Are there any ISPs or email clients that only accept text emails and reject HTML emails?

Key findings

• No SMTP blocking: There is no widespread evidence or industry standard protocol indicating that ISPs or email clients outright block emails solely because they contain HTML content.
• Client rendering, not rejection: Some older or specialized email clients may only display the plain text portion of a multipart message. However, this is a rendering preference or limitation, not a rejection of the email itself. The HTML part is still delivered.
• Spam filtering: While HTML content can be a factor in spam scoring (e.g., suspicious links, image-only emails, or malformed HTML), it rarely leads to a direct rejection solely based on its presence. Instead, it contributes to an aggregate spam score that determines inbox placement or filtering to spam folders. For more on how email deliverability issues can lead to spam folders, see Why your emails are going to spam.
• Multipart/alternative: The standard practice is to send emails using the multipart/alternative format, which includes both HTML and plain text versions. This ensures compatibility across diverse email clients. This approach can also improve deliverability and conversion rates.

Key considerations

• Content quality: Focus on creating high-quality, relevant content that adheres to email best practices. This is more critical for deliverability than the choice between HTML and plain text alone.
• Accessibility: Always include a well-formatted plain text version as a fallback, ensuring your message is accessible to all recipients, regardless of their email client's capabilities.
• HTML coding: Ensure your HTML is clean, valid, and optimized for email clients. Poorly coded HTML or malformed HTML can lead to rendering issues or trigger spam filters, not outright rejection by ISPs. Email on Acid provides a detailed overview of when to use plain text email effectively.
• Image-to-text ratio: While not a rejection criterion for HTML itself, an extremely high image-to-text ratio, or image-only emails, can be a red flag for spam filters. Learn more about protecting deliverability for image-only emails.

Key opinions

• HTML for engagement: Marketers largely agree that HTML emails are crucial for branding, visual appeal, and driving engagement through rich content, images, and calls to action.
• Plain text fallback: Despite the prevalence of HTML, many marketers still emphasize the necessity of including a plain text alternative. This ensures the message is readable even if the HTML doesn't render, or for users who prefer text-only viewing. Campaign Monitor offers insights into the benefits of both HTML and plain text emails.
• Complexity of multipart: Some marketers question the value of sending multipart/alternative messages due to the added complexity and potential for errors, especially given the low number of recipients who exclusively view plain text.
• Simplicity for specific use cases: For specific types of communications, like monitoring alerts or internal reports, plain text is often deemed sufficient and even preferred for its directness.

Key considerations

• Audience preference: Consider your audience. While most modern users view HTML, some niche or professional audiences might still use text-only clients or prefer plain text for quick information consumption. ISPs like Google and Yahoo also track email engagement, which can be influenced by how well your emails display.
• Deliverability impact: The way ISPs interpret and filter emails is complex, combining content analysis, sender reputation, and user engagement. While an image-only HTML email can increase spam flagging risks, HTML itself is not inherently bad. For further insights into ISP tracking of email engagement, refer to our guide on how internet service providers track email engagement.
• Testing: Always test your emails across various clients and devices to ensure proper rendering for both HTML and plain text versions. Dyspatch highlights the factors to consider when choosing between HTML and plain text emails.

Key opinions

• No direct SMTP blocking for HTML: Experts confirm that they have not observed SMTP services specifically blocking emails based solely on the presence of HTML content. Such a rule would be highly unusual and inefficient.
• Rendering limitations vs. rejection: The primary issue with HTML for some users is that their email client might not render it, defaulting to the plain text version. This is a client-side display choice, not a server-side rejection.
• Spam score contribution: HTML elements can contribute to a spam score in tools like SpamAssassin, but it's typically a low score that combines with other factors. It's not usually the sole reason for an email being marked as spam or blocked.
• Legacy client preference: Some users, particularly those with a technical background, prefer plain text for certain types of emails (e.g., monitoring reports), but this is a personal preference for readability, not a technical requirement for deliverability.

Key considerations

• Focus on reputation: A sender's overall reputation, including factors like DMARC, SPF, and DKIM authentication, list hygiene, and engagement metrics, far outweighs the HTML vs. plain text debate in terms of deliverability. Ensuring proper email authentication is paramount.
• Adaptability for specialized content: For specific, non-marketing communications such as system alerts or monitoring reports, plain text can be more efficient and suitable, as recipients are often looking for concise, factual information.
• HTML best practices: When sending HTML, adhere to coding best practices for email (e.g., inline CSS, simple tables for layout) to ensure broad compatibility and reduce the likelihood of rendering issues. Word to the Wise offers additional insights into email sending challenges that extend beyond HTML content.
• User experience matters: Ultimately, the goal is for the recipient to successfully read and interact with your email. This means considering how your email looks and behaves across various platforms. SpamResource.com provides valuable information on new authentication requirements that impact all email types.

Key findings

• HTML limitations: Documentation frequently highlights that email clients have varying support for HTML and CSS, which means complex web-like designs may not render consistently. This necessitates simplified HTML structures.
• Multipart/alternative recommendation: Many guides recommend sending emails with both HTML and plain text parts to ensure broad compatibility. This is crucial for reaching all subscribers, especially those with accessibility needs or older clients.
• Spam avoidance: Documentation often advises against practices that can trigger spam filters, such as using excessive images with little text or poorly formatted HTML, as these can lead to messages being sent to the junk folder, but not typically outright rejected.
• Plain text for core message: Some technical guides suggest creating the plain text version first to ensure the core message is clear and concise before adding HTML styling. This prioritizes content over design complexity.

Key considerations

• HTML coding standards: Adhere to specific HTML and CSS practices recommended for email, such as using inline styles and table-based layouts, to maximize rendering consistency across various email clients. Mailchimp's documentation on limitations of HTML email provides valuable insights.
• Content optimization: Regardless of format, email content should be clear, concise, and provide value to the recipient to minimize spam complaints and improve engagement. This directly impacts how ISPs manage email deliverability. For more information about ISP data and deliverability, refer to our page on what ISP information is most useful.
• Image-text balance: Be mindful of the image-to-text ratio. While images enhance HTML emails, an over-reliance on images without sufficient text can trigger spam filters, as highlighted by Tower Marketing's blog on image-only emails.