Does using base64 vs UTF-8 HTML/Text impact email deliverability?

Key findings

• Encoding distinction: UTF-8 defines character representation, while Base64 and Quoted-Printable are content-transfer encodings for transmitting data safely via email.
• Historical spam tactic: Base64 encoding of plain text (or HTML) was used by spammers in the past to circumvent content filtering, leading to negative reputation associations.
• Modern filter capabilities: Current email filters can decode Base64 content, reducing its direct impact on deliverability from a spam evasion perspective.
• Size implications: Base64 encoding can increase email size by approximately 33%, which can indirectly affect deliverability if the total email size becomes excessive, potentially leading to slower loading times or triggering size limits. Learn more about how email code quality and size impact deliverability.
• Image embedding: Base64 is commonly used for embedding images directly into HTML emails, which can help ensure images display even if external linking is blocked. However, this contributes to email size, and support for Base64 images varies across clients.

Key considerations

• Prefer quoted-printable for text: For text-based content (plain text or HTML), Quoted-Printable is generally preferred over Base64 as it’s more human-readable when inspected and often results in smaller file sizes.
• Use base64 for binary data: Base64 is appropriate for encoding binary attachments or embedded images. For tips on managing embedded images, see how to protect email deliverability for image-only emails.
• UTF-8 for character encoding: Always ensure your email uses UTF-8 for character encoding, as it supports a wide range of characters and is widely supported by email clients. As discussed by Email on Acid, it's the standard for modern email.
• Developer responsibility: The choice of content-transfer encoding is typically handled at the MTA or mail submission engine level by developers, rather than directly by email marketers.

Key opinions

• Encoding types: Many marketers initially view UTF-8 and Base64 as comparable encoding methods, though they serve distinct purposes, one for character sets and the other for transfer encoding.
• Legacy perception: Some marketers recall a time when Base64 encoding of plain text was associated with negative sender reputation due to spammers' attempts to bypass early filters. This history can lead to a general recommendation against it for text.
• Current relevance: While the direct deliverability impact might be minimal today, it’s still considered a bad practice to Base64 encode textual content unnecessarily.
• Readability concerns: If email content is ever manually reviewed by an ISP or a human filter, using Quoted-Printable for text makes it easier to read and can foster goodwill, as it signals transparency.
• HTML email preference: Generally, marketers prefer HTML emails over plain text, provided they are well-coded and optimized for deliverability. This includes ensuring proper encoding.

Key considerations

• Content-transfer encoding for text: For any content that is primarily ASCII text (or extended ASCII, like Latin-1 characters), Quoted-Printable is the recommended content-transfer encoding. It’s more efficient and human-readable. Learn more about content-transfer encoding.
• Impact on email size: Be mindful that Base64 encoding increases the size of the email, which can affect loading times and potentially trigger size limits in some email clients or servers. This is particularly important for marketers dealing with email size and image-to-text ratio.
• Handling images: For embedded images and other binary data, Base64 encoding is standard and appropriate. This helps ensure that attachments and embedded content are transmitted correctly.
• Technical collaboration: Marketers should communicate best practices to their development teams who manage the email submission pipeline, as these encoding decisions are often made at a technical level.

Key opinions

• Functional distinction: Experts stress that UTF-8 is a character set encoding, while Base64 and Quoted-Printable are methods to encode bytes for transmission in a 7-bit ASCII environment. They are not interchangeable in purpose.
• Quoted-printable for text: For email content that is primarily text (plain text or HTML), experts strongly recommend using Quoted-Printable encoding. It's more efficient for text and aids in human readability during manual inspection.
• Base64 for binary: Base64 is considered the correct encoding for binary data, such as images, attachments, or non-western characters/fonts where the raw encoded string's readability is irrelevant.
• Minimal modern impact: Direct deliverability penalties for Base64 encoding text are largely a thing of the past because modern spam filters can easily decode it. However, it's still considered a suboptimal practice.
• User experience: Large email sizes, potentially exacerbated by excessive Base64 encoding of text or images, can negatively impact email loading times and user experience, which indirectly affects engagement metrics that ISPs monitor.

Key considerations

• Optimize encoding per content type: Ensure your Mail Transfer Agent (MTA) or email platform correctly applies Quoted-Printable for text and Base64 for binary data. This is a technical configuration crucial for optimal deliverability.
• Monitor email size: Regardless of encoding, keep email size in check. Large emails can be slow to load and may be truncated or refused by some email clients or servers. You can use tools to test your email deliverability.
• UTF-8 as standard: Always specify UTF-8 as the character set encoding for your emails to ensure broad compatibility and proper rendering of diverse characters across different email clients.
• Human review factor: While automated filters are advanced, some emails may still undergo human review (e.g., during a complaint investigation). Using appropriate encoding (Quoted-Printable for text) can foster trust and simplify the review process.

Key findings

• MIME standards: MIME dictates the use of content-transfer encodings like Base64 and Quoted-Printable to convert non-ASCII data into a 7-bit format suitable for email transmission.
• Purpose of encodings: Documentation specifies that Base64 is designed for binary data (e.g., images, attachments), while Quoted-Printable is optimized for text that is largely ASCII but may contain some non-ASCII characters.
• Size impact: Technical specifications confirm that Base64 encoding generally increases the size of the original data by about 33% (4 Base64 characters for every 3 original bytes). This increase can be significant for large emails.
• UTF-8 as character set: UTF-8 is widely documented as the preferred character encoding for email content, offering broad support for international characters and preventing display issues.

Key considerations

• Adherence to rfc standards: Following RFC (Request for Comments) standards for email encoding ensures maximum compatibility across various email systems and clients. This includes correct declaration of Content-Type and Content-Transfer-Encoding headers.
• Optimizing for load time: Excessive use of Base64 encoding for content that could be more efficiently encoded (or externally linked) can lead to larger email sizes, impacting load times and potentially affecting deliverability or client processing. This is a crucial aspect of technical solutions for email deliverability.
• Header importance: Properly setting the Content-Type header (e.g., text/html; charset=UTF-8) and Content-Transfer-Encoding is critical for email clients to correctly interpret the message body. Misconfiguration can lead to display issues or spam flagging.