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

Key findings

• Minimal Direct Impact: Base64 encoding for the plain text part of an email generally has no direct negative effect on modern spam filter scores or deliverability. Filters are sophisticated enough to process standard encodings.
• Purpose of Encoding: Encoding is often a necessary technical measure to accommodate non-ASCII characters or to break long lines to conform to SMTP protocol line length requirements.
• Historical Context: While some older spam filters, such as SpamAssassin, might have had minor heuristics or preferred certain encodings, this is largely outdated. Modern filters focus more on sender reputation and content analysis rather than just the encoding type.
• Size Increase: Base64 encoding results in a slightly larger email size (approximately 25% larger than quoted-printable or 7-bit ASCII), but this marginal increase is rarely a primary factor for spam filtering on its own.

Key considerations

• Modern Client Behavior: Most email clients today support HTML and will display the HTML part of a multipart/alternative message preferentially. This makes the plain text version less critical for visual display to the end-user.
• Encoding Choice: While Base64 is a valid and acceptable encoding, Quoted-Printable or 7-bit ASCII are often preferred for plain text content if compatible with the character set, due to their smaller footprint.
• Unsubscribe Token Length: Long unsubscribe tokens or links can sometimes force the plain text part into Base64 encoding. Shortening these tokens, especially if used in mailto: unsubscribe headers, can be beneficial for broader compatibility.
• Overall Deliverability Strategy: Rather than focusing on minor encoding details, prioritize broader deliverability factors. These include maintaining a strong sender reputation, implementing proper email authentication (SPF, DKIM, DMARC), and ensuring high-quality, relevant content to avoid spam folders.

Key opinions

• Limited Impact Observed: Many marketers report that their plain text emails, even when Base64 encoded, do not experience significant deliverability issues or increased spam flagging in their testing.
• Preference for Simplicity: Despite not seeing issues, some marketers still prefer plain text to be encoded as 7-bit ASCII or Quoted-Printable if possible, viewing it as more friendly, even without a concrete technical reason for better performance.
• Outdated Guidance: The idea that explicitly having a plain text version (or a specific encoding for it) significantly boosts spam scores is sometimes considered historical or only relevant to older, less sophisticated filters.
• Focus on HTML: Given that most email clients display the HTML version, marketers tend to prioritize the accessibility and rendering quality of their HTML content.

Key considerations

• Platform Specifics: The encoding behavior of plain text (e.g., triggering Base64 encoding for long strings) can be specific to the sending platform or script (like MailApp in Google Apps Script), so understanding your tool's defaults is useful.
• Real-World Testing: While general principles apply, conducting small-scale tests within your specific sending environment can provide practical confirmation of deliverability with various encodings. This can help you understand how your messages perform across different ISPs.
• Encoding vs. Content: Deliverability issues are more commonly tied to factors like sender reputation, email content quality, and proper authentication (SPF, DKIM, DMARC) than the specific transfer encoding of the plain text part. For example, template changes in the HTML body are more impactful.

Key opinions

• Acceptable Encoding: Base64 encoding for plain text is deemed acceptable, though it typically results in a larger message size compared to Quoted-Printable encoding.
• Heuristic Choice: Email sending systems often employ heuristics to decide when to use specific encodings (e.g., for handling long lines or non-ASCII characters) to ensure compliance with SMTP line length limits.
• Dot Stuffs and Broken Systems: Issues like dot stuffing (where a line starting with a dot might be truncated) are rare and primarily occur with very poorly configured or broken smarthosts, not as a direct result of the encoding choice itself.
• Plain Text Visibility: On the modern internet, the plain text version of a multipart/alternative message is almost never seen by recipients, as contemporary email clients universally prefer to display the HTML part. This means that focusing on accessible HTML is a better use of effort.

Key considerations

• RFC Compliance: Encoding choices, including Base64, are primarily driven by RFC standards to ensure that email content can be transmitted reliably and without corruption over SMTP, not to influence spam scores directly.
• Efficiency vs. Necessity: While Quoted-Printable can be more efficient for text composed predominantly of ASCII characters, Base64 becomes necessary for certain content types, such as binary data or text containing non-ASCII characters, like Asian languages.
• Unsubscribe Link Constraints: If mailto: unsubscribe links are included in the List-Unsubscribe header, their local part is limited to 64 characters. This might necessitate shorter tokens or different handling compared to links within the email body. For more on unsubscribe mechanisms, see discussions on email encoding implications.
• Advanced Authentication: While encoding is a formatting detail, overall email security and deliverability heavily rely on robust authentication mechanisms. Our advanced guide to email authentication details how protocols like DMARC, SPF, and DKIM safeguard your emails.

Key findings

• SMTP Line Limits: RFCs, such as RFC 5321 (Simple Mail Transfer Protocol), explicitly define maximum line lengths for SMTP transmissions (e.g., 998 characters, plus CRLF). Encoding methods like Base64 are critical for ensuring that email content, including plain text, adheres to these limits when it contains very long lines or non-ASCII characters.
• Purpose of Content-Transfer-Encoding: According to RFC 2045 (MIME), this header field's primary purpose is to allow for the reliable transmission of arbitrary data over channels that may not be 8-bit clean, such as a 7-bit SMTP channel. It is a mechanism for transport, not an inherent indicator of spam.
• Multipart/Alternative Structure: The multipart/alternative MIME type, as defined in RFC 2046, is designed to offer multiple representations of the same content (e.g., plain text and HTML). Mail clients are instructed to display the richest version they can handle, meaning HTML is typically preferred.
• Unsubscribe Header Limitations: RFC 2369, which specifies the List-Unsubscribe header, implies that mailto: URIs used within it must adhere to specific syntax and length constraints for the email address's local part, which can affect the maximum allowable length of unsubscribe tokens.

Key considerations

• RFC 5321 and Line Lengths: This RFC directly addresses the 998-character limit for mail data lines (excluding CRLF). Understanding this standard helps explain why Base64 encoding may be automatically applied when content exceeds certain lengths, ensuring protocol compliance.
• Standard Practice: Employing Base64 encoding for specific types of content, such as binary files or text with non-ASCII characters, is a standard and expected practice within MIME email composition. This is not a deviation that would typically flag an email as spam.
• Client Interpretation: Email clients are designed to correctly interpret all standard encodings and multipart structures. Their ability to render the appropriate part of a multipart message is a core function, ensuring that the recipient sees the intended content.
• Deliverability Perspective: While RFCs focus on the technical mechanisms of email transmission and formatting, they indirectly support deliverability by ensuring messages are well-formed and can be processed by receiving systems. Compliance with these technical standards, such as those discussed in our article What RFC 5322 Says vs. What Actually Works, contributes to a positive sending reputation.