Summary
SpamAssassin hexadecimal sequence errors, often indicated by rules like URI_HEX or FROM_LOCAL_HEX, occur when email links or 'From' addresses contain hexadecimal encoding. This is primarily because spammers frequently use such sequences to obfuscate content or bypass filters, making them a red flag for deliverability. Resolving these errors involves ensuring that URLs are clean, canonical, and use minimal, standard encoding. For 'From' addresses, the solution is to ensure they are plain, unencoded, and conform to standard email address formatting. Senders should inspect their raw email content, correct any unnecessary hex sequences, and review their email sending software configurations to prevent future occurrences, as adhering to clear and standards-compliant email formatting is crucial for maintaining sender trust and deliverability.
Key findings
- Hexadecimal Triggers: SpamAssassin rules such as URI_HEX, FROM_LOCAL_HEX, HEX_IN_URL, and HEX_IN_URI_FROM trigger when hexadecimal sequences (like %20) are present in email links or 'From' addresses.
- Spammer Obfuscation: These errors are flagged because spammers frequently use hexadecimal encoding to obfuscate malicious links, bypass filters, or identify spamtraps, making their presence suspicious to email security systems.
- Format-Specific Errors: The issue is specific to the format and encoding of a URL's hostname or the 'From' address, not general email content like embedded tables. Hex presence in a 'From' address strongly indicates a malformed or suspicious header.
- Legitimate Use vs. Misuse: While hexadecimal encoding (%xx) is a legitimate part of URLs for specific characters, senders should use it minimally and only when strictly necessary, as its misuse for obfuscation is a primary reason for flags.
- Root Cause: Sender Practices: Even though mail servers apply SpamAssassin rules, the underlying cause of these errors typically lies with the sender's email content creation practices, including how URLs are generated and how 'From' addresses are formatted.
Key considerations
- Inspect and Correct Encoding: Carefully inspect the raw email source to pinpoint the exact hexadecimal sequences causing the error. For URLs, this often means removing unnecessary encoding, especially for common characters like spaces (e.g., replacing %20 with '+' or ensuring no encoding if not required). For 'From' addresses, remove any hex encoding entirely.
- Adhere to Standards: Ensure all URLs and 'From' addresses strictly follow established internet standards (RFCs). This includes using standard URL encoding practices and confirming that 'From' addresses are plain, valid email addresses, devoid of any unusual or excessive encoding.
- Clean URL Practices: Prioritize 'clean' URL construction. This means using canonical, direct links and avoiding excessive or unusual encoding that could be perceived as obfuscation by spam filters. If using tracking links, leverage platform-provided solutions optimized to avoid triggering such rules.
- Software Configuration Review: Examine your mail server configuration, CMS plugins, or email marketing software settings. These applications might be incorrectly encoding URLs or 'From' headers, requiring adjustments to ensure they produce standard, unencoded email content.
- Assess Score Impact: If the SpamAssassin score increase from these hexadecimal sequence errors is minimal (e.g., 1.3 points), it might be ignorable, especially if overall deliverability remains strong. However, addressing it is best practice for long-term health.
What email marketers say
10 marketer opinions
Addressing SpamAssassin hexadecimal sequence errors in email links and 'From' addresses is critical for strong email deliverability. These flags frequently arise when URLs contain excessive or unusual hexadecimal encoding, or when 'From' addresses are malformed with such sequences. To effectively resolve these issues, email marketers should prioritize adherence to clean URL practices, ensuring links are canonical and use minimal, standard encoding, for example, by substituting '%20' with '+' for spaces where appropriate. For 'From' addresses, the solution is straightforward: ensure they are presented in a plain, unencoded, and valid format. Careful inspection of the email's raw source, coupled with adjustments to sending software or CMS plugins, helps correct these problematic sequences and ensures content appears legitimate to spam filters.
Key opinions
- Targeted Hex Sequences: SpamAssassin rules like URI_HEX and HEX_IN_URL specifically target overly long hexadecimal sequences in URI hostnames or malformed 'From' addresses.
- Legitimate Encoding Impact: Even legitimate URL encoding, such as '%20' for spaces, can trigger flags, particularly if a sender's reputation is low.
- From Address Indicators: Hexadecimal encoding in a 'From' address is a strong indicator of a malformed or suspicious header, signaling potential issues to spam filters.
- Software-Induced Errors: Errors often originate from CMS plugins or email marketing platforms incorrectly encoding URLs or email headers during the sending process.
- Obfuscation Perception: Excessive or unusual hexadecimal encoding is frequently perceived by email filters as an attempt to obfuscate content, thereby reducing sender trust and impairing deliverability.
Key considerations
- Direct String Inspection: Examine the raw email source to identify the precise hexadecimal sequences triggering the errors in both links and 'From' addresses.
- URL Encoding Standards: When necessary to encode spaces in URLs, prioritize using '+' over '%20', or decode characters programmatically if possible to minimize perceived obfuscation.
- From Address Purity: Ensure the 'From' email address is in a standard, unencoded format, as any hexadecimal sequences here typically signify a malformed or suspicious header.
- Software Configuration Adjustments: Configure CMS plugins, email marketing software, or other sending applications to adhere to standard URL encoding practices and to use plain, unencoded email addresses in headers.
- Clean Content Creation: Adopt clean content practices, focusing on short, readable URLs that are canonical and direct, and avoid any unnecessary or unusual encoding perceived as suspicious by filters.
Marketer view
Marketer from Email Geeks suggests that the SpamAssassin rule 'URI_HEX', which describes a URI hostname having a long hexadecimal sequence, might be the cause of the error. He provides a regular expression, `https?://[^/?]*\b[0-9a-f]{6,}`, to search for such sequences within an email using a text editor like Sublime Text.
25 Jan 2024 - Email Geeks
Marketer view
Email marketer from Stack Overflow explains that if legitimate URLs contain hexadecimal sequences, for instance %20 for spaces, it's often better to URL-encode spaces as '+' or decode them programmatically where possible. For 'From' addresses, the solution is to ensure no hex encoding is present as it's typically a sign of a malformed or suspicious header.
15 Feb 2022 - Stack Overflow
What the experts say
1 expert opinions
SpamAssassin flags hexadecimal sequences in email links, particularly in hostnames, and 'From' addresses, as these are often utilized by spammers to obfuscate content or identify spamtraps. The 'FROM_LOCAL_HEX' rule specifically points to issues with the 'From' address. Resolving these errors requires removing any hexadecimal encoding from the 'From' address, replacing it with a clean, standard email address. This fix may involve adjusting mail server configurations, not just email templates. While it's best practice to address such issues, a minimal increase in SpamAssassin score, like 1.3 points, might sometimes be acceptable.
Key opinions
- Spammer Identification Tactics: SpamAssassin flags hexadecimal sequences in domain names or 'From' addresses because spammers frequently use these to encode data, such as identifying spamtraps.
- Format-Specific Errors: This type of error is specific to the formatting of a link's hostname or the 'From' address itself, and does not apply to other email elements like embedded tables.
- 'FROM_LOCAL_HEX' Rule: The SpamAssassin rule 'FROM_LOCAL_HEX' specifically indicates a problem with hexadecimal content found within the 'From' address of an email.
- Non-Template Origin: The presence of hexadecimal sequences in 'From' addresses may stem from mail server configuration rather than an issue with the email template itself.
Key considerations
- Rectify From Address: Eliminate any hexadecimal sequences from the 'From' address, ensuring it is a standard, unencoded email address format.
- Mail Server Configuration: Review your mail server settings, as hexadecimal sequences in the 'From' address might be a configuration issue at the server level, not just a template error.
- Evaluate Score Impact: Consider the overall SpamAssassin score. A low score increase, such as 1.3 points, might be ignorable if your deliverability remains otherwise strong.
Expert view
Expert from Email Geeks explains that a spam rate check error regarding hexadecimal sequences means the checker dislikes a domain name or a 'From' address because spammers often encode things in hostnames or 'From' addresses to identify spamtraps. This error is specific to the format of a link or the 'From' address, not elements like embedded tables. She identifies the error as a SpamAssassin rule, potentially 'FROM_LOCAL_HEX', which indicates a problem with the 'From' address. To fix it, remove any hexadecimal sequences from the 'From' address and replace it with an actual email address, noting that this might be a mail server configuration rather than a template issue. She also advises that a low score (e.g., 1.3) might be ignorable.
18 Aug 2022 - Email Geeks
What the documentation says
6 technical articles
Email deliverability often hinges on adhering to strict formatting standards, and SpamAssassin's flags for hexadecimal sequences, particularly in email links and 'From' addresses, highlight a common pitfall. These warnings, triggered by rules like URI_HEX or HEX_IN_URL, arise because such encoding is frequently exploited by spammers to hide malicious content or bypass filters, making them suspicious. To resolve these issues, senders must ensure that all URLs are constructed cleanly, using minimal and standard encoding only when strictly necessary, rather than for general characters. Crucially, 'From' addresses should always be plain, unencoded email addresses that fully comply with email formatting specifications. The responsibility lies with the sender to adopt clean email practices and verify that their sending platforms or mail servers correctly format headers and links, aligning with established internet standards to maintain sender reputation and ensure messages reach the inbox.
Key findings
- Obfuscation Indicator: Hexadecimal sequences like %xx are flagged by SpamAssassin rules (e.g., URI_HEX, HEX_IN_URL) as they are frequently used by spammers to obfuscate links, bypass filters, or identify spamtraps.
- Strict 'From' Rule: The presence of any hexadecimal encoding in a 'From' address is a strong indicator of a malformed or suspicious header, often triggering specific rules like HEX_IN_URI_FROM.
- Sender Responsibility: The root cause of these errors typically lies with the sender's practices, including how URLs are generated and how 'From' addresses are formatted, rather than with the mail server's application of rules.
- Legitimate vs. Malicious Use: While hex encoding is a legitimate part of URLs for non-alphanumeric characters, its excessive or unnecessary use, especially for common characters, is perceived as an obfuscation technique.
- Standard Deviations: Any unnecessary hexadecimal encoding, particularly in 'From' addresses, deviates from established email and URL formatting standards (e.g., RFC 5322, RFC 2396), leading to flags.
Key considerations
- Streamline URL Encoding: Ensure URLs are properly constructed, avoiding unnecessary hexadecimal encoding. Prioritize minimal, standard encoding, and strip hex sequences for common characters where they are not strictly required.
- Plain 'From' Addresses: Verify that the email address in the 'From:' header is a standard, unencoded format, entirely free of hexadecimal sequences.
- Adhere to RFC Standards: Strictly follow IETF RFCs for email formatting and URL encoding. This means ensuring that both email headers and URIs conform to defined specifications to avoid raising flags.
- Review Sending Systems: Inspect the configuration of your email marketing platform, CMS, or mail server to confirm they are not inadvertently adding unnecessary hexadecimal encoding to links or 'From' addresses.
- Direct Content Examination: For immediate resolution, examine the raw email content to pinpoint and manually correct any problematic hexadecimal sequences in links or the 'From' address.
Technical article
Documentation from SpamAssassin Wiki explains that SpamAssassin rules like URI_HEX, HEX_IN_URL, HEX_IN_URI_FROM, and HEX_IN_URI_EMAIL trigger when hexadecimal sequences (like %20 for space) are present in URIs or email addresses. These are flagged because spammers frequently use such encoding to obfuscate links or bypass filters, making them suspicious.
1 Aug 2021 - SpamAssassin Wiki
Technical article
Documentation from Apache SpamAssassin explains that rules detecting hexadecimal escapes within URIs or email addresses, such as '%xx', are designed to catch obfuscation techniques used by spammers. To resolve this, legitimate content must ensure that URLs are properly constructed without unnecessary hex sequences and that email addresses in the 'From' header are plain and unencoded.
16 Apr 2022 - Apache SpamAssassin Documentation
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