Summary
The consensus from experts, marketers, and email documentation is that using quoted-printable encoding in the List-Unsubscribe header is generally discouraged due to potential compatibility issues. While RFC 2369 doesn't explicitly forbid it, RFC 2047 restricts encoded words to specific header fields, excluding List-Unsubscribe. Many email clients and ISPs may not correctly interpret encoded characters, leading to unsubscribe failures and deliverability problems. Keeping headers simple, using plain ASCII characters, or URL-encoding URLs within the header are recommended best practices. Thorough testing across different email clients and ISPs is crucial if quoted-printable encoding is used.
Key findings
- Compatibility Problems: Inconsistent support for quoted-printable encoding in List-Unsubscribe across different email clients (e.g., Hotmail) and ISPs.
- Deliverability Risks: Poorly encoded headers increase the risk of emails being filtered as spam or rejected.
- RFC 2047 Restrictions: RFC 2047 limits the use of encoded words to specific header fields, excluding List-Unsubscribe.
- Simplicity Best Practice: Maintaining simple, easily parsable headers enhances deliverability.
- URL Encoding Necessity: Any URLs used within the List-Unsubscribe header must be URL-encoded.
- Non-Standard Encoding: Avoid using non-standard encoding methods in headers.
Key considerations
- Comprehensive Testing: Perform extensive testing across diverse email clients and ISPs to identify compatibility issues.
- ASCII/URL Encoding Alternatives: Prefer plain ASCII or URL-encoded URLs as safer alternatives to quoted-printable encoding.
- Standards Compliance: Ensure adherence to all relevant email standards (RFCs).
- ESP Configuration: Understand how your ESP handles header encoding and adjust settings as needed.
- Broken Links: Check for any broken unsubscribe links related to encoding issues
What email marketers say
11 marketer opinions
While technically feasible, the consensus among email marketers is that using quoted-printable encoding in the List-Unsubscribe header is generally discouraged due to potential compatibility issues across different email clients and ISPs. Plain ASCII characters or URL-encoded URLs are preferred for maximum compatibility and deliverability. Testing across various email clients and ISPs is crucial if quoted-printable encoding is used.
Key opinions
- Compatibility Issues: Quoted-printable encoding can lead to compatibility issues with certain email clients (e.g., Hotmail) and ISPs, causing unsubscribe links to fail.
- Deliverability Risks: Improperly encoded headers can negatively impact email deliverability, leading to emails being filtered or rejected.
- URL Encoding: If the List-Unsubscribe header includes a URL, the URL itself should be properly URL-encoded (percent-encoded) to handle special characters.
- Simplicity Preferred: Keeping email headers as simple as possible, ideally using plain ASCII characters, improves deliverability.
- Non-Standard Encoding: Avoid using non-standard encoding methods in headers as it increases the chances of deliverability problems.
Key considerations
- Testing: Thoroughly test the rendering of emails with quoted-printable encoding in the List-Unsubscribe header across various email clients and ISPs.
- Alternatives: Consider using plain ASCII characters or URL-encoded URLs in the List-Unsubscribe header as safer alternatives to quoted-printable encoding.
- Standards Compliance: Ensure compliance with email standards for headers to avoid deliverability issues.
- Error Handling: Implement robust error handling to catch encoding-related issues and prevent broken unsubscribe links.
- ESP Defaults: Be aware of ESP's default encoding practices and how they may impact header compatibility.
Marketer view
Email marketer from Litmus Blog emphasizes the importance of testing email header compatibility across various email clients. He adds, that if you must use quoted-printable encoding, double-check how different clients render it.
5 Dec 2022 - Litmus Blog
Marketer view
Email marketer from Mailchimp specifies that while encoding the entirety of the List-Unsubscribe is not generally recommended, the associated URL should always be URL-encoded.
28 Nov 2023 - Mailchimp
What the experts say
3 expert opinions
Experts generally advise against using quoted-printable text in the List-Unsubscribe header. While technically possible, it's considered problematic due to potential compatibility issues with various email clients and ISPs. The consensus is that using quoted-printable can lead to misinterpretation of characters and unsubscribe failures. Sticking to ASCII compatibility is the recommended best practice.
Key opinions
- Compatibility Issues: Many ISPs and email clients may not correctly interpret quoted-printable encoded characters in the List-Unsubscribe header.
- Unsubscribe Failures: Using quoted-printable in the List-Unsubscribe header can lead to unsubscribe failures due to misinterpretation by email clients.
- RFC Limitations: RFC 2047 specifies limited header fields where encoded words are permitted, and List-Unsubscribe isn't one of them.
- Badly Formed Header: Using quoted-printable outside of the specified fields can result in a badly formed header that may only work accidentally at some ISPs.
- ASCII Preference: Maintaining ASCII compatibility is the recommended best practice for the List-Unsubscribe header.
Key considerations
- Alternatives: Consider using alternatives to quoted-printable, such as plain ASCII or URL-encoded characters, in the List-Unsubscribe header.
- Testing: If quoted-printable is unavoidable, test thoroughly across different email clients and ISPs to ensure correct interpretation.
- Standards Compliance: Adhere to email standards and RFC specifications to ensure proper formatting and avoid potential issues with deliverability.
Expert view
Expert from Word to the Wise advises against using quoted-printable in the List-Unsubscribe header. While technically possible, it can create compatibility issues across different email clients, and best practice is to maintain ASCII compatibility.
7 Nov 2023 - Word to the Wise
Expert view
Expert from Email Geeks explains that quoted printable text is likely not allowed in the List-Unsubscribe header in any useful way, based on RFC 2047 and 2369. He clarifies that RFC 2047 encoded words are only allowed in specific header fields like Subject, Comments, or within a 'phrase' before an email address in the From header. Using it elsewhere results in a badly formed header that may only work accidentally at some ISPs.
19 May 2024 - Email Geeks
What the documentation says
5 technical articles
Email documentation suggests caution when using quoted-printable text in the List-Unsubscribe header. While RFC 2369 doesn't explicitly prohibit it, RFC 2047 restricts encoded words to specific header fields, excluding List-Unsubscribe. General header formatting rules and the importance of valid headers for deliverability are emphasized. Testing compatibility across different email clients is crucial.
Key findings
- RFC 2047 Restrictions: RFC 2047 limits encoded words to specific header fields, and List-Unsubscribe is not explicitly listed.
- RFC 2369 Ambiguity: RFC 2369 doesn't explicitly prohibit encoded words in the List-Unsubscribe header, but implementations must ensure validity and parsability.
- Formatting Rules: Header fields must adhere to specific formatting rules to ensure proper interpretation by mail clients.
- Deliverability Impact: Improper header encoding can cause filtering or rejection of emails.
- MIME Encoding Caution: While MIME encoding is supported for email content, its use in headers should be approached with caution.
Key considerations
- Compatibility Testing: Thoroughly test header encoding across different email clients to ensure compatibility.
- Validity and Parsability: Ensure that the resulting List-Unsubscribe header remains valid and parsable if using encoded words.
- Standards Adherence: Adhere to email standards and RFC specifications for header formatting.
- General Guidance: Apply general header formatting guidelines to the List-Unsubscribe header, even if it's not explicitly addressed in documentation.
Technical article
Documentation from ietf.org explains that RFC 2047 defines how to represent non-ASCII characters in email headers. It specifies that encoded words (using quoted-printable or base64 encoding) are only permitted in specific header fields like Subject, Comments, or certain address fields. The List-Unsubscribe header is not among those explicitly mentioned.
7 Dec 2021 - ietf.org
Technical article
Documentation from Microsoft highlights that valid and properly formatted email headers are critical for email deliverability. Improper encoding in headers can cause filtering or rejection. Though it doesn't specifically discuss List-Unsubscribe, this principle applies broadly.
28 Nov 2024 - Microsoft
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