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

Summary

Using base64 encoding for text content instead of UTF-8 or quoted-printable negatively impacts email deliverability. Base64 increases message size, can trigger spam filters due to historical association with spam, and is generally inappropriate for text. Quoted-printable is preferred for text due to readability and smaller size. Proper email setup (SPF, DKIM, DMARC), clean HTML coding, good sender reputation, and avoiding spam trigger words are crucial. Character set encoding (especially UTF-8 and non-ASCII) needs correct implementation.

Key findings

  • Encoding Impact: Base64 negatively impacts deliverability when used for text.
  • Spam Triggers: Excessive/inappropriate encoding triggers spam filters.
  • Message Size: Base64 increases message size.
  • Encoding Alternatives: Quoted-printable is preferred for text; UTF-8 requires proper handling.
  • Authentication: SPF, DKIM, DMARC are essential for authentication and deliverability.
  • HTML & Content: Clean HTML, no spam trigger words, and sender reputation are vital.

Key considerations

  • Choose Encoding Wisely: Use quoted-printable for text, base64 only for binary.
  • Minimize Message Size: Avoid unnecessary bloat from base64 encoding.
  • Implement Authentication: Set up SPF, DKIM, and DMARC to improve trust.
  • Clean HTML: Ensure clean, well-formatted HTML to avoid spam triggers.
  • Monitor Reputation: Maintain a good sender reputation.
  • Test Emails: Test emails across clients to ensure proper rendering and avoid spam filters.
  • Character Sets: Correctly encode character sets, particularly non-ASCII characters and UTF-8.

What email marketers say
9Marketer opinions

Using base64 encoding for text content instead of UTF-8 can negatively impact email deliverability. Base64 increases message size, potentially causing rejection by servers or impacting sender reputation. Excessive or unnecessary encoding can trigger spam filters, as it appears suspicious. Proper email setup (SPF, DKIM, DMARC) and clean HTML coding are crucial for deliverability. Maintaining a good sender reputation and avoiding spam trigger words also play a significant role.

Key opinions

  • Message Size: Base64 encoding increases message size, which can lead to deliverability issues.
  • Spam Filters: Excessive encoding can trigger spam filters.
  • Authentication: Proper email authentication (SPF, DKIM, DMARC) is essential for deliverability.
  • HTML Coding: Clean and well-formatted HTML is preferable for deliverability.
  • Sender Reputation: A good sender reputation is crucial for emails to reach the inbox.
  • Spam Trigger Words: Using spam trigger words can negatively impact deliverability.

Key considerations

  • Encoding Choice: Prefer UTF-8 for text content and avoid unnecessary base64 encoding.
  • Email Size: Keep email size to a minimum to avoid rejection by servers.
  • HTML Structure: Use clean and simple HTML structures.
  • Authentication Setup: Implement SPF, DKIM, and DMARC for email authentication.
  • Sender Reputation Management: Maintain a good sender reputation by warming up IP addresses, cleaning up contact lists, and avoiding spam traps.
  • Content Optimization: Avoid spam trigger words and test email rendering across different clients.
Marketer view

Email marketer from Sendinblue recommends keeping HTML email designs simple and using inline CSS for better compatibility across email clients. Avoid complex or unusual HTML structures, which could inadvertently trigger spam filters. Clean, well-formatted HTML is preferable for deliverability.

June 2022 - Sendinblue
Marketer view

Email marketer from Reddit shares that excessively encoding an email message may make it appear suspicious. Suggests that it can trigger spam filters if the encoding is seen as an attempt to hide the content.

March 2024 - Reddit
Marketer view

Email marketer from Mailjet shares that using clean HTML code, avoiding excessive use of images without alt text, and maintaining a good sender reputation are key deliverability factors. While not directly mentioning base64, impliedly avoid complex or unusual encoding of HTML which could negatively impact the rendering or perception of the email.

March 2022 - Mailjet
Marketer view

Email marketer from Stack Overflow explains that base64 encoding increases message size by approximately 33%, which can be significant for large attachments or complex emails. Increased size can contribute to deliverability issues if it causes emails to be rejected by servers with size limits or if it negatively impacts sender reputation due to increased bandwidth usage. Using UTF-8 and specifying the correct character set is generally preferable for text content.

December 2022 - Stack Overflow
Marketer view

Email marketer from HubSpot stresses the importance of a good sender reputation. If your emails are flagged as spam, the ESPs will automatically move your future emails to the spam folder, regardless of the encoding used. You can avoid this by warming up your IP address, cleaning up your contact list, and authenticating your emails.

March 2022 - HubSpot
Marketer view

Email marketer from EmailToolTester recommends using email testing tools to check how your emails render across different email clients and whether they trigger spam filters. These tools can help identify issues related to encoding, HTML structure, or content that might affect deliverability. The tool will help you confirm if you have implemented the encodings and HTML correctly.

May 2024 - EmailToolTester
Marketer view

Email marketer from Litmus shares that proper HTML coding is important. Improperly formatted or excessively complex HTML can trigger spam filters. Litmus recommends using well-formed HTML and avoiding techniques that are commonly associated with spam, such as obfuscated code.

June 2023 - Litmus
Marketer view

Email marketer from Reddit emphasizes the importance of using a proper email setup (SPF, DKIM, DMARC) as a prerequisite for any encoding considerations. Shares that focusing on authentication is generally more impactful than specific encoding choices for deliverability.

April 2023 - Reddit
Marketer view

Email marketer from Constant Contact recommends avoiding spam trigger words in your email content and subject line. Spam trigger words may cause email provider spam filters to flag your email as spam, which would affect your deliverability rate.

February 2025 - Constant Contact

What the experts say
6Expert opinions

Experts agree that using base64 encoding for text instead of quoted-printable or UTF-8 negatively impacts email deliverability. Base64's historical association with spam and its inappropriateness for text-based content raise red flags for spam filters. Quoted-printable is recommended for text, especially non-ASCII, due to better readability and smaller size. Proper character set declaration is also crucial. The choice between quoted-printable and base64 should be based on content type, with base64 reserved for binary data.

Key opinions

  • Historical Spam Association: Base64 encoding was previously used by spammers, leading to negative reputation.
  • Encoding Appropriateness: Base64 is less suitable for text content than quoted-printable or UTF-8.
  • Readability and Size: Quoted-printable offers better readability and smaller size for text-based emails.
  • Spam Filter Triggers: Encoding content unnecessarily can trigger spam filters.
  • Character Set: Properly declaring the character set is essential for accurate display.

Key considerations

  • Content Type: Choose encoding based on content type: quoted-printable for text, base64 for binary.
  • Encoding Best Practices: Avoid base64 encoding of text; use quoted-printable unless there's a specific need for base64.
  • Spam Filter Avoidance: Refrain from unnecessary encoding to prevent triggering spam filters.
  • Character Set Declaration: Ensure proper character set declaration (e.g., UTF-8) for accurate text display.
  • Human Review: Consider human review: quoted-printable allows easier content inspection.
Expert view

Expert from Spam Resource shares that email filters look at a variety of things but in general if you are encoding things that should not be encoded that it will likely be flagged negatively by spam filters. If you base64 encode text that should not be encoded you will negatively impact your deliverability.

June 2023 - Spam Resource
Expert view

Expert from Email Geeks explains that there was a time when base64 encoding plain text was assigned negative reputation due to spammers using it to bypass early content filters. Filters started cracking the Base64 encoding. Recommends Quoted Printable encoding if high ASCII is needed.

May 2024 - Email Geeks
Expert view

Expert from Word to the Wise explains that quoted-printable is the preferred encoding for text, particularly if the character set is not ASCII, and recommends using it unless there is a specific reason to use base64. Shares that base64 is more appropriate for binary content. Properly declaring the character set is also essential.

April 2023 - Word to the Wise
Expert view

Expert from Email Geeks shares that it's bad practice to base64 encode something that is text, use quoted-printable encoding.

September 2023 - Email Geeks
Expert view

Expert from Email Geeks shares that while base64 and quoted-printable do the same thing at a high level, quoted-printable is better for text content because it's more readable and smaller, while base64 is better for non-western text content where readability doesn't matter and base64 is smaller. Also indicates that, in 2020, these encoding choices don't directly affect deliverability, but using quoted-printable makes it easier for humans reviewing the email to see that no shenanigans are being attempted, which can help if you're in a situation where someone is looking at your mail to make a human decision about it.

December 2021 - Email Geeks
Expert view

Expert from Email Geeks simplifies the advice to "Always use quoted-printable for anything that looks vaguely like ascii text". Explains that utf8 is a character encoding to describe glyphs via bytes, like utf16, ascii, etc., while Base64 and quoted-printable "armor" byte streams for email transmission by converting them to 7-bit ascii characters. You always need both a character encoding and a content-transfer encoding.

January 2023 - Email Geeks

What the documentation says
5Technical articles

Technical documentation emphasizes the importance of using appropriate content transfer and character encodings for email deliverability. Base64 and quoted-printable serve different purposes: base64 for binary data and quoted-printable for text. Proper character set encoding, especially for non-ASCII characters, is critical to avoid display issues and spam filters. UTF-8 is a widely supported character encoding, but it requires correct implementation. Furthermore, email authentication (SPF, DKIM, DMARC) is crucial for establishing trust and preventing emails from being marked as spam, particularly for Gmail users.

Key findings

  • Encoding Purpose: Base64 is for binary data, quoted-printable for text.
  • Character Set Encoding: Proper encoding of character sets (especially non-ASCII) is crucial.
  • UTF-8 Importance: UTF-8 is dominant but requires correct implementation.
  • Display Issues: Incorrect encoding can cause display issues (mojibake) that affect deliverability.
  • Email Authentication: SPF, DKIM, and DMARC are crucial for deliverability, especially for Gmail.

Key considerations

  • Encoding Choice: Select content transfer encoding based on data type (binary vs. text).
  • Character Set Implementation: Implement character set encoding correctly, following RFC 2047 guidelines for headers.
  • UTF-8 Handling: Ensure proper handling of UTF-8 encoding to prevent display errors.
  • Authentication Setup: Set up SPF, DKIM, and DMARC to authenticate emails and improve deliverability.
  • Email Testing: Test emails to ensure correct rendering and avoid spam filters.
Technical article

Documentation from Google shares that authenticating your email with SPF, DKIM, and DMARC is crucial for deliverability to Gmail users. A strong authentication setup improves trust and prevents your emails from being marked as spam. Ensure your domain meets Google's email authentication standards.

July 2023 - Google
Technical article

Documentation from MDN explains that UTF-8 is a dominant character encoding for the web and email. Using UTF-8 ensures broad compatibility and correct display of text across different systems. Improper handling of UTF-8 can lead to garbled text, which might affect deliverability if it makes the email appear unprofessional or suspicious.

July 2021 - Mozilla Developer Network
Technical article

Documentation from Microsoft shares that choosing the correct character encoding is critical for displaying text correctly. Using incorrect character encodings can lead to mojibake (garbled text), which may negatively impact the user experience and potentially affect deliverability if the content becomes unreadable or suspicious.

November 2024 - Microsoft
Technical article

Documentation from IETF explains that content-transfer-encodings (like base64 and quoted-printable) are used to represent arbitrary binary data in a format suitable for transport over email. The choice of encoding depends on the nature of the data being transmitted. Base64 is designed for binary data, while quoted-printable is intended for mostly text data.

March 2022 - RFC 1341
Technical article

Documentation from Oracle explains that character sets used in mail headers must be encoded according to RFC 2047, especially when using non-ASCII characters. Failure to properly encode these headers can lead to display issues and potentially trigger spam filters. UTF-8 is often recommended but needs correct implementation.

October 2024 - Oracle Communications Messaging Server