How to identify and resolve MIME encoding issues in List-Unsubscribe headers?

Key findings

• Invalid encoding: The List-Unsubscribe header should not be MIME encoded; its value (URI) must remain plain, unencoded text according to RFCs. An indicator of this encoding is the presence of strings like =?us-ascii?Q?=3C.
• Misleading displays: Email clients, notably Gmail's Show Original view, can silently MIME decode headers, falsely presenting them as correctly formatted, even when the raw email file shows the encoding issue.
• Accurate diagnosis: Specialized email analysis tools or direct inspection of the downloaded raw EML file are necessary to reveal actual MIME encoding issues not visible in standard client views.
• Platform-induced: Email service providers (ESPs) or sending platforms can sometimes incorrectly apply MIME encoding to headers like List-Unsubscribe, treating them as if they contain display names requiring encoding.
• Deliverability impact: While some mailbox providers might tolerate MIME encoding, it is not compliant with RFC compliance and can lead to unsubscribe failures, affecting sender reputation and potentially leading to blocklisting or blacklisting.

Key considerations

• Vendor assessment: If you detect MIME encoding issues, investigate your ESP's header generation logic, as this is often an automated process on their end.
• Header formatting: The List-Unsubscribe header should simply contain mailto: or https: URIs, enclosed in angle brackets, without any character set or encoding prefixes (e.g., =?us-ascii?Q?). Learn more about header encoding behavior.
• Testing strategy: Implement regular, thorough testing using tools that analyze the raw email payload to catch hidden header issues before they impact deliverability.
• Compliance upkeep: Stay updated on RFCs and industry best practices for email headers to ensure ongoing compliance and optimal inbox placement.

Key opinions

• Diagnostic tool reliance: Marketers frequently use specialized tools to validate email headers because native email client views can hide underlying encoding issues.
• Gmail's 'original' view: There's widespread agreement that Gmail's Show Original often silently decodes headers, presenting a misleading view of the raw email structure. This makes it difficult to view headers accurately.
• Hidden encoding: MIME encoding in List-Unsubscribe headers can be subtle, not always causing immediate failures, but posing a risk for compliance and future deliverability.
• Platform responsibility: Marketers often point to their email sending platforms (ESPs) as the source of unintended MIME encoding, particularly when the platform treats header values like friendly From addresses.

Key considerations

• Raw email inspection: To accurately diagnose header issues, always download the original email file (EML) and inspect its raw content.
• ESP troubleshooting: If MIME encoding is detected, open a dialogue with your ESP, providing detailed examples and asking about their header generation processes for List-Unsubscribe, particularly in relation to SendGrid unsubscribe issues.
• Proactive monitoring: Regularly check your email headers, especially after platform updates or configuration changes, to catch and resolve issues promptly.
• Unsubscribe link validation: Ensure that the functionality of your unsubscribe links, whether mailto: or HTTP/HTTPS, is not compromised by incorrect header encoding, which could impact sender reputation.

Key opinions

• Gmail's deceptive display: Experts commonly find that Gmail's Show Original silently decodes headers, making it unreliable for genuine header inspection, as highlighted by Word to the Wise.
• Non-compliant encoding: MIME encoding in List-Unsubscribe is a clear sign of non-compliance with email standards, even if some mailbox providers might process it without explicit errors.
• DKIM signing necessity: For compliance with RFC 8058, the List-Unsubscribe and List-Unsubscribe-Post headers must be included in the DKIM signature, which presupposes their correct, unencoded format. See Microsoft's support for RFC 8058.
• Platform-generated data: The reported data from analysis tools is typically generated at email delivery time, meaning that historical links to test results should remain consistent unless the sending platform changes its behavior.

Key considerations

• Prioritize raw headers: Always inspect the raw message source, preferably by downloading the EML file, to ensure complete accuracy in header validation, bypassing any client-side transformations.
• ESP collaboration: Work closely with your ESP to ensure their system properly handles List-Unsubscribe headers according to standards, preventing unintended encoding. Also, understand why your ESP DKIM signs certain headers.
• Continuous validation: Regularly validate your email headers using reliable analysis tools to promptly detect any new or recurring encoding issues.
• Stay informed: Keep abreast of RFC updates and best practices for email authentication and header formatting to maintain optimal deliverability.

Key findings

• RFC 2369 specifications: This RFC defines the List-Unsubscribe header, stating it should contain URIs (either mailto: or http/https) enclosed in angle brackets, with no mention of MIME encoding for the URI value itself. Find more at IETF Datatracker for RFC 2369.
• RFC 2047 on encoded words: This RFC specifies the encoded-word syntax (=?charset?encoding?encoded_text?=) for MIME-encoding, explicitly stating it applies to 'display-names' and not to structured field bodies like addresses or message-IDs, reinforcing that List-Unsubscribe values should not be encoded this way.
• Header folding vs. encoding: RFCs allow headers to be folded (broken into multiple lines) to manage length without introducing character set or transfer encodings into the header value itself. This is distinct from MIME encoding.
• MIME-Version header context: The presence of a MIME-Version header indicates that MIME encoding was used for the message content, but it does not imply that structured header fields should also be MIME encoded.

Key considerations

• Strict URI adherence: The value for List-Unsubscribe must strictly follow URI syntax (RFC 3986), ensuring it's a plain string and not subject to character set encoding.
• Character set limitations: For standard email headers like List-Unsubscribe, only US-ASCII characters are expected. The presence of non-ASCII characters or complex encoding suggests a misconfiguration. Also, consider if quoted printable text is allowed.
• Header field specificity: Different header fields have distinct syntax rules. It's critical not to apply MIME encoding universally to all headers, as this contradicts the specific definitions for fields like List-Unsubscribe. Pay attention to what RFC 5322 says versus what actually works.
• Tool interpretation: Understand how email analysis tools interpret and display headers, as some may perform decoding before presenting the data, potentially masking underlying encoding issues in the raw message.