Does a SpamAssassin LONG_HEX_URI warning impact email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 5 Aug 2025
Updated 18 Aug 2025
8 min read
Many email marketers and developers encounter various warnings when testing their email campaigns, and one that occasionally surfaces is the SpamAssassin "LONG_HEX_URI" flag. This warning typically indicates that a URL within your email contains a lengthy sequence of hexadecimal characters, often a byproduct of tracking parameters added by email service providers (ESPs). It's a technical flag, and it raises a crucial question: how much does it actually impact your email deliverability?
When I see this flag, my initial thought is always to understand its significance in the broader context of email authentication and content filtering. SpamAssassin, while still used by some mail servers, is one of many layers that determine whether an email lands in the inbox or the spam folder. Understanding this specific warning is key to determining if it requires immediate attention or if it's merely a minor point in an otherwise healthy email.
What is a LONG_HEX_URI warning?
The "LONG_HEX_URI" warning is exactly what it sounds like: SpamAssassin has detected a Uniform Resource Identifier (URI), commonly a URL, that contains an unusually long string of hexadecimal characters. These characters often appear as part of tracking codes appended to links by marketing automation platforms or ESPs to monitor clicks, opens, and other engagement metrics. The rule essentially flags these URLs because they can sometimes be an indicator of suspicious or obfuscated content, although in legitimate marketing emails, they're usually harmless.
SpamAssassin functions by assigning a score to each email based on various rules, and if the total score exceeds a certain threshold, the email is flagged as spam. The SpamAssassin rules (or tests) are designed to identify characteristics often found in unwanted bulk email. A SpamAssassin score is the accumulation of these points. The `LONG_HEX_URI` rule adds a small penalty to an email's overall score. How significant this penalty is depends on the specific configuration of the SpamAssassin instance being used, as each server can have custom rules and weightings.
It's worth noting that the rule can be triggered by a combination of factors. For instance, a URL might need to contain 128 consecutive hexadecimal digits in a row, coupled with the email body being over 1024 bytes. This highlights that it's not just the hex string itself, but its context within the email's structure and size that can trigger the flag.
Generally, the points assigned for this flag are relatively low, meaning it rarely pushes an otherwise clean email over the spam threshold by itself. For more information on how these rules function, you can review our guide on how SpamAssassin rules affect deliverability.
Does it truly affect inbox placement?
This is often the core question on everyone's mind. In my experience, for major inbox providers like Gmail and Yahoo, a `LONG_HEX_URI` warning by itself does not significantly impact deliverability. These providers use far more sophisticated algorithms that look at a multitude of factors, including sender reputation, engagement metrics, email authentication (like DMARC, SPF, and DKIM), and content quality, rather than relying heavily on a single SpamAssassin rule.
However, it's not entirely benign. While it might not single-handedly trigger a spam classification, it contributes a small negative score. If your email is already on the edge due to other issues, such as poor sender reputation or other content-related flags, this additional point could theoretically push it over the threshold. It's often treated as a signal that the email might be marketing-related, and if the recipient isn't expecting marketing content, it could slightly increase the chance of filtering, especially for stricter B2B spam filters.
The relevance of SpamAssassin itself in modern deliverability is a topic we've explored further in our article: How relevant is SpamAssassin scoring. The general consensus is that while it provides valuable diagnostic information, it's less of a direct determinant of inbox placement than it once was for major consumer ISPs. Still, it's a piece of the puzzle, especially for smaller mail servers or those with customized spam filtering rules.
LONG_HEX_URI in isolation
Low impact: Rarely causes an email to go to spam on its own for major inbox providers.
Common for ESPs: Often results from standard tracking links added by email service providers.
Minor score addition: Contributes a small, usually negligible, number of points to the SpamAssassin score.
LONG_HEX_URI with other flags
Compounding effect: Can increase the risk if other spam triggers are present, pushing the total score above the threshold.
Reputation matters: A poor sender reputation combined with this flag is more problematic.
Strict filters: More aggressive corporate or B2B filters might be more sensitive to it.
Why you might encounter this warning
The primary cause of a `LONG_HEX_URI` warning is almost always the use of tracking links embedded by an ESP. These links are designed to capture detailed analytics about user interactions, and the hexadecimal strings are part of the unique identifiers for each click or interaction. Since ESPs handle the underlying URL structure, direct modification by the sender is typically not possible.
Sometimes, the issue is not just the length of the hexadecimal string, but how it interacts with the overall email content. For example, some SpamAssassin rules are triggered when a very long hexadecimal URL is found in a relatively small email. This can make the long URL stand out more to the filter. It's a balance between URL complexity and email design.
If you're encountering this warning and are concerned, the best approach is to check if it's accompanied by other, more severe SpamAssassin flags. Minor flags like `LONG_HEX_URI` are usually benign if your email is otherwise well-constructed and your sender reputation is solid. Our article on resolving hexadecimal sequence errors provides more context on this specific type of issue, while how URL length affects deliverability covers the broader implications of long links.
Mitigating the warning
Since `LONG_HEX_URI` often stems from your ESP's tracking mechanisms, a direct "fix" by shortening the hexadecimal part of the URI is generally not something you can implement. These are functionalities built into the platform to provide essential analytics. Trying to manually encode or shorten these parts could break your tracking or even lead to malformed URLs.
Instead of focusing on this specific flag in isolation, it's far more productive to concentrate on overall email deliverability best practices. This includes maintaining a healthy sender reputation, ensuring proper email authentication (SPF, DKIM, DMARC), sending relevant content to engaged subscribers, and avoiding other common spam triggers. If your overall email strategy is sound, a minor SpamAssassin flag like this one is unlikely to cause significant issues.
Focus on the bigger picture
While `LONG_HEX_URI` can raise an eyebrow, it’s usually a low-scoring flag. Don't let it distract you from the more critical aspects of email deliverability, such as list hygiene, sender reputation, and email authentication. These factors have a much greater impact on whether your emails reach the inbox.
Consistently monitoring your blocklist status and proactively managing your sender reputation are far more impactful strategies. Learn more about common email deliverability issues and how to address them.
The example below shows a theoretical LONG_HEX_URI pattern that SpamAssassin might flag. While this is just an illustration, it highlights the type of long, alphanumeric sequences often added by tracking systems.
Example of a long URI with hexadecimal charactershtml
Maintain a robust email authentication setup including SPF, DKIM, and DMARC to build strong sender trust.
Regularly clean your email list to remove inactive or invalid addresses, reducing bounces and spam trap hits.
Monitor your sender reputation using tools like Google Postmaster Tools for Gmail and Yahoo data.
Ensure email content is relevant and provides value to your subscribers to boost engagement rates.
Common pitfalls
Over-optimizing for minor SpamAssassin flags while neglecting core deliverability factors.
Sending emails to unengaged segments, leading to low open rates and increased spam complaints.
Ignoring DMARC reports, which provide critical insights into email authentication failures and potential spoofing.
Using generic 'no-reply' sender addresses instead of branded, monitored email addresses.
Expert tips
Focus on the cumulative effect: A single low-scoring flag like LONG_HEX_URI is rarely an issue; look for multiple, higher-scoring flags.
Understand your ESP: Most ESPs use tracking URLs, making this flag unavoidable. Focus on other areas where you have more control.
Test thoroughly: Use deliverability testing tools to get a comprehensive view of how your emails perform across various filters.
Segment and personalize: Highly relevant emails are less likely to be marked as spam, even with minor technical flags.
Marketer view
Marketer from Email Geeks says a LONG_HEX_URI warning is probably related to tracking parameters, but usually isn't a problem if other email elements are fine, especially with major inbox providers. Strict B2B filters might be an exception, and fixing it often depends on the ESP or tool used.
2018-08-23 - Email Geeks
Expert view
Expert from Email Geeks says the additional points for this flag are likely low, as each SpamAssassin instance has its own rules, learning, and score thresholds. If this is the only flag triggered, there's no need to worry. If an ESP causes this, all their users are affected, and they would address it if it became a major issue, as long URL strings are common for tracking.
2018-08-24 - Email Geeks
Key takeaways
The SpamAssassin `LONG_HEX_URI` warning is generally a minor flag that, by itself, is unlikely to cause significant email deliverability problems for most senders, especially with major inbox providers. It's often a side effect of legitimate email tracking employed by ESPs. While it adds a small penalty to your SpamAssassin score, it rarely pushes an email into the spam folder on its own.
My advice is to view this warning as an informational note rather than an urgent red flag. Focus your efforts on the foundational elements of email deliverability: maintaining a strong sender reputation, ensuring proper email authentication, sending engaging content, and cleaning your email lists. Addressing these core areas will have a much greater positive impact on your inbox placement than attempting to eliminate a minor SpamAssassin flag that you may not even have direct control over.
Gmail and 
Yahoo, a `LONG_HEX_URI` warning by itself does not significantly impact deliverability. These providers use far more sophisticated algorithms that look at a multitude of factors, including sender reputation, engagement metrics, email authentication (like DMARC, SPF, and DKIM), and content quality, rather than relying heavily on a single SpamAssassin rule.
