Summary
The appearance of '<undefined>' in the List-Unsubscribe header within Gmail is a perplexing issue for many senders. This anomaly can manifest inconsistently, sometimes showing a valid unsubscribe URL and other times displaying this undefined value, even for the same message. This behavior is particularly noticeable in Gmail's Show Original or Download Message views, leading to confusion about its root cause. Given Google's recent emphasis on one-click unsubscribe compliance, such inconsistencies raise questions about potential system bugs or ongoing internal testing as the June 1st enforcement date approaches.
Key findings
- Inconsistent display: The List-Unsubscribe header can sometimes show as <undefined> in Gmail's header views, impacting sender reputation and potentially leading to messages being routed to spam or even a blocklist.
- Transient behavior: This issue is often temporary, with the correct unsubscribe URL appearing later for the same message without any sender-side changes.
- Recipient-specific: The problem appears to be tied to specific recipient email addresses or domains within Google's ecosystem (e.g., Gmail.com versus Google Workspace addresses).
- Impact of upcoming changes: This peculiar behavior may be a symptom of Gmail's ongoing testing related to its upcoming one-click unsubscribe enforcement, as discussed on MailBluster.
Key considerations
- Verify headers: Always check both Show Original and Download Message in Gmail for the most accurate header information, as one may display differently from the other, potentially impacting why Gmail sometimes does not display the List-Unsubscribe header.
- Monitor changes: Stay informed about ongoing changes and updates by major mailbox providers like Gmail regarding sender requirements.
- Test across accounts: Conduct thorough testing across various recipient email types (personal Gmail, Google Workspace, other providers) to identify inconsistencies in List-Unsubscribe header display.
- Review sender configuration: While this may appear to be a Gmail issue, always ensure your own List-Unsubscribe header implementation is robust and compliant with RFC standards, particularly concerning the requirements for List-Unsubscribe headers to comply with Yahoo and Gmail.
What email marketers say
Email marketers grappling with deliverability issues often encounter bewildering anomalies. The sudden appearance of '<undefined>' in a crucial header like List-Unsubscribe without any changes to their sending infrastructure can be particularly frustrating. These observations highlight the dynamic and often unpredictable nature of email deliverability, especially when dealing with major mailbox providers like Gmail who are constantly refining their systems. Marketers are left to meticulously track these inconsistencies, often concluding that the issue lies beyond their control.
Key opinions
- Sudden appearance: The <undefined> value can unexpectedly appear in headers, even when no sender-side changes have been made to the List-Unsubscribe mechanism.
- Inconsistent experience: The same email campaign might exhibit the problem for one recipient address but not another, even within the same Gmail domain, which adds to the difficulty of troubleshooting why the Gmail unsubscribe button does not always appear in emails.
- Temporarily resolved: The List-Unsubscribe URL can sometimes magically reappear correctly after initially displaying <undefined>.
- Troubleshooting challenges: The transient and inconsistent nature of the issue makes it very challenging for senders to pinpoint a clear root cause on their end.
Key considerations
- Document observations: Maintain detailed logs of when and where the <undefined> issue occurs to assist in identifying patterns and potential triggers.
- Communicate with ESPs: If you suspect issues originating from your Email Service Provider (ESP) platform, open a support ticket with them.
- Stay updated: Follow industry news and official announcements regarding Gmail and Yahoo's new sender requirements, as detailed in articles like Gmail launches "Manage subscriptions" directly in Gmail - everything you need to know.
- Test thoroughly: Prioritize testing List-Unsubscribe header behavior across a diverse range of Gmail recipient types to catch inconsistencies.
Marketer view
Email marketer from Email Geeks observes issues with Gmail and list-unsubscribe, noting the List-Unsubscribe URL appearing as <undefined> in email headers for some recipients. This was reported to be an intermittent problem over several days, causing confusion and requiring constant monitoring.
Marketer view
Email marketer from Reddit notes that sporadic List-Unsubscribe header issues can make it extremely difficult to accurately diagnose broader deliverability problems. They highlight that such inconsistencies lead to wasted time in troubleshooting.
What the experts say
Email deliverability experts agree that the appearance of a non-standard value like '<undefined>' in a critical header usually signals an underlying problem. While senders should always ensure their own configurations are flawless, the transient and inconsistent nature of this specific issue, especially when it spontaneously resolves, points strongly towards potential bugs or ongoing system adjustments within Gmail itself. This is particularly relevant given Google's recent rollout of new sender requirements and one-click unsubscribe mandates.
Key opinions
- Sign of a bug: The presence of undefined in email headers is generally a clear indicator of a software bug somewhere in the email processing pipeline, not a valid value.
- Internal google issue: If the List-Unsubscribe header value actually changes from <undefined> to a valid URL within the same message over time, it strongly suggests internal Google code is not yet fully stable.
- Pre-enforcement testing: This behavior could be part of Gmail's testing of its compliance confirmation mechanisms, particularly ahead of the June 1st one-click unsubscribe enforcement, which relates to why List-Unsubscribe requests from Gmail are increasing suddenly.
- Focus on sender pipeline: While Google might be experiencing issues, experts still recommend that senders meticulously review their message composition and sending infrastructure for any potential vulnerabilities or misconfigurations.
Key considerations
- Distinguish display issues: It's crucial to determine whether the <undefined> issue is a mere display bug in Gmail's interface or an actual corruption of the header in transit, impacting the unsubscribe header functionality on Gmail and other email services.
- Anticipate changes: Recognise that large mailbox providers frequently roll out changes and tests that can cause temporary anomalies and require constant adaptation from senders.
- Validate List-Unsubscribe format: Ensure your List-Unsubscribe header adheres strictly to RFC 2369 for proper parsing and avoid common DMARC issues from Google and Yahoo.
- Monitor gmail postmaster tools: Actively use Google Postmaster Tools to identify any wider trends or issues impacting your domain's deliverability, including potential blacklisting or blocklisting.
Expert view
Expert from Email Geeks questioned where the <undefined> was being observed, emphasizing the need to distinguish between Show Original and Download Message views for true header accuracy.
Expert view
Expert from Spam Resource emphasizes that inconsistent header display issues can sometimes be a symptom of broader mail transfer agent (MTA) problems at the receiving end, rather than solely a sender-side issue. This requires careful diagnosis.
What the documentation says
The foundational documentation for email, specifically RFCs, outlines precise requirements for headers like List-Unsubscribe. These specifications do not include '<undefined>' as a valid state or value, implying that its appearance is an aberration rather than an intended outcome. Furthermore, Google's own sender guidelines reinforce the need for properly formatted and functional unsubscribe mechanisms. The discrepancy between documented standards and observed behavior points to a system-level issue that deviates from established norms.
Key findings
- RFC compliance: The List-Unsubscribe header is strictly defined by RFC 2369, requiring specific URL or mailto formats, explicitly not an <undefined> value.
- One-click unsubscribe: RFC 8058 specifies the List-Unsubscribe-Post header for one-click unsubscribe, which Gmail and Yahoo are now actively enforcing for bulk senders.
- Gmail's sender requirements: Gmail's official guidelines stipulate proper authentication (SPF, DKIM, DMARC) and correctly implemented List-Unsubscribe headers for all bulk senders to ensure a positive user experience.
- Error handling: Standard email documentation does not describe <undefined> as a valid response or state for a parsed header, inherently implying that its appearance indicates an error or unexpected parsing failure.
Key considerations
- Adhere to RFCs: Always construct your List-Unsubscribe headers in strict accordance with relevant RFCs (2369 and 8058) to ensure proper interpretation by mailbox providers, and consult a simple guide to DMARC, SPF, and DKIM to bolster your email authentication.
- Implement one-click: Ensure both List-Unsubscribe (URL/mailto) and List-Unsubscribe-Post headers are correctly implemented to facilitate one-click unsubscribe functionality and avoid issues like why Klaviyo DKIM signs the List-Unsubscribe header.
- Review gmail's guidelines: Regularly check Google's official sender guidelines for any updates or clarifications on header requirements and best practices.
- Monitor DMARC reports: Utilize DMARC reports to gain insights into how your email headers are being interpreted by receiving mail servers, which can help troubleshoot unexpected behaviors or common DMARC issues.
Technical article
Documentation from RFC 2369 explains that the List-Unsubscribe header provides a method for automated unsubscribe requests, which should typically contain either a mailto URI, a URL, or both, enabling straightforward user opt-out.
Technical article
Documentation from RFC 8058 specifies that for true one-click unsubscribe functionality, the List-Unsubscribe-Post: List-Unsubscribe=One-Click header must be included alongside a List-Unsubscribe header that specifically supports a POST request.
Related resources
1 resources
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