Summary
The consensus from experts and documentation is that ISPs do not outright reject HTML emails. Email standards (RFC 2046, RFC 5322) support both plain text and HTML formats. While some users prefer text-only email clients or configure their email clients (Outlook, Thunderbird, Apple Mail) to view emails in plain text, this is due to user preference and is not a server-level block. Additionally, poorly coded or excessively large HTML emails can trigger spam filters, emphasizing the importance of providing a plain text alternative. Sending emails in a multi-part MIME format (both HTML and plain text) is recommended for improved deliverability and catering to diverse user preferences.
Key findings
- No Rejection of HTML: Major ISPs and email clients generally do not reject HTML emails.
- Standards Support Both: Email standards and protocols (RFC 2046, RFC 5322) support both plain text and HTML formats.
- User Preference for Text: Some users prefer text-only email clients or configure their clients to view emails in plain text.
- Spam Filter Triggers: Poorly coded or excessively large HTML emails can trigger spam filters.
- Improved Deliverability with Plain Text: Providing a plain text alternative improves deliverability by avoiding spam filters and catering to different user preferences.
- Multi-Part MIME Recommended: Using a multi-part MIME format (both HTML and plain text) is recommended for compatibility and deliverability.
Key considerations
- Deliverability Strategy: Prioritize email deliverability by always including a plain text version alongside HTML.
- Account for User Preferences: Cater to users who prefer or require plain text emails by providing a clear and accessible alternative.
- Optimize HTML Code: Optimize HTML code to avoid triggering spam filters, ensuring clean and lightweight design.
- Test Email Rendering: Test emails in different email clients to ensure they render correctly in both HTML and plain text formats.
- Ensure Content Accessibility: Ensure the key content of your email is accessible in both formats, giving all users a good experience.
What email marketers say
9 marketer opinions
While no major ISPs or email clients outright reject HTML emails, certain factors influence how they are handled. Some users prefer text-only clients for security or customization reasons, and while they *can* read HTML emails, they choose not to. Poorly coded or excessively large HTML emails can trigger spam filters, making it advisable to include a plain text alternative for improved deliverability. Some email clients also give users the *option* to view emails in plain text. It's technically possible to configure a server to reject HTML emails, but it is not common. Having both HTML and plain text versions in a multi-part MIME format caters to different user preferences and helps avoid spam filters.
Key opinions
- No outright rejection: Major ISPs generally do not reject HTML emails.
- User preference: Some users choose text-only email clients.
- Spam filters: Poorly coded HTML can trigger spam filters.
- Deliverability boost: Plain text alternatives improve deliverability.
- MIME format: Multi-part MIME format (HTML and text) is recommended.
- Client configuration: Email clients allow users to choose how they view emails, with some clients enabling users to configure emails to be viewed in plain text.
- Server Configuration: It is technically possible for email servers to only accept text emails but it is not a common configuration.
Key considerations
- Deliverability: Always include a plain text version to improve deliverability and avoid spam filters.
- User experience: Cater to users who prefer or require plain text emails.
- Code quality: Ensure HTML emails are well-coded and not excessively large to avoid spam triggers.
- Client compatibility: Be aware that some email clients may display emails in plain text by default or based on user settings.
- Content structure: Provide clear plain text alternatives which enable users to understand key information.
Marketer view
Email marketer from Campaign Monitor shares that while not rejecting HTML outright, some spam filters penalize emails that don't have a plain text version. They suggest using a multi-part MIME format.
20 Jul 2021 - Campaign Monitor
Marketer view
Email marketer from Email on Acid explains that some email clients allow users to choose whether to view HTML emails, so emails should have a text version.
10 Apr 2023 - Email on Acid
What the experts say
3 expert opinions
Experts generally agree that ISPs don't outright reject HTML emails. While some clients display only the plain text part due to user preference, this isn't the same as blocking HTML. There's debate on whether multipart/alternative emails are worth the complexity, with some suggesting sending only text/html. However, having a plain text alternative can improve deliverability and cater to users who prefer or require it.
Key opinions
- No HTML blocking: ISPs generally do not block HTML emails.
- User choice: Some clients display only plain text based on user settings.
- Plain text benefits: Plain text versions can improve deliverability and cater to specific users.
- Multipart Debate: The value of multipart/alternative emails is debated.
Key considerations
- Deliverability: Weigh the deliverability benefits of a plain text alternative.
- Complexity vs. Benefit: Consider the complexity of multipart/alternative vs. its benefits.
- User preferences: Cater to users who prefer or require plain text.
- Test Clients: Test your email content on different email clients to ensure it renders correctly.
Expert view
Expert from Word to the Wise, Laura Atkins, discusses the importance of sending both HTML and plain text versions of emails. While she doesn't state that ISPs outright reject HTML emails, she emphasizes that having a plain text alternative can improve deliverability as it caters to users who prefer or require plain text and can help avoid spam filters.
23 Apr 2022 - Word to the Wise
Expert view
Expert from Email Geeks states she has never heard of SMTP blocking like that, but some clients only display the plain text part of the email due to user choice, not blocking.
8 Nov 2022 - Email Geeks
What the documentation says
5 technical articles
Email standards (RFC 2046, RFC 5322) support both 'text/plain' and 'text/html' content types, indicating widespread support for both formats. Popular email clients like Outlook, Thunderbird, and Apple Mail allow users to configure their settings to view messages in plain text, but this is a user-level preference, not a server-level rejection of HTML. Documentation confirms that HTML emails are not blocked by default; users choose to view emails as plain text.
Key findings
- Standards Support HTML: Email standards support both plain text and HTML content.
- Client Configuration: Email clients allow users to choose to view messages in plain text.
- User Preference: Plain text viewing is a user-level preference, not a server-level block.
- No Default Blocking: HTML emails are not blocked by default in major email clients.
Key considerations
- Client Settings: Be aware of client settings that may display your HTML email as plain text.
- Content Delivery: Ensure essential content is accessible in plain text format if users choose to view emails this way.
- Accessibility: Prioritize accessibility by creating both HTML and plain text versions to accommodate diverse user preferences.
- Email testing: Test email in both HTML and plain text formats to check for correct rendering in different clients.
Technical article
Documentation from Apple outlines how users can view all messages in plain text format within the Apple Mail application. It does not block HTML emails, it is a user preference setting.
12 Aug 2021 - Apple Support
Technical article
Documentation from Mozilla clarifies how Thunderbird users can configure the email client to display messages in plain text rather than HTML for security or preference reasons.
21 Jan 2024 - Mozilla Support
Are HTTP links penalized by spam filters in email marketing?
Are image-based emails a good practice, and what are the deliverability and accessibility implications?
Are image-only emails bad for deliverability?
Do images in emails affect deliverability?
Does base64 encoding of plain text emails impact spam filter scores?
Does including a plain text version of an email improve deliverability and conversion rates?
Does using base64 vs UTF-8 HTML/Text impact email deliverability?
How do HTML email size, TLD, attachments, and domain reputation affect deliverability and open rates?
