How to resolve SpamAssassin hexadecimal sequence errors in email links or from addresses?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 May 2025
Updated 19 Aug 2025
9 min read
Email deliverability is a complex landscape, and one of the more cryptic issues you might encounter is a SpamAssassin hexadecimal sequence error in your email links or 'From' addresses. These warnings, such as URI_HEX or FROM_LOCAL_HEX, indicate that parts of your email, specifically URLs or the local part of your 'From' address (the portion before the '@' symbol), contain long sequences of hexadecimal characters. While sometimes harmless, these patterns are often associated with spamming tactics and can significantly impact your email's journey to the inbox. Understanding what triggers these errors and how to address them is crucial for maintaining good sender reputation and ensuring your messages reach their intended recipients.
The main goal is to minimize any red flags that SpamAssassin identifies, which contributes to a lower spam score and better overall deliverability. Ignoring these warnings, even if they seem minor, can lead to your emails being flagged as spam or blocked outright by various email providers and their anti-spam filters.
Understanding SpamAssassin's hexadecimal rules
SpamAssassin, a widely used open-source anti-spam platform, employs a set of rules to score incoming emails. Each rule, if triggered, adds points to an email's spam score. If the total score exceeds a certain threshold, the email is marked as spam. Hexadecimal sequence rules are designed to catch patterns commonly used by spammers to obfuscate malicious links, evade detection, or track compromised accounts.
The URI_HEX rule, for instance, typically flags URIs (links) where the hostname or path contains a long sequence of hexadecimal characters. Spammers might use this to generate unique, untraceable links or to encode information that helps them identify active email addresses, sometimes related to spam traps. Similarly, the FROM_LOCAL_HEX rule applies to the 'From' address itself, if its local-part contains suspicious hexadecimal strings.
These rules are part of a broader effort to filter out unwanted mail. You can find comprehensive lists of SpamAssassin's tests in their official documentation, which provides insight into the various mechanisms they use, including header and text analysis, Bayesian filtering, and DNS blocklists. It's worth remembering that SpamAssassin rules directly affect email deliverability, and understanding them is key to improving your inbox placement.
Example SpamAssassin URI_HEX Rule
uri URI_HEX m%^https?://[^/?]*\b[0-9a-f]{6,}\b%i
describe URI_HEX URI hostname has long hexadecimal sequence
Common causes of hexadecimal sequence errors
Hexadecimal sequence errors typically arise from dynamic content generation within your email campaigns. While they often point to issues with links, sometimes the 'From' address itself can be the culprit. I've seen these issues pop up from a variety of sources, ranging from email service providers (ESPs) to custom tracking setups.
One common cause for URI_HEX is the use of unique tracking parameters appended to URLs. Many marketing automation platforms or link shorteners add distinct identifiers to links to track clicks and user behavior. If these identifiers are long, appear in hexadecimal format, and are inserted into the hostname or main path of the URL, SpamAssassin may flag them as suspicious. This is because spammers often use such techniques to create unique links for each recipient to bypass filters or identify active addresses. You should also ensure that URLs are properly encoded, as malformed encoding can also lead to unintended hexadecimal sequences.
For FROM_LOCAL_HEX errors, the issue lies within the local part of your 'From' email address (e.g., somehexsequence@example.com). This can happen if your mail server or ESP is adding unique, hex-encoded identifiers to the 'From' address for internal routing or tracking purposes, and these identifiers become visible to SpamAssassin. It's less common for legitimate senders to intentionally use such patterns in their 'From' addresses, which is why SpamAssassin flags them as suspicious. Resolving this often involves checking your email platform's settings or consulting with your ESP.
Some email service providers or marketing automation platforms append unique identifiers to URLs for tracking purposes. These might appear as hexadecimal sequences. For example, a link might look like https://example.com/track/1a2b3c4d5e6f. If these are not overly long or malformed, they might be acceptable depending on the filter's strictness.
Suspicious use cases
Spammers often encode malicious payloads or obfuscate their origin using long, random-looking hexadecimal sequences in domains or email addresses. For example, http://4f8c9b2e.phishing.ru or a1b2c3d4e5f6@scam.com. These patterns are a strong indicator of spam and will severely impact deliverability.
Diagnosing and identifying the issue
The first step to resolving these errors is to accurately diagnose where the hexadecimal sequence is appearing. SpamAssassin provides detailed reports that typically point to the specific rule being triggered and the offending part of the email. Many email deliverability testing tools also leverage SpamAssassin and will show you similar warnings, providing a score and specific rule names like URI_HEX or FROM_LOCAL_HEX.
Once you have the specific rule, you'll need to examine your email's raw source (email headers and body) to find the problematic string. For URI_HEX, look closely at all the links within your email, especially any that involve tracking or dynamic parameters. Check the hostname (the domain part of the URL) and the path for sequences of 6 or more hexadecimal characters (0-9, a-f). For FROM_LOCAL_HEX, inspect the 'From' header field of your email. The local part of the email address is where you'll find the suspicious string.
This process involves a bit of detective work, but it's essential. If you're using an email platform or marketing automation system, check its settings related to link tracking, URL shortening, and sender identity. These systems might be injecting these sequences automatically. Remember, a long hexadecimal sequence in either the hostname of a URL or the local part of an email address can trigger these warnings, regardless of whether it's intentionally added or a side effect of a system configuration.
Pinpointing the problematic element
To effectively diagnose the issue, consider using a free online email deliverability tester. Send your email through it and analyze the detailed report. Look for specific SpamAssassin rule hits, then examine the corresponding section of your email's raw source. This will help you narrow down if the problem is with a link, an email address, or something else.
Strategies to resolve hexadecimal errors
Once you've identified the source of the hexadecimal sequence error, you can implement targeted solutions. The approach differs slightly depending on whether the issue is with a link or a 'From' address.
For email links (URI_HEX)
If the hexadecimal sequence is in one of your email links, consider these strategies:
Adjust tracking parameters: If your ESP or marketing platform adds hexadecimal strings for tracking, see if you can configure them to use different types of identifiers or shorten the length of the hex sequence. Some platforms offer options for white-labeling or custom tracking domains which might circumvent this issue.
Review URL encoding: Ensure all URLs are correctly encoded. Improper encoding can sometimes introduce characters that SpamAssassin misinterprets as hexadecimal sequences.
Avoid unusual subdomains: If the hexadecimal sequence appears in a subdomain (e.g., 4a5b6c.yourdomain.com), try to use a cleaner, more standard subdomain for your links. This is a common tactic for spammers.
For 'From' addresses (FROM_LOCAL_HEX)
If the issue is with your 'From' address, consider these actions:
Standardize 'From' address format: Ensure your 'From' address strictly adheres to RFC 5322 standards. Avoid any unusual characters or dynamically generated strings in the local part of the address. Sometimes, this can also be related to issues like Gmail's 'sender's email address uses abnormal characters' error, indicating a similar underlying problem with formatting.
Check mail server configuration: If your email server or ESP is adding these hexadecimal sequences, you may need to adjust its configuration. This might involve changing how it handles sender addresses or internal routing IDs. Contact their support if you cannot find these settings.
Maintaining a clean email reputation
While resolving hexadecimal sequence errors can be specific, the broader context of email deliverability involves continuous monitoring and adherence to best practices. Many email senders face challenges when their emails are unexpectedly sent to the spam folder, even with proper configurations like SPF, DKIM, and DMARC. Regular email deliverability tests are essential to catch issues like these before they impact your campaigns.
Maintaining a healthy sender reputation is an ongoing effort that involves multiple factors beyond just technical configurations. It includes managing your sender score, avoiding blacklists (or blocklists), and ensuring your email content and sending practices align with industry best practices. By addressing specific warnings like hexadecimal sequence errors, you contribute to a stronger overall email program that fosters better engagement and consistent inbox delivery.
Views from the trenches
Best practices
Always test your email campaigns with an email deliverability tool before sending to identify potential SpamAssassin issues.
If using an ESP, explore their settings for link tracking and ensure it generates clean, non-hexadecimal URLs.
Regularly review your 'From' address format to confirm it adheres to standard RFC guidelines, avoiding unusual characters or dynamic insertions.
Monitor your sender reputation and blocklist status to proactively address any deliverability challenges.
Common pitfalls
Overlooking hexadecimal sequence warnings, assuming they are minor and won't impact deliverability.
Using generic link shorteners that might introduce problematic hexadecimal strings into your URLs.
Failing to check mail server configurations that automatically add hex sequences to 'From' addresses or link domains.
Ignoring the collective impact of multiple small spam score penalties, which can lead to emails landing in spam folders.
Expert tips
If a hex error persists despite template changes, investigate your email sending infrastructure (e.g., mail server, ESP) as the issue may stem from their configuration.
For 'URI_HEX', try temporarily disabling link tracking on problematic links to confirm if it resolves the issue, then re-evaluate your tracking setup.
A SpamAssassin score of 1.3 for a single rule like 'FROM_LOCAL_HEX' is relatively low, but multiple minor flags can accumulate, so it's best to address what you can.
Consider segmenting your email sends and testing small batches if you suspect a specific campaign or link is causing deliverability issues.
Expert view
Expert from Email Geeks says a hexadecimal sequence error means the email checker dislikes your domain name, possibly due to spammers encoding information like spam traps in hostnames.
2019-10-14 - Email Geeks
Expert view
Expert from Email Geeks says this is likely a specific SpamAssassin error, possibly related to the 'URI hostname has long hexadecimal sequence' rule, often triggered by unusual link formats.
