Summary
SpamAssassin hexadecimal sequence errors typically indicate that your email contains links or a 'From' address with patterns that resemble hexadecimal encoding. This often leads to increased spam scores, impacting email deliverability. These patterns are often flagged because spammers use them to encode information, such as identifying spam traps or tracking recipients. Resolving these issues is crucial for maintaining a good sender reputation and ensuring your emails reach the inbox.
Key findings
- Specific error: The error is primarily associated with SpamAssassin, a widely used open-source spam filtering system.
- Primary causes: The error points to long hexadecimal sequences found in the hostnames of email links (URIs) or within the local part of the 'From' address.
- Spammer tactics: Spammers often encode data in these patterns to bypass filters or track interactions, leading legitimate emails with similar structures to be flagged.
- Rule specificity: The error is very specific to these hexadecimal patterns and is not typically a custom message or related to other email formatting elements like embedded tables.
- Scoring impact: Hitting such a rule contributes to an email's overall spam score. While some individual scores may be low (e.g., 1.3), cumulative points can push an email into the spam folder.
Key considerations
- Identify the source: Determine whether the hexadecimal sequence is in an email link or the 'From' address. Inspect all URLs and the sender's email address for unusual character strings.
- Review links: Examine links within your email content, especially those generated by marketing automation platforms or tracking systems, which may contain long, complex identifiers. For more general advice on link warnings, see our guide on why legitimate emails sometimes trigger inconsistent suspicious link warnings.
- Check 'From' address: Verify the local part (the segment before the '@' symbol) of the 'From' address for any non-standard or automatically generated hexadecimal-like patterns. This could also be related to mail server configurations rather than email template code.
- Consult SpamAssassin rules: Refer to the SpamAssassin rules documentation to understand the specific regex patterns that trigger URI_HEX or FROM_LOCAL_HEX rules.
- Evaluate overall impact: If the score from this rule is low, it might not severely affect deliverability on its own. However, it's a good practice to resolve it to reduce any unnecessary spam points. Learn more about how to diagnose email deliverability issues more broadly.
What email marketers say
Email marketers often encounter these hexadecimal sequence errors when using spam checkers, and their initial reaction is typically confusion about the meaning and origin of such technical warnings. They are keen to understand if the issue stems from their email design, such as the number of tables, or more technical aspects like how links are generated. Their focus is on finding clear, actionable steps to resolve the problem and prevent their emails from being marked as spam. Marketers need to bridge the gap between their creative and campaign management roles and the underlying technical configurations that affect deliverability.
Key opinions
- Seeking clarification: Marketers frequently ask for explanations of cryptic spam checker errors, demonstrating a need for simplified technical insights.
- Testing reliance: They often rely on spam testing tools to identify potential deliverability issues before campaign deployment.
- Internal vs. external: Marketers tend to wonder if errors are custom messages from their email service providers or standard, widely recognized spam filter rules.
- Design vs. technical: There's often a question about whether design elements (e.g., HTML structure, images) could cause such errors, as opposed to technical components like URLs or headers. Read more about how email code analysis errors impact deliverability.
- Solution-oriented: The primary goal for marketers is a straightforward solution to avoid spam folders, such as simply replacing a problematic link.
Key considerations
- Understand the checker: Identify which specific spam checker or system is generating the warning, as different tools might have varying rule sets or thresholds.
- Collaborate with technical teams: Work closely with email developers or IT support to pinpoint and resolve technical issues that trigger spam filter warnings.
- Review link generation: If using an email platform, investigate how it generates tracking links, as these are often the source of long hexadecimal sequences. HubSpot's community forum discusses how URI hostname issues impact email scores.
- Prioritize fixes: While any spam score is undesirable, consider the weight of the specific rule. Low-scoring rules might be less critical to address immediately if other, higher-scoring issues exist. For general deliverability issues, consider how to troubleshoot transactional emails going to spam.
Marketer view
Email marketer from Email Geeks asks about the meaning of a spam rate check error, indicating a general lack of clarity when encountering unfamiliar technical alerts in email testing tools. This highlights the need for more user-friendly explanations of deliverability diagnostics.
Marketer view
Email marketer from HubSpot Community discusses receiving a low rating from mail-tester.com due to 'URI hostname has long hexadecimal sequence.' This reflects a common challenge for marketers who rely on external tools for pre-send spam checks.
What the experts say
Email deliverability experts agree that SpamAssassin's hexadecimal sequence errors are specific indicators of suspicious patterns in email content, most commonly in links or the 'From' address. These patterns are often associated with spammers trying to conceal information or track users covertly. While the score impact of a single such rule might be low, experts caution that cumulative low scores can still lead to emails being blocked or landing in the spam folder. They advise a precise approach to identifying and correcting these patterns, distinguishing between email template issues and potential mail server configuration problems.
Key opinions
- Specific domain dislike: The error suggests the spam checker dislikes a particular domain name, which spammers often use to encode information for spam trap identification.
- Problem with hostnames: The issue is precisely about hostnames within email links (URIs) or the 'From' address, not other email content elements like tables.
- Targeted rule: The error likely triggers specific SpamAssassin rules, such as FROM_LOCAL_HEX or URI_HEX, which are designed to detect suspicious hexadecimal patterns.
- Low score context: While the rule might assign a low score, which may be ignorable in isolation, it still contributes to the overall spam score, which can be critical for deliverability.
- Mail server vs. template: The problem could stem from mail server configuration settings that introduce these patterns, rather than directly from the email template code.
Key considerations
- Precise identification: It is crucial to precisely identify where the hexadecimal sequence appears, whether it's in a link's hostname or the local part of the 'From' address. This requires inspecting the raw email source.
- Review 'From' addresses: Since emails can have multiple 'From' addresses (e.g., header From, envelope From), all should be checked for suspicious patterns. This can be complex, as discussed in our article What RFC 5322 Says vs. What Actually Works.
- Correct the pattern: The most direct fix is to remove or alter the hexadecimal-like sequence in the problematic link or 'From' address, replacing it with standard, non-encoded text.
- Consider server configuration: If the pattern is automatically generated, it might be an issue with the mail server's configuration or the email service provider's system, requiring changes beyond the email template itself. Familiarity with general spam-fighting techniques like greylisting can also provide context.
- Regex utilization: Use regular expressions (regex) to search for specific patterns like https?://[^/?]*\b[0-9a-f]{6,}\b within the email's raw source to identify the problematic strings. A SpamAssassin rules description provides examples of these patterns.
Expert view
Deliverability expert from Email Geeks states that the error indicates a domain name disliked by the checker, often because spammers have been encoding information in hostnames to identify spam traps. This provides a clear explanation for the filter's sensitivity.
Expert view
Email deliverability consultant from WordToTheWise explains that automated spam filters frequently flag unusual character patterns in URLs, even if they are part of legitimate tracking. These patterns can closely mimic those used in malicious activities, leading to increased scrutiny.
What the documentation says
Official documentation for spam filtering systems, particularly SpamAssassin, clearly defines rules that target hexadecimal sequences in email content. These rules are designed to identify patterns commonly used by spammers for various malicious purposes, such as evading detection or tracking illicitly. The documentation specifies how these rules are triggered, often using regular expressions to match suspicious strings in URI hostnames or the local part of the 'From' address. Understanding these documented rules is key to diagnosing and resolving the underlying issues that cause legitimate emails to be flagged.
Key findings
- Defined rules: SpamAssassin includes specific rules, such as URI_HEX and FROM_LOCAL_HEX, to detect long hexadecimal sequences.
- Pattern matching: These rules use regular expressions to identify hexadecimal patterns longer than a specified number of characters (e.g., six or more).
- Location of detection: The patterns are specifically looked for in URI hostnames or the local part of the 'From' email address.
- Spam rationale: The presence of such sequences is considered suspicious because spammers frequently employ them for obfuscation, tracking, or to uniquely identify recipients for illicit purposes.
- Score assignment: When triggered, these rules contribute a specific score to the email, increasing its likelihood of being marked as spam or junk.
Key considerations
- Understand rule specifics: Familiarize yourself with the exact definitions and regex patterns of SpamAssassin rules like URI_HEX and FROM_LOCAL_HEX by consulting official documentation. This helps in precise identification.
- Analyze email structure: Review your email's raw source to identify any links or 'From' addresses that might contain long hexadecimal sequences that match these rules. This is similar to troubleshooting DKIM selector name examples.
- Modify generation processes: If your email platform or server automatically generates such patterns in links or addresses, explore options to modify these processes to produce cleaner, non-suspicious strings. Some platforms, like ActiveCampaign, have documentation on common spam check errors that can be helpful.
- Monitor spam scores: Regularly monitor your emails' spam scores using testing tools to ensure that any changes implemented have resolved the hexadecimal sequence error and improved overall deliverability.
Technical article
SpamAssassin documentation states that the URI_HEX rule triggers when a URI hostname contains a long hexadecimal sequence, indicating potential spammer activity. This rule is designed to catch covert tracking or evasion techniques.
Technical article
Support documentation from Mark-i Support notes that a 'URI hostname has long hexadecimal sequence' error is a common SpamAssassin check error, often related to encoded tracking links generated by email marketing platforms. This highlights a prevalent issue for users of such systems.
Related resources
3 resources
Related pages
How to diagnose email deliverability issues and prevent emails from going to spam?
Do email code analysis report errors impact deliverability?
How does DCC (Distributed Checksum Clearinghouse) function with SpamAssassin and Rspamd for email scoring?
Why your emails are getting a 'DMARC verification failed' error
Understanding email greylisting and how it works
What RFC 5322 Says vs. What Actually Works
Why do legitimate emails sometimes trigger inconsistent suspicious link warnings in Gmail?
Why Your Emails Fail: Expert Guide to Improve Email Deliverability [2025]
How to troubleshoot transactional emails going to spam?
