What does the SpamAssassin rule FONT_INVIS_MSGID test for?
Michael Ko
Co-founder & CEO, Suped
Published 3 Jun 2025
Updated 18 Aug 2025
7 min read
SpamAssassin is a widely used open-source spam filtering system that employs various rules to identify and score unsolicited emails. It assigns a numerical score to incoming messages based on characteristics that often indicate spam. The higher an email's score, the more likely it is to be flagged or quarantined.
Among its many rules, some are more complex and less intuitive than others. One such rule that frequently prompts questions from senders is FONT_INVIS_MSGID. Understanding these specific rules is crucial for anyone managing email campaigns or system administration, as they directly influence whether your legitimate messages reach the inbox or end up in the spam folder (or blocklist).
The FONT_INVIS_MSGID rule is a 'meta-rule,' meaning it triggers based on the simultaneous activation of several other, more granular tests. This particular rule flags messages that combine the use of hidden text within the email body with a suspiciously formatted or inconsistent Message-ID header. It targets a common tactic used by spammers to evade detection and hide malicious content or keywords.
Understanding the FONT_INVIS_MSGID rule
The FONT_INVIS_MSGID rule in SpamAssassin is designed to catch a specific pattern of spamming behavior. It's a combination (or "meta-rule") that activates when two distinct conditions are met: the presence of invisible text in the email content and a problematic Message-ID header. This dual check makes it a robust defense against certain deceptive email practices.
Spammers often employ invisible text to embed keywords or phishing links that are hidden from the human eye but still detectable by some older or less sophisticated spam filters. This technique aims to manipulate content analysis without alerting the recipient. When SpamAssassin detects such hidden elements, it marks the email as suspicious, contributing to its overall spam score.
Simultaneously, the rule scrutinizes the Message-ID header. A Message-ID (or Message-ID) is a unique identifier for each email, typically generated by the sending server. If this identifier appears "suspicious"—perhaps it's malformed, non-standard, or doesn't align with the expected sending patterns for the purported sender—SpamAssassin flags it. This combination of hidden text and an irregular Message-ID strongly suggests a deceptive intent, increasing the likelihood of the email being classified as spam.
The mechanics of invisible text
Invisible text refers to content within an email that is intentionally made unreadable to the human recipient. This is commonly achieved by setting the font color to match the background color of the email, using extremely small font sizes, or employing zero-width spaces between characters. The goal for spammers is often to stuff keywords that help them bypass simple content filters or to hide phishing URLs.
From a deliverability perspective, any deliberate attempt to conceal text is a major red flag for spam filters. Legitimate emails should have clear and visible content. Even if the hidden text isn't explicitly malicious, its mere presence signals deceptive practices, leading to a higher spam score. You can read more about how hidden HTML affects filtering on our page about malformed HTML impacts deliverability.
SpamAssassin's internal tests look for these hidden elements. When a message body contains text styled to be invisible, it triggers specific sub-rules that then feed into the FONT_INVIS_MSGID meta-rule. For instance, rules might check for specific HTML/CSS properties like color: background-color or font-size: 0px.
Best practice for email content
Transparency is key: Always ensure all text in your emails is visible and intentionally placed. Avoid any CSS or HTML tricks designed to hide content, as these are strong spam indicators.
Accessibility focus: Design emails for readability by all users, including those with accessibility needs. This naturally aligns with avoiding hidden text.
The suspicious message-ID component
The Message-ID header (or Message-ID) serves as a unique identifier for an email message, helping to track its journey across mail servers. A typical Message-ID looks something like <unique-string@sending-domain.com>. When this identifier is "suspicious," it can mean several things in the context of the FONT_INVIS_MSGID rule.
Example of a valid Message-ID header
<1234567890.example.com@mail.example.com>
A Message-ID can be deemed suspicious if its format deviates significantly from standard conventions, if it appears to be forged, or if it doesn't align with the Received headers, which document the path an email takes. For example, if the domain within the Message-ID does not match the actual sending domain or the IP address from which the email originated, it raises a flag. This mismatch indicates a potential attempt at spoofing or obfuscation, which is a common tactic in phishing and spam campaigns.
Valid message-ID characteristics
Unique identifier: Each email has a distinct Message-ID generated by the sending server.
Standard format: Follows RFC standards, typically including a local part and a domain part (e.g., <id@domain>).
Consistent with headers: The domain in the Message-ID aligns with other sending headers, like From and Received fields.
Suspicious message-ID triggers
Malformed format: Missing parts, incorrect characters, or an unusual structure.
Domain mismatch: The domain in the Message-ID does not match the actual sending domain or the server IP.
Inconsistency with path: Message-ID suggests a sender that doesn't align with the servers listed in the Received headers.
This aspect of the rule emphasizes the importance of proper email authentication and consistent email headers. Mail servers expect these identifiers to be well-formed and verifiable. Inconsistencies or malformations in the Message-ID can lead to your emails being flagged, even if the content itself is benign. Ensuring that your email infrastructure generates valid and consistent Message-IDs is a key step in maintaining good email deliverability. For more on how email headers influence spam classification, consider exploring what Microsoft email headers reveal.
Impact on email deliverability
When the FONT_INVIS_MSGID rule is triggered, it contributes a specific number of points to an email's total SpamAssassin score. The exact score varies depending on the SpamAssassin configuration, but any positive score increases the likelihood of an email being marked as spam. If the cumulative score exceeds a predefined threshold (typically around 5.0), the email is likely to be filtered.
This can severely impact your email deliverability, preventing legitimate communications from reaching their intended recipients. Even if the message isn't outright blocked, it might be diverted to the recipient's junk or spam folder, reducing engagement and potentially damaging your sender reputation. It's crucial to understand how SpamAssassin rules impact deliverability more broadly. You can also review how SpamAssassin 4.0 rules are configured in practice.
To avoid triggering this and similar blocklist (or blacklist) rules, you should adopt transparent email practices. This means avoiding any form of hidden text in your email campaigns and ensuring all email headers, especially the Message-ID, are correctly formatted and consistent with your sending domain. Regularly auditing your email content and sending practices helps maintain a positive sender reputation and ensures your messages land in the inbox.
Views from the trenches
Best practices
Ensure your email templates do not use font colors that blend with the background or extremely small font sizes to hide content.
Always verify that your email sending platform generates unique and properly formatted Message-ID headers for every outgoing email.
Consistently align your Message-ID domain with your sending domain to prevent mismatches that might flag your emails.
Regularly test your email campaigns against various spam filters, including SpamAssassin, to identify and rectify potential issues proactively.
Common pitfalls
Using hidden text to stuff keywords for SEO purposes, which can inadvertently trigger spam filters.
Relying on outdated email sending software that may generate malformed or inconsistent Message-ID headers.
Not checking your email headers for consistency between the Message-ID and the Received headers.
Ignoring SpamAssassin warnings, assuming they only affect a small portion of your audience.
Expert tips
Investigate the specific SpamAssassin rule definitions and source code when troubleshooting, as public documentation may be limited or outdated.
Utilize tools that provide detailed SpamAssassin scores to pinpoint which rules are being triggered in your emails.
Implement strong email authentication (SPF, DKIM, DMARC) to bolster your sender reputation and reduce the impact of minor scoring issues.
Monitor DMARC reports for insights into email authentication failures and potential spoofing attempts that could affect Message-ID integrity.
Marketer view
Marketer from Email Geeks says: I once encountered an issue where my valid Message-ID was still flagged, and it turned out to be related to the presence of invisible text elsewhere in the email.
2021-03-16 - Email Geeks
Expert view
Expert from Email Geeks says: The FONT_INVIS_MSGID rule is a metarule that combines checks for invisible text and a suspicious Message-ID, where "suspicious" means the Message-ID format doesn't match the sender's expected pattern in the received headers.
2021-03-16 - Email Geeks
Minimizing risks from FONT_INVIS_MSGID
The SpamAssassin FONT_INVIS_MSGID rule is a sophisticated detection mechanism for spam that leverages hidden text and suspicious Message-ID headers. Understanding its components—invisible text and inconsistent Message-IDs—is key to ensuring your legitimate emails aren't incorrectly flagged.
By adhering to transparent email design practices and ensuring the technical integrity of your email headers, you can significantly reduce the risk of triggering this rule. This involves avoiding hidden content and verifying that your Message-ID headers are always valid and consistent with your sending infrastructure. A robust email authentication setup (DMARC, SPF, DKIM) can also mitigate issues.
Proactive monitoring and testing of your email campaigns are essential steps in maintaining optimal deliverability. Regularly check your email deliverability with tools to identify potential issues before they impact your recipient engagement or sender reputation. Learn more about why your emails fail to improve your sending practices.
