Summary
The SpamAssassin rule FONT_INVIS_MSGID is a metarule, meaning it triggers based on the combination of other underlying rules. Specifically, it tests for the presence of invisible text within an email (often a tactic used by spammers to hide keywords or content) alongside a suspicious Message-ID header. A Message-ID header is a unique identifier assigned to each email, and a suspicious one might indicate forgery, a malformed format, or inconsistencies with the email's other headers, such as the Received headers.
Key findings
- Metarule operation: The FONT_INVIS_MSGID rule combines two independent spam indicators: invisible font usage and unusual Message-ID headers.
- Invisible text detection: This part of the rule targets hidden content, often achieved by setting text color to match the background, or using very small font sizes. This is a common spamming technique to evade filters while including keywords. Learn more about malformed HTML's impact on spam filtering.
- Suspicious Message-ID: A Message-ID is flagged as suspicious if its format is unexpected or if it doesn't align with other email headers, which could indicate spoofing or a malicious origin.
- Spam score impact: When both conditions are met, the rule adds a significant score to the email, increasing its likelihood of being classified as spam. Understanding how SpamAssassin rules affect deliverability is crucial.
- Purpose: This rule aims to catch sophisticated spam attempts that try to hide content while manipulating standard email headers.
Key considerations
- Content design: Avoid any use of invisible text in your emails, even if unintentional. This includes setting text to the same color as the background or using very small font sizes that are practically unreadable. Consider alternative ways to structure your email if you have content that is not meant to be immediately visible but serves a technical purpose, as discussed in the long invisible text context.
- Message-ID integrity: Ensure your email sending system generates valid and consistent Message-ID headers that align with email standards and other authentication protocols like SPF, DKIM, and DMARC. Improperly formatted headers can negatively affect your domain's reputation.
- Testing: Regularly test your emails with tools that provide SpamAssassin scores to identify potential issues before they impact your deliverability. This can help you understand how SpamAssassin scores are calculated.
What email marketers say
Email marketers often encounter SpamAssassin scores and seek to understand the underlying rules that influence them. When it comes to FONT_INVIS_MSGID, marketers typically focus on ensuring their email templates do not inadvertently include hidden text and that their email sending platforms generate compliant Message-ID headers. The general sentiment is that while specific rule details can be obscure, adhering to best practices in HTML formatting and header integrity is key to avoiding penalties from rules like this one.
Key opinions
- Rule obscurity: Many marketers find it challenging to get detailed, up-to-date information on specific SpamAssassin rules without diving into technical source code or archives. This can make troubleshooting difficult. This is similar to how SpamAssassin 'try it' messages can be confusing.
- Focus on fundamentals: Rather than obsessing over every individual rule, marketers tend to prioritize clean HTML, clear content, and proper email authentication to improve overall deliverability. This reduces the chances of triggering less common or complex rules.
- Template hygiene: There's a strong emphasis on reviewing email templates for any hidden elements that might inadvertently appear as invisible text, even if not maliciously intended. This is critical for avoiding issues with commented code or similar hidden content.
- Monitoring tools: Marketers rely on deliverability testing tools to get SpamAssassin scores and identify which rules are being hit, allowing them to adjust their campaigns as needed.
Key considerations
- HTML validation: Always validate your email HTML to ensure it's well-formed and doesn't create unintended hidden content through CSS or deprecated tags. This helps prevent triggering rules related to invisible text.
- Message-ID generation: While usually handled by the sending platform, it's good to be aware that your Message-ID should appear valid and consistent. If you have control over this, ensure it's unique and follows standard conventions. Sometimes, a poorly chosen envelope domain could also raise flags indirectly.
- Testing practices: Before sending to a large audience, run your emails through a testing service that provides SpamAssassin scores. This provides a clear report on which rules are being triggered and how to address them.
- Transparency: Aim for transparency in your email content. Hidden text, even for SEO or tracking, is a red flag for spam filters. Ensure all content is visible and readable to recipients.
Marketer view
Marketer from Email Geeks asks for reliable online resources to better understand specific SpamAssassin rules, particularly what FONT_INVIS_MSGID is designed to test for.
Marketer view
Marketer from Email Geeks questions whether the Message-ID header itself contains any unusual characteristics that could contribute to this rule's activation.
What the experts say
Email deliverability experts often delve into the intricacies of spam filtering systems like SpamAssassin to understand how rules are triggered and what factors contribute to an email being marked as spam. For the FONT_INVIS_MSGID rule, experts confirm its nature as a metarule, combining a content-based check (invisible text) with a header-based anomaly (suspicious Message-ID). They stress the importance of understanding the internal workings of such rules, even if public documentation is scarce, to diagnose and resolve complex deliverability problems. They may also suggest newer, faster alternatives to SpamAssassin.
Key opinions
- Rule interpretation: Experts clarify that FONT_INVIS_MSGID is a compound rule. It only triggers when both invisible text and a suspicious Message-ID are detected.
- Suspicious Message-ID meaning: A Message-ID is suspicious if it doesn't match expected sender patterns or if it conflicts with received headers, often pointing to potential spoofing attempts.
- Source code review: Due to limited public documentation, experts often resort to reviewing SpamAssassin's source code to understand the precise logic behind various rules, including FONT_INVIS_MSGID.
- Alternative solutions: Some experts suggest that SpamAssassin is outdated and slower compared to modern solutions like rspamd, which offers more advanced features and better performance. This is a common theme when discussing email deliverability issues.
Key considerations
- Holistic deliverability: While specific rules matter, experts emphasize that overall email deliverability is a combination of content quality, sender reputation, and proper authentication (like SPF, DKIM, and DMARC). A single rule like FONT_INVIS_MSGID is just one factor.
- Header consistency: Ensuring Message-ID and other headers (like From and Received) are consistent and correctly formatted is critical. This is a common area for spam flags, similar to what Microsoft email headers reveal about classification.
- Evolving threats: Spam filters continually adapt to new spamming techniques. What might trigger a rule today could be refined or replaced tomorrow. Staying updated with best practices and filter behaviors is crucial for long-term deliverability.
- Proactive monitoring: Regular use of deliverability testing platforms helps identify if your emails are triggering any SpamAssassin rules, including FONT_INVIS_MSGID, allowing for prompt adjustments.
Expert view
Expert from Email Geeks notes that reliable, up-to-date documentation for SpamAssassin rules can be scarce, often requiring users to directly consult the source code for comprehensive details.
Expert view
Expert from Email Geeks clarifies that FONT_INVIS_MSGID is a metarule that activates when an email contains invisible text and its Message-ID appears suspicious, such as a mismatch with expected sender formats or received headers.
What the documentation says
Technical documentation for SpamAssassin often defines rules based on regular expressions and heuristic logic. While specific detailed documentation for every rule, especially older or dynamically generated metarules like FONT_INVIS_MSGID, can be challenging to find in a single, comprehensive public resource, the core principles are consistent. Documentation typically highlights that invisible content is a strong indicator of spam, and any irregularities in email headers, including the Message-ID, are scrutinized to prevent spoofing and detect malicious intent. The combination of these factors is designed to catch sophisticated spamming techniques.
Key findings
- Rule definition: The FONT_INVIS_MSGID rule typically checks for patterns of hidden text (e.g., <font color="#FFFFFF"> on a white background) combined with specific irregularities in the Message-ID.
- Message-ID validation: RFC 5322 (Internet Message Format) defines the Message-ID as a globally unique identifier. Filters like SpamAssassin analyze its format and domain part for common spammer patterns or inconsistencies with the Received headers.
- Dynamic nature of rules: SpamAssassin's rules are continuously updated to combat new spam tactics, which means specific rule definitions (especially metarules) may not always be static or fully documented in older public archives. The Apache SpamAssassin project provides some historical data.
- Combined heuristic: The rule's strength lies in combining two suspicious behaviors. Neither invisible text nor a slightly off Message-ID might trigger a high score individually, but together they suggest a deliberate attempt to evade filters.
Key considerations
- RFC compliance: Adhering strictly to RFCs (Request for Comments) for email formatting and headers, particularly for Message-ID and Received headers, is fundamental. Deviation from these standards can contribute to a suspicious score. You can read more about what RFC 5322 says.
- MIME structure: The correct structure of MIME parts in an email can also play a role, as malformed MIME might be interpreted as an attempt to conceal content or manipulate parsing, indirectly impacting how invisible text is perceived. This relates to issues like MIME_NO_TEXT in SpamAssassin.
- Regular expression logic: Understanding that SpamAssassin rules are often based on regular expressions that scan for specific patterns in headers and body text can help diagnose why a rule is being triggered.
Technical article
Documentation from GitLab's local.cf file illustrates how SpamAssassin rules are configured locally, allowing administrators to customize or add new rules based on observed spam patterns, including those that might incorporate checks for invisible fonts and header anomalies.
Technical article
Documentation from Apache SpamAssassin's historical rule archives indicates that rules evolve, and while specific compound rules like FONT_INVIS_MSGID might not be explicitly listed in older versions, the underlying checks for hidden text and header manipulation are fundamental to spam detection.
Related resources
4 resources
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What do Microsoft email headers reveal about spam classification?
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