When your email is scanned by our mail server, it goes through a series of spam filter checks. These checks assign scores to different characteristics of the message. A higher combined score increases the likelihood that the message will be flagged as spam.
Below is a breakdown of common filter results you may see in email headers, what they mean, and why they might appear.
Meaning: The Bayesian spam filter thinks there’s a 50% chance the message is spam.
Why it happens:
Bayesian analysis is based on statistical patterns of known spam vs. legitimate messages.
A BAYES_50 score is neutral — it indicates the system is unsure (half spam / half ham).
This usually occurs with short, generic emails or content that doesn’t strongly match either spam or ham patterns.
✅ Tip: Add more meaningful, personalised content to help the filter categorise it correctly.
Meaning: The message contains invisible font elements but no actual HTML <html> tag.
Why it happens:
Spammers often use tiny or invisible text to include hidden keywords, which can influence filters or bypass simple checks.
The lack of a proper HTML structure combined with hidden text is suspicious.
✅ Tip: Make sure your email templates are properly formatted with standard HTML tags and avoid using invisible text (e.g. white text on white background).
Meaning: The email contains very long lines of invisible text.
Why it happens:
Hidden or obfuscated text is a classic spam tactic to trick filters.
Very long lines of invisible text are often used to pad out messages to dilute spammy content.
✅ Tip: Check your email signature or template for unnecessary or hidden formatting. Use visible, properly styled text.
Meaning: The message is in HTML format.
Why it happens:
This is a neutral or low-impact flag.
Most modern legitimate emails are HTML formatted, so this alone isn’t a problem.
However, combined with other suspicious traits, it may contribute to the spam score.
✅ Tip: Ensure your HTML is clean, properly structured, and not overly complex.
Meaning: The email’s MIME type says it’s HTML, but there’s no <html> tag in the content.
Why it happens:
This is common in poorly formatted emails or some marketing platforms that strip tags.
Spam filters consider this sloppy or deceptive coding, often used in mass-mailed spam.
✅ Tip: Use properly built email templates that include <html> and <body> tags.
Meaning: The body text matches a known pattern from a KAM marketing blacklist (PCCC = “Possible Commercial Content Code”).
Why it happens:
The message content contains phrases, URLs, or patterns commonly found in unsolicited marketing emails.
KAM rulesets are community-maintained SpamAssassin rules that detect known spam/marketing wording.
✅ Tip: Rework overly “salesy” phrases, avoid spammy buzzwords (e.g. “Act now”, “Guaranteed results”), and ensure proper unsubscribe links and business details are included.
Meaning: This rule relates to DMARC (Domain-based Message Authentication, Reporting & Conformance) policy checks.
Why it happens:
This indicates a DMARC evaluation was performed. Depending on result, it may be informational or contribute to scoring.
If DMARC fails (alignment or authentication), your email is more likely to be treated as spam.
✅ Tip: Ensure your sending domain has correctly configured SPF, DKIM, and DMARC records.
Meaning: The message only includes an HTML part — no plain text version.
Why it happens:
Legitimate emails usually include both HTML and plain text versions (multipart/alternative).
Spammers often send HTML-only messages to embed tracking or hide text.
✅ Tip: Always include a plain text alternative version of your email, especially in bulk or marketing sends.
Meaning: The sending domain passed SPF authentication.
Why it happens:
The server sending the message is authorised by the domain’s SPF DNS record.
This is a good sign, and typically lowers the spam score.
✅ Tip: Make sure your SPF records stay up to date to avoid future delivery issues.
Spam filtering involves multiple factors. One single rule (e.g. HTML_MESSAGE) usually won’t cause delivery issues. However, several small issues together can push an email’s score over the threshold.
To improve deliverability:
✅ Use proper HTML templates
🧠 Avoid hidden or deceptive formatting
📄 Include plain text alternatives
🌐 Authenticate with SPF, DKIM, and DMARC
📬 Avoid spammy language and follow marketing email best practices