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

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.

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.

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.

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

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.

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.

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.

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.

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.

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 Might still be true in Spam Assassin, but it’s outdated guidance.

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

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.

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

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.

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.

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.

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.