Does Base64 encoding of plain text emails increase spam scores?

No, generally it does not. Modern spam filters are sophisticated enough to decode Base64 content before scanning. While spammers have used Base64 for obfuscation in the past, legitimate use of it, even for plain text (often due to long lines or specific character sets), does not typically trigger high spam scores.

When is Base64 encoding typically used in emails?

Base64 encoding is primarily used for binary data, such as images and attachments, and for text that contains non-ASCII characters that cannot be easily represented otherwise. It can also be automatically applied by sending systems when plain text lines exceed a certain length, such as the RFC-recommended 998 characters.

Does Base64 encoding affect email size, and does that impact deliverability?

While Base64 encoding plain text does increase message size by about 33%, this alone is unlikely to cause deliverability issues unless emails are exceptionally large. The focus should be on overall email size and content, rather than just the encoding of the plain text part.

Is Quoted-Printable better than Base64 for plain text emails?

For plain text emails that consist primarily of ASCII characters, Quoted-Printable is often more efficient as it results in smaller message sizes while still allowing for special characters. However, if your sending system automatically uses Base64 for plain text, it's generally not a problem for deliverability.

Is it still important to include a plain text version of an email?

The existence of a plain text alternative, regardless of its specific encoding (Base64 or Quoted-Printable), is still considered a best practice. It ensures your email can be displayed by older or text-only email clients, although most modern clients prioritize the HTML version.

Does base64 encoding of plain text emails impact spam filter scores? - Technical - Email deliverability - Knowledge base

When sending emails, especially those with diverse content or character sets, encoding mechanisms are crucial for ensuring the message is delivered and displayed correctly across various email clients. Base64 is one such encoding scheme that converts binary data into an ASCII string format, making it safe for transmission over email protocols that traditionally handle text.

A common concern among email marketers and technical teams is whether the choice of encoding, specifically Base64 for plain text emails, can negatively impact email deliverability or spam filter scores. It is generally understood that spam filters prefer to see a plain text alternative in multipart emails.

The role of email encoding

Email messages, particularly those that are not purely 7-bit ASCII, require encoding to ensure they traverse various mail transfer agents (MTAs) without corruption. The Content-Transfer-Encoding header specifies how the body of an email message has been encoded. Common types include 7-bit, 8-bit, binary, Quoted-Printable, and Base64.

For plain text content, Quoted-Printable is often the preferred choice when non-ASCII characters are present, as it remains largely human-readable. Base64, on the other hand, converts every character into an encoded string, making the original text unreadable without decoding. This is typically used for binary data, such as images or attachments, or when handling character sets that cannot be represented by Quoted-Printable.

Another factor that often leads to Base64 encoding of plain text is line length. SMTP (Simple Mail Transfer Protocol) has a long-standing recommendation (though not a strict requirement for all modern servers) for lines not to exceed 998 characters, including the CRLF. If a plain text email contains very long lines of text, the sending system might automatically encode it to Base64 to comply with these implicit or explicit line length limitations, as outlined in RFC 5321, section 4.5.3.1.6. You can review the RFC 5321 documentation for more details on these specifications.

Here's a quick look at the main encoding types:

Encoding type	Description	Typical use
7-bit ASCII	Standard ASCII characters, no special encoding.	Simple plain text emails without special characters.
Quoted-Printable	Encodes non-ASCII characters into hexadecimal sequences. Largely readable.	Plain text with special characters (e.g., accents).
Base64	Encodes all data into a text-based format, increasing size by about 33%. Unreadable.	Binary attachments, images, or heavily non-ASCII content.

Historical perceptions vs. modern reality

Historically, there was a perception that Base64 encoding for plain text emails could negatively influence spam filter scores. This suspicion stemmed from the fact that spammers sometimes used encoding to obfuscate malicious content, making it harder for simple filters to detect keywords or patterns. For example, Base64 encoding was often a technique used in image spam to embed text within images, bypassing text-based filters.

However, modern spam filters (or blocklists), including prominent ones like SpamAssassin, are sophisticated enough to decode Base64 and Quoted-Printable content before applying their filtering rules. This means that the encoding itself for the plain text part is unlikely to be a significant factor in your spam score, provided the content is legitimate. For more on this, read how relevant SpamAssassin scoring is.

The primary concern with Base64 in older systems or less advanced filters was often associated with rules like MIME_BASE64_TEXT. These rules would flag emails where pure ASCII text was unnecessarily Base64 encoded, as this could be an indicator of a sender attempting to hide content. However, these rules typically contribute a very low spam score and are usually combined with other, more significant indicators of spam before an email is blocked or sent to the junk folder.

Therefore, while the concept of Base64-encoded plain text raising flags is rooted in past spam-fighting techniques, its impact on your current deliverability is minimal compared to other factors. Focusing on a positive sender reputation and legitimate content is far more impactful than the specific encoding for plain text.

Practical implications for plain text emails

For most standard textual content, especially in English or European languages, using Quoted-Printable for your plain text part is generally more efficient because it results in a smaller email size. Base64 encoding increases the size of the data by approximately 33%, which can contribute to overall email size, potentially impacting deliverability, particularly with large volumes or for recipients on slower connections.

However, if your sending system automatically encodes long lines of plain text or includes complex characters into Base64, it's typically not a cause for alarm regarding spam filtering. The key is that the email remains compliant with MIME standards and that the content itself is not spammy.

Plain text email encoding and spam filters

Size increase: Base64 encoding increases message size. While this isn't a direct spam trigger, large emails can sometimes face deliverability challenges or be perceived negatively by some receivers.
Readability: Quoted-Printable is more human-readable, which is often considered 'friendlier' for plain text parts, though modern mail clients rarely show the plain text version anyway.
Primary focus: Your overall email content, sender reputation, and authentication (SPF, DKIM, DMARC) are far more critical for inbox placement than the specific encoding of your plain text alternative.

The choice between Base64 and Quoted-Printable for plain text often comes down to the sending system's default behavior, especially when handling long strings or non-ASCII characters. For most marketing or transactional emails, whether the plain text version is Base64 encoded or not will have negligible impact on your spam score or deliverability.

When Base64 encoding is expected and safe

While we've discussed that Base64 encoding plain text isn't ideal for typical messages, it is absolutely necessary and expected for other parts of an email. When an email contains binary data, such as images, videos, or other file attachments, Base64 is the standard encoding method. This ensures that the binary data is transmitted safely over text-based email protocols without corruption. You can learn more about how images in emails can cause them to go to spam.

Furthermore, if your email (especially the HTML part) includes characters outside the standard ASCII range (e.g., emojis, characters from non-Latin alphabets), Base64 encoding is often used to ensure these characters display correctly across all email clients. For a deeper dive into how different encodings affect deliverability, check out our article on Base64 vs. UTF-8.

In these common scenarios, Base64 encoding is not only acceptable but necessary for proper email rendering and functionality. Spam filters are designed to recognize these legitimate uses of Base64 and will not penalize an email simply for containing Base64-encoded images, attachments, or international characters. The focus remains on the overall reputation of the sender and the nature of the content itself.

Views from the trenches

Best practices

Always include a plain text alternative, regardless of its encoding, as it is still considered a best practice for email clients that might not render HTML.

Prioritize email content quality and sender reputation over minor technical optimizations like plain text encoding.

Ensure your sending system is configured to handle various content transfer encodings correctly based on the email's content and character set.

Keep unsubscribe tokens and other URLs concise where possible, as excessively long strings can trigger automatic Base64 encoding or other issues.

Common pitfalls

Over-optimizing encoding when modern spam filters are advanced enough to decode content regardless of the `Content-Transfer-Encoding`.

Believing that Base64 encoding of plain text alone will significantly impact spam filter scores, overlooking more critical factors.

Neglecting the overall email deliverability strategy by focusing too much on encoding types rather than sender reputation, authentication, and content relevance.

Using Base64 encoding for simple ASCII text, leading to unnecessarily larger email sizes.

Expert tips

Focus on the content and authentication first. These are the primary drivers of inbox placement, not whether plain text is Base64 encoded.

For most common plain text, Quoted-Printable is often more efficient than Base64 due to smaller message size, but both are generally fine.

Base64 is essential for binary attachments, images, or non-ASCII characters in HTML parts and is not flagged as spam in these contexts.

If long strings in your plain text (like unsubscribe tokens) cause Base64 encoding, consider shortening them for efficiency, though it's unlikely to be a deliverability blocker.

Marketer view

A marketer from Email Geeks says they found that having a string longer than 256 characters in the email body caused their plain text version to be encoded to Base64 when sending with MailApp in Google Apps Script.

2023-08-04 - Email Geeks

Marketer view

A marketer from Email Geeks says their small-scale testing never found deliverability issues with Base64 encoded plain text, noting that they had good deliverability even when HTML was Base64 encoded.

2023-08-04 - Email Geeks

Summary of Base64 encoding and deliverability

In summary, while Base64 encoding of plain text emails might seem like a concern for spam filters due to historical practices of obfuscation, its impact in modern email environments is generally minimal. Advanced spam filtering technologies are designed to decode content before analysis, meaning the encoding type itself rarely triggers a high spam score or causes an email to be added to a blocklist (or blacklist).

Your focus should remain on maintaining a strong sender reputation, adhering to email authentication best practices (SPF, DKIM, DMARC), and ensuring your content is valuable and relevant to your recipients. These factors collectively have a far greater influence on your email deliverability than whether your plain text email is Base64 encoded.