Why is Gmail showing '<undefined>' in the List-Unsubscribe header?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Jun 2025
Updated 17 Aug 2025
8 min read
Recently, I've observed a puzzling issue where Gmail sometimes displays <undefined> in the List-Unsubscribe header, instead of the expected unsubscribe URL. This can be quite alarming for senders, especially when emails from the same campaign or domain show inconsistent behavior across different recipient addresses. It's a critical element for user experience and maintaining good sender reputation, as a functional unsubscribe option helps prevent recipients from marking your emails as spam.
Understanding the List-Unsubscribe header
The List-Unsubscribe header is a vital component of legitimate email marketing. It provides a simple, one-click method for recipients to opt out of future mailings, significantly improving user experience and reducing the likelihood of spam complaints. A properly configured header allows mail clients like Gmail to display an unsubscribe button directly within the email interface, offering a convenient alternative to searching for a link within the message body.
This header typically supports two main formats: a mailto URI for email-based unsubscribes, and an HTTP(S) URL for web-based one-click unsubscribes. While both formats are valid, recent mandates from major mailbox providers emphasize the HTTP(S) URL for a seamless one-click experience. When this header is malformed or missing, Gmail may not display the unsubscribe option at all, or worse, display something like <undefined>, indicating a parsing error.
The proper implementation of this header is now more critical than ever, especially with the new sender requirements imposed by Gmail and Yahoo. These changes mandate an easy one-click unsubscribe process for bulk senders to maintain good deliverability and avoid being flagged. A non-functional or undefined List-Unsubscribe header could lead to significant deliverability issues, including emails going to spam or even being rejected.
Example of a compliant List-Unsubscribe headertext
Seeing <undefined> in the List-Unsubscribe header is certainly an anomaly. This string isn't part of any standard email header specification and typically indicates a problem in the parsing or processing of the header value. It points to a bug, either within your email sending system's message composition or within the recipient's mail client, such as Gmail.
One primary reason for this could be a misconfiguration on the sender's side. If the List-Unsubscribe header is being dynamically generated, and a variable or parameter used to construct the URL is null or empty, it could result in an undefined value. This often happens if the email service provider (ESP) or marketing automation platform fails to correctly inject the unsubscribe URL into the header, or if there's an issue with the data used to personalize the link. You might also encounter this if there are MIME encoding issues with the header itself.
Sender-side issues
Malformed header: Incorrect syntax or missing parts of the URL. Ensure both mailto and HTTP(S) links are valid and correctly formatted.
Dynamic content errors: Variables for the unsubscribe URL or email address not resolving correctly in your ESP or MTA.
ESP configuration: Your email service provider might be inserting a placeholder that isn't being replaced properly before sending.
Gmail processing quirks
Internal bug: Temporary glitches within Gmail's system that cause incorrect header parsing, especially during rollouts or testing phases.
Reputation-based filtering: For lower reputation senders, Gmail might intentionally suppress or alter the unsubscribe link, although 'undefined' is an unusual manifestation of this.
Testing new features: Gmail is constantly refining its unsubscribe enforcement. Anomalies can occur during these testing periods.
Inconsistent behavior, as reported in the search results and by some users, suggests that Google's own systems might be experiencing a temporary bug or are in a testing phase. Some users have reported that the unsubscribe button may not appear until Gmail has assessed the domain's reliability, or that it is more specific to the email address receiving the message. For instance, Gmail's support forum discusses the button's conditional display. This could explain why you see <undefined> in one instance and a correct URL in another, even for the same campaign. It implies that while the header might be correctly formed, Google's dynamic processing might be faltering at times.
To accurately diagnose the issue, always check the raw email headers, specifically the List-Unsubscribe and List-Unsubscribe-Post fields, by downloading the original message rather than just viewing Show Original. This provides the unaltered header as received. If it consistently shows <undefined> in the raw message, the problem is likely on your end. If it varies, or if the raw message is correct but Gmail's UI shows <undefined>, it points to an issue with Gmail’s rendering or processing. You can see discussions about this on Stack Overflow, which highlight the complexities of this behavior.
Impact on deliverability and user trust
The appearance of <undefined> in your List-Unsubscribe header, or any issue preventing the unsubscribe button from showing, has direct consequences for your email deliverability and overall sender reputation. Mailbox providers, especially Gmail and Yahoo, heavily weigh the ease of unsubscribing when assessing a sender's trustworthiness. A broken or non-obvious unsubscribe path encourages recipients to mark your emails as spam instead, leading to a surge in spam complaints.
High spam complaint rates, even if unintended by the recipient, signal negative engagement to mailbox providers. This can lead to your emails being directed to the spam folder, or even result in your sending IP or domain being added to a blacklist (or blocklist). Our guide on why emails go to spam details common factors, including poor unsubscribe experiences. Furthermore, if your List-Unsubscribe is problematic, you might see 'needs work' status in Google Postmaster Tools, directly impacting your sender reputation.
Increased spam complaints, negative impact on sender reputation, potential blacklisting, emails landing in spam folder.
One-click unsubscribe via List-Unsubscribe-Post
Mandatory for bulk senders to comply with new regulations, critical for maintaining good deliverability with Gmail and Yahoo.
The stricter requirements from mailbox providers mean that any failure in the unsubscribe process, including the mysterious <undefined> error, could lead to your domain or IP address being added to an email blacklist (or blocklist). This can severely hinder your ability to reach the inbox, as many mail servers use these lists to filter out potentially malicious or low-quality senders. Our guide on what happens when your domain is on a blacklist provides more detail.
Ultimately, the trustworthiness of your brand is at stake. When recipients cannot easily unsubscribe, they lose trust, making them less likely to engage with your emails in the future. This impacts not only immediate campaign performance but also long-term brand perception and customer loyalty. Ensuring your List-Unsubscribe header is always functional is paramount for maintaining a positive sender reputation and respecting user preferences.
Ensuring proper implementation and monitoring
Preventing the <undefined> issue and ensuring consistent List-Unsubscribe functionality requires a proactive approach. First, meticulously verify that your email sending platform correctly generates both the mailto and HTTP(S) unsubscribe links in the headers. Use tools to inspect your sent emails' raw headers to confirm that the URLs are fully formed and accessible.
Consistent testing across various recipient environments, including different Gmail account types (personal vs. Google Workspace), is crucial. This can help you identify inconsistencies like the one leading to the <undefined> display. Implement monitoring for your email headers to catch any anomalies immediately. Staying informed about updates to Google Postmaster Tools and other postmaster guidelines is also vital, as changes can affect how headers are processed.
Best practices for List-Unsubscribe compliance
Ensure both formats are present: Include both mailto and HTTP(S) URLs in your List-Unsubscribe header to maximize compatibility and compliance.
Implement One-Click Unsubscribe: Add the List-Unsubscribe-Post: List-Unsubscribe=One-Click header for true one-click functionality, as required by Gmail and Yahoo.
Regularly test headers: Send test emails and inspect raw headers in multiple mailbox providers to ensure correct formatting and functionality.
Monitor deliverability metrics: Keep an eye on spam complaint rates, inbox placement, and unsubscribe request volume (including Gmail Actions user agent data) for any anomalies.
If the issue persists despite thorough checks on your end, it might indicate a transient bug or a unique processing behavior within Gmail. This type of situation highlights the complex nature of email deliverability, where even compliant senders can face unexpected challenges due to receiving server logic. Continuous monitoring and quick reaction to such anomalies are key to maintaining optimal email performance.
Views from the trenches
Best practices
Always include both mailto: and http(s):// URLs in your List-Unsubscribe header for maximum compatibility and compliance.
Utilize the List-Unsubscribe-Post header to enable one-click unsubscribe functionality, which is now mandatory for bulk senders by major mailbox providers.
Regularly test your email headers by sending test campaigns to various email clients, including different Gmail accounts and Google Workspace domains, to catch any inconsistencies early.
Monitor your Google Postmaster Tools for any unusual spikes in spam complaints or issues flagged under the 'Honor Unsubscribe' status.
Common pitfalls
Relying solely on mailto: for unsubscribes, as it may not be sufficient for new one-click requirements and can sometimes be malformed.
Overlooking dynamic variable resolution in your email platform, which can lead to empty or 'undefined' unsubscribe URLs in the header.
Not checking raw email headers (via 'Download Message' in Gmail) but instead relying on 'Show Original', which might not always reflect the true header content.
Assuming that a correctly formatted header will always display perfectly, ignoring potential, temporary bugs or specific processing behaviors on the recipient's end.
Expert tips
If you see inconsistent 'undefined' behavior, especially for the same campaign, consider it might be a temporary processing quirk on the mailbox provider's end, particularly during enforcement periods.
A sudden increase in unsubscribe requests from Gmail, identifiable by the Gmail Actions user agent, might indicate that the unsubscribe button is now prominently displayed and functional.
Always align your email authentication protocols (SPF, DKIM, DMARC) with your sending domain; strong authentication can influence how mail providers display or process headers like List-Unsubscribe.
Implement a DMARC policy with reporting to gain visibility into email authentication failures, which can indirectly affect reputation and header processing.
Marketer view
Marketer from Email Geeks says they noticed that all emails from a specific sender had the List-Unsubscribe URL as '<undefined>' in the header, then later saw the URLs present, and observed inconsistent behavior in test messages.
May 22, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that '<undefined>' is indicative of a bug somewhere, suggesting it shouldn't be visible. They would initially focus on the sender's message composition but acknowledge that dynamic changes in the same message point to Google's internal code.
May 22, 2024 - Email Geeks
Ensuring a clear unsubscribe path
The appearance of <undefined> in your List-Unsubscribe header is a clear signal that something isn't right, whether it's an issue stemming from your sending platform or a temporary quirk within Gmail's processing. Given the new, stricter requirements for bulk senders, a functional and easily discoverable unsubscribe option is no longer just a best practice, but a critical compliance factor. Prioritize ensuring your List-Unsubscribe headers are always valid, test them rigorously, and stay vigilant for any anomalies to safeguard your deliverability and maintain recipient trust.
Gmail to display an unsubscribe button directly within the email interface, offering a convenient alternative to searching for a link within the message body.
Google's own systems might be experiencing a temporary bug or are in a testing phase. Some users have reported that the unsubscribe button may not appear until Gmail has assessed the domain's reliability, or that it is more specific to the email address receiving the message. For instance, Gmail's support forum discusses the button's conditional display. This could explain why you see <undefined> in one instance and a correct URL in another, even for the same campaign. It implies that while the header might be correctly formed, Google's dynamic processing might be faltering at times.
