deliverability. Without tracking pixels, you lose the ability to measure open rates. You can't include compelling visual elements that might drive engagement.

HTML Email Deliverability Challenges

HTML emails face more scrutiny from spam filters, but understanding the specific triggers can help you avoid problems.

Common HTML Spam Triggers

Email providers flag HTML emails for several reasons.

Emails with too many images compared to text are more likely to be flagged. If your HTML email is mostly images, spam filters assume you're trying to hide spammy content from text-based filters.

If there are broken tags in your HTML code, email providers and users can mark it as spam. Clean, valid HTML is essential.

Excessive styling, embedded JavaScript, or unusual formatting structures often raise red flags. Keep your HTML simple and standard.

HTML emails packed with links, especially if they point to different domains, trigger spam filters.

Authentication Requirements

HTML emails especially benefit from proper email authentication. If you're sending formatted emails, you absolutely need SPF (Sender Policy Framework) to verify you're authorized to send from your domain, DKIM (DomainKeys Identified Mail) to add a digital signature to your emails, and DMARC (Domain-based Message Authentication) to tell email providers how to handle authentication failures.

Without these authentication protocols in place, your HTML emails are much more likely to be filtered as spam.

When HTML Is Worth the Risk

Despite the deliverability challenges, HTML emails make sense in certain situations.

When you're emailing people who already know your brand, HTML helps with recognition and branding.

For eCommerce, product images and visual layouts are essential for showcasing items and driving purchases.

Subscribers who opted in for your newsletter expect formatted content with images and design.

When the design itself carries important information that can't be conveyed in plain text, HTML is the right choice.

The Best of Both Worlds: Multipart MIME

You don't have to choose between plain text and HTML. You can send both in the same email using multipart MIME.

Multipart MIME emails contain both a plain text version and an HTML version. The recipient's email client automatically displays whichever version it prefers.

This approach signals to ISPs that you're following best practices, ensures compatibility with all email clients, provides a fallback for recipients with images disabled, and maintains accessibility for screen readers.

Most email service providers make it easy to send multipart emails. They automatically generate a plain text version from your HTML, or let you customize both versions.

Email Client Market Share and Deliverability Impact

Understanding which email clients your audience uses helps you make format decisions.

As of 2024, Apple Mail commands 56% of email consumption and favors clean, simple HTML or plain text. Gmail has a large share, especially in business, and uses aggressive Promotions tab filtering for HTML. Outlook is significant in enterprise settings with strict HTML rendering and filtering. Mobile clients handle 61% of all email opens.

For mobile-dominant audiences, plain text has a significant advantage. It loads instantly with no rendering issues.

Best Practices by Use Case

Your email format strategy should match your use case.

Cold Email Outreach

Plain text is non-negotiable for cold outreach. HTML formatting immediately signals bulk email and tanks your deliverability. Keep it simple, personal, and text-only.

Marketing Campaigns to Existing Lists

Your subscribers expect designed emails, but sending both versions ensures maximum deliverability and accessibility. Use multipart MIME with HTML plus a plain text fallback.

Transactional Emails

Order confirmations, password resets, and account notifications can use simple HTML for branding, but keep it minimal. Plain text works great here too.

B2B Newsletters

Consider the HubSpot approach: minimal HTML that looks like plain text. This maintains a personal feel while allowing for tracking and simple formatting.

eCommerce

Product emails need images, but always include a plain text version for deliverability and accessibility.

Testing and Monitoring Deliverability

You can't improve what you don't measure.

Key Metrics to Monitor

Track your inbox placement rate: what percentage of your emails reach the inbox versus spam or promotions folders.

Watch your bounce rate. Hard bounces indicate deliverability issues that might be format-related.

Monitor spam complaint rates. If recipients mark your emails as spam, it signals a format or content problem.

Check engagement rates. Opens, clicks, and replies indicate your emails are reaching engaged recipients.

A/B Testing Format Impact

The best way to determine optimal format for your audience is A/B testing.

Send identical content in plain text to half your list. Send the HTML version to the other half. Track inbox placement, opens, and clicks. Repeat with different email types.

What works for HubSpot's audience might not work for yours.

Tools for Deliverability Testing

Use deliverability testing tools to check your emails before sending to your full list. These tools can test inbox placement across different providers, identify spam trigger words and formatting issues, verify authentication (SPF, DKIM, DMARC), check HTML rendering across clients, and analyze image-to-text ratios.

Technical Factors That Impact Deliverability

Beyond format choice, several technical factors affect whether your emails reach the inbox.

Sender Reputation

Your sending domain's reputation matters more than any single email format. Email providers track bounce rates, spam complaint rates, engagement rates, sending consistency, and authentication compliance.

A strong sender reputation helps both plain text and HTML emails reach the inbox.

IP Warming

If you're increasing email volume or switching to HTML from plain text (or vice versa), warm up your IP address gradually. Sudden changes in sending patterns or format can trigger spam filters.

List Hygiene

Sending to invalid addresses or inactive recipients hurts deliverability regardless of format.

Regularly clean your list by removing hard bounces immediately, re-engaging or removing inactive subscribers, validating new email addresses, and implementing double opt-in.

The Bottom Line on Deliverability

Plain text emails generally have better deliverability than HTML emails, especially for cold outreach and B2B communication. But the best approach for most senders is multipart MIME, sending both formats in the same email.

This strategy maximizes deliverability by avoiding HTML spam triggers, maintains branding and visual appeal for recipients who can view HTML, ensures accessibility for all recipients, and signals to ISPs that you're following email best practices.

If you're struggling with deliverability, start by switching to plain text or multipart emails. Combined with proper authentication (SPF, DKIM, DMARC) and good list hygiene, format optimization can significantly improve your inbox placement rate.