Does base64 encoding of plain text emails impact spam filter scores?

Summary

The consensus suggests that Base64 encoding of plain text emails doesn't directly cause spam filtering issues, but it can indirectly affect deliverability. Increased email size, particularly with images or unoptimized content, is a primary concern as spam filters may penalize overly large messages. Proper email authentication (SPF, DKIM, DMARC) is crucial in preventing emails from being marked as spam. Poor Base64 implementations, such as long lines, can also trigger filters. While historically providing both plain text and HTML versions was beneficial, it's now largely outdated as most clients support HTML. Spam filters consider multiple factors, with unusual encoding potentially contributing to a higher score. Overall email design, structure, and adherence to best practices have a more significant impact on deliverability than the encoding method itself.

Key findings

  • Base64 Indirect Impact: Base64 encoding itself isn't a direct spam trigger, but can indirectly affect deliverability.
  • Email Size Matters: Increased email size due to Base64 (especially with images) can negatively impact deliverability.
  • Authentication is Key: Proper email authentication (SPF, DKIM, DMARC) is crucial for avoiding spam filters.
  • Implementation Quality: Poor Base64 implementation (e.g., long lines) can trigger spam filters.
  • Plain Text Outdated: Providing both plain text and HTML is largely outdated, with HTML now being the standard.
  • Holistic Analysis: Spam filters consider multiple factors, with unusual encoding potentially contributing negatively.
  • Design and Structure: Overall email design, structure, and best practices have a greater impact than encoding.

Key considerations

  • Optimize Content: Optimize email content to minimize the impact of Base64 encoding on email size.
  • Implement Authentication: Prioritize implementing SPF, DKIM, and DMARC to authenticate emails and improve deliverability.
  • Review Encoding: Ensure proper Base64 implementation to avoid triggering spam filters due to poor encoding practices (e.g., long lines).
  • Focus on Overall Design: Concentrate on crafting well-designed, structured emails that adhere to best practices for better deliverability.
  • Monitor Headers: Regularly analyze email headers to identify potential spam triggers and understand how filters are treating your emails.

What email marketers say
7Marketer opinions

The impact of Base64 encoding on plain text emails and spam filter scores is nuanced. While Base64 encoding itself may not be a direct trigger for spam filters, it can indirectly affect deliverability. Factors like increased email size due to encoding, especially if the content isn't optimized, can negatively impact deliverability. Proper email authentication methods (SPF, DKIM, DMARC) are crucial for preventing emails from being flagged as spam. Additionally, poor implementations of Base64, such as long lines, are more likely to trigger spam filters. Some experts suggest focusing on overall email design, structure, and adherence to best practices for better deliverability.

Key opinions

  • Encoding & Size: Base64 encoding increases email size, potentially affecting deliverability.
  • Authentication: Proper email authentication (SPF, DKIM, DMARC) is crucial for avoiding spam filters.
  • Implementation Matters: Poor Base64 implementation (e.g., long lines) can trigger spam filters.
  • Holistic Approach: Email design, structure, and best practices are more impactful than encoding alone.
  • Plain text versions: Spam filters may not really care for plain text versions, as they are rarely seen

Key considerations

  • Optimize Content: Ensure email content is optimized to minimize the impact of increased email size due to Base64 encoding.
  • Implement Authentication: Prioritize implementing SPF, DKIM, and DMARC to authenticate emails and improve deliverability.
  • Review Implementation: Avoid poor Base64 implementations that can trigger spam filters.
  • Focus on Design: Concentrate on crafting well-designed and structured emails that adhere to best practices.
  • Analyze Headers: Analyze email headers for spam triggers to understand how spam filters are treating your emails.
Marketer view

Email marketer from Reddit shares that providing both plain text and HTML versions is useful, although spam filters do not really care for plain text versions, as they are rarely seen. If there are deliverability issues, ensure email authentication is setup.

November 2021 - Reddit
Marketer view

Email marketer from Mailjet explains that Base64 encoding increases email size, which can negatively impact deliverability, particularly if the content isn't optimized. Larger emails take longer to download, and some spam filters may penalize overly large messages.

July 2021 - Mailjet
Marketer view

Email marketer from Stackoverflow states that typically spam filters will score an email based on its content, the use of Base64 in an email makes the email larger, which could potentially trigger spam scores. However, this is not always the case.

August 2021 - Stackoverflow
Marketer view

Email marketer from Campaign Monitor shares that encoding is not an important area of focus for design and deliverability, more important is the actual design and structure of your email. Following best practices has a much greater impact on deliverability

February 2022 - Campaign Monitor
Marketer view

Email marketer from Sendinblue shares that while Base64 encoding itself might not directly trigger spam filters, using proper email authentication methods (SPF, DKIM, DMARC) are important to ensure emails aren't flagged as spam.

February 2023 - Sendinblue
Marketer view

Email marketer from GlockApps explains that analyzing email headers for spam triggers helps understand how spam filters treat emails. While not directly related to Base64, it helps identify other potential factors that may cause emails to land in the spam folder.

September 2022 - GlockApps
Marketer view

Email marketer from Litmus shares that Base64 encoding is not a problem directly, but poor implementations are, for example long lines are more likely to trigger spam filters.

September 2024 - Litmus

What the experts say
7Expert opinions

The impact of Base64 encoding on plain text emails concerning spam filter scores appears to be multi-faceted. While direct issues stemming from the use of Base64 have not been widely found in testing, certain factors can contribute to deliverability concerns. Increased email size due to Base64 encoding can become a factor, particularly when incorporating images, potentially triggering spam filters due to size thresholds. Employing quoted-printable encoding can sometimes circumvent issues related to European text. The importance of plain text versions is diminishing as HTML-capable mail clients become ubiquitous. Spam filters consider many factors, unusual encoding may contribute negatively, and historical practices of including both HTML and text versions for better spam scores are now largely outdated.

Key opinions

  • Base64 Not a Direct Issue: Direct problems with Base64 encoding are not widely confirmed in testing.
  • Email Size Impact: Increased email size, especially with images, due to encoding, can impact deliverability.
  • Quoted-Printable Alternative: Quoted-printable encoding can be used to avoid issues related to European text.
  • Plain Text Version Diminishing: The relevance of plain text versions is decreasing due to widespread HTML support.
  • Spam Filter Multifactorial Analysis: Spam filters consider multiple factors; unusual encoding might negatively impact the score.
  • HTML and text versions historical practice: Historically having both HTML and text is no longer important.

Key considerations

  • Minimize Email Size: Reduce email size, especially when using Base64 encoding, to avoid triggering spam filters.
  • Consider Encoding Alternatives: Evaluate the use of quoted-printable encoding for European text to potentially mitigate issues.
  • Focus on HTML Content: Prioritize well-crafted HTML content given the diminishing importance of plain text versions.
  • Holistic Spam Filter Awareness: Understand that spam filters analyze multiple factors, so avoid any unusual encoding practices.
Expert view

Expert from Spam Resource generally explains spam filters look at many factors. While they don't specifically call out base64 encoding of plain text emails, they imply that any unusual encoding might contribute to a higher spam score.

February 2024 - Spam Resource
Expert view

Expert from Email Geeks explains that encoding everything to quoted-printable avoids issues for European text, and systems often use heuristics to decide when to use it to avoid lines that are too long for SMTP.

June 2022 - Email Geeks
Expert view

Expert from Email Geeks says that historically having both text and HTML would get a better score in Spam Assassin and other low-rent filters. Might still be true in Spam Assassin, but it’s outdated guidance.

November 2021 - Email Geeks
Expert view

Expert from Word to the Wise explains that one encoding issue with the increase in using images, is that encoded images can increase the overall size of your message. This could lead to deliverability issues depending on the thresholds of spam filters

April 2024 - Word to the Wise
Expert view

Expert from Email Geeks shares testing experience and states that they didn't find any issues with base64 encoding. They mention seeing something long ago about spam filters looking for base64 encoded content that didn’t need to be, but this is not a broad issue.

April 2022 - Email Geeks
Expert view

Expert from Email Geeks says that Base64 encoded plain text is OK, but it makes mail 25% bigger than quoted printable.

November 2023 - Email Geeks
Expert view

Expert from Email Geeks states that on the modern internet, no one will see the text/plain version of a multipart/alternative message, because all normal mail clients support text/html and will display it in preference to the text/plain part.

November 2021 - Email Geeks

What the documentation says
3Technical articles

The documentation provides insights into various aspects of email and spam filtering. RFC 2045 defines Base64 encoding but does not address its direct impact on spam filters. SpamAssassin utilizes heuristic rules considering factors like encoding, size, and patterns; Base64 might be one of them. DKIM improves deliverability and prevents spoofing, indirectly influencing spam filter results but not directly related to Base64 encoding.

Key findings

  • RFC 2045: Defines Base64 encoding but doesn't discuss spam filter impacts.
  • SpamAssassin Heuristics: SpamAssassin uses rules that may consider encoding, size, and patterns, potentially including Base64.
  • DKIM Impact: DKIM improves deliverability and prevents spoofing, indirectly influencing spam filter results.

Key considerations

  • RFC Context: Base64 encoding is a standard, but its effects on spam aren't defined there.
  • SpamAssassin Review: Be aware that SpamAssassin may consider Base64 as part of its heuristic analysis.
  • DKIM Implementation: Implement DKIM for improved deliverability, even if it doesn't directly address Base64 encoding concerns.
Technical article

Documentation from DKIM explains that DKIM provides a method for verifying the sender's domain identity. While not directly about Base64 encoding, implementing DKIM can improve deliverability and prevent spoofing, which affects spam filter results.

January 2025 - DKIM
Technical article

Documentation from RFC Editor explains that RFC 2045 defines standards for internet message bodies. It describes Base64 encoding as a way to represent binary data in ASCII format. It does not mention specific impacts on spam filtering.

March 2022 - RFC Editor
Technical article

Documentation from Apache SpamAssassin shares that while specific rules change, SpamAssassin uses heuristic rules that may consider various factors, including encoding types, message size, and the presence of specific patterns, in scoring emails. Base64 encoding may be one of many factors considered.

February 2022 - Apache SpamAssassin