How do I interpret SpamAssassin DKIM test results and troubleshoot DKIM signature issues?

Key findings

• Interpretation nuances: SpamAssassin's DKIM tests, like DKIM_SIGNED (indicating the presence of a DKIM header) and DKIM_INVALID_DKIM (signaling an invalid signature), are individual scoring elements. A low score (e.g., 0.1) for an invalid DKIM doesn't necessarily mean the email will go to spam, as SpamAssassin aggregates scores.
• Limited documentation: Comprehensive official documentation for all SpamAssassin rule descriptions is scarce, with older wikis being partial and incomplete. The most detailed information often resides directly within the installed rules or the Perl code.
• Custom rule prevalence: Many SpamAssassin installations use custom rules that are not publicly documented, making it challenging to understand all score triggers without direct access to the server's configuration.
• Diagnostic importance: When SpamAssassin reports an invalid DKIM, the next step should be to examine the email's full headers, particularly the Authentication-Results header, for more precise details on the failure reason.
• Alignment considerations: DKIM issues might arise if the envelope domain (the actual sending domain) differs from the From address domain, which is a common cause for DMARC alignment failures even if DKIM itself passes technical validation.

Key considerations

• Holistic view: Do not solely rely on SpamAssassin scores. While useful for diagnostics, a positive score on a DKIM rule does not necessarily mean an email will be marked as spam. Focus on overall email authentication health (SPF, DKIM, DMARC) for true deliverability improvements.
• Header analysis: Always analyze the full email headers for a complete picture of authentication results. Tools that break down header information can provide clear insights into DKIM validation outcomes. Learn how to troubleshoot DKIM issues with header analysis.
• DMARC alignment: Even if DKIM is technically valid, a domain mismatch between the From header and the DKIM signing domain can lead to DMARC alignment failures. This is often the actual cause of deliverability problems. Fix DKIM from domain mismatch to improve DMARC results.
• Consult official documentation: For specific rule interpretations, refer to the official Apache SpamAssassin test definitions and resources. While not exhaustive, it's the primary source for understanding standard rules. Find more at the Apache SpamAssassin Project website.

Key opinions

• Confusion over scores: Marketers frequently find the numerical scores in SpamAssassin tests confusing, especially when small scores like 0.1 appear for what seems like a correctly configured DKIM. They often wonder if these minor flags genuinely impact deliverability.
• Focus on actual delivery: The main goal for marketers is inbox placement. If a SpamAssassin test shows a DKIM issue, but DMARC reports show passes, they prioritize real-world delivery over tool-specific warnings.
• Seeking clearer explanations: There is a desire for more straightforward explanations of SpamAssassin test descriptions beyond the technical jargon found in official documentation.
• Distrust of generic tools: Some marketers express skepticism about the utility of generic SpamAssassin scoring tools because they do not reflect the diverse and often custom configurations of real-world mail servers.

Key considerations

• Understand tool limitations: Remember that online SpamAssassin checkers are diagnostic tools and may not perfectly replicate how every ISP or mail server filters mail. Focus on the core authentication mechanisms (SPF, DKIM, DMARC) as outlined in a simple guide to DMARC, SPF, and DKIM.
• Verify DKIM independently: If SpamAssassin reports an invalid DKIM, verify its validity using dedicated DKIM validation tools, as seen in this Server Fault discussion on DKIM signing, to rule out false positives or minor misconfigurations.
• Address underlying issues: If a DKIM issue is consistently reported, investigate potential causes such as body hash mismatches or header canonicalization problems. For more in-depth troubleshooting, review how to fix DKIM body hash mismatch failures.

Key opinions

• Scoring engine context: Experts highlight that SpamAssassin's rules are part of a scoring system; a single rule firing doesn't automatically mean a problem, as it contributes to an overall spam score.
• Documentation limitations: There's a consensus among experts that thorough, up-to-date documentation for all SpamAssassin tests, particularly custom ones, is largely unavailable. The best resources are often the installed rules themselves.
• Interpreting DKIM scores: For DKIM tests, DKIM_SIGNED indicates the presence of a DKIM-Signature header, while DKIM_INVALID_DKIM signifies that the signature is not valid, regardless of other positive configurations.
• Full header analysis: To truly troubleshoot DKIM, experts recommend examining the full email headers, specifically the Authentication-Results header, for detailed failure reasons.
• Vanilla SpamAssassin irrelevance: Many experts state that vanilla SpamAssassin is not widely used for email filtering by major players, rendering online SpamAssassin scores mostly useless for real-world deliverability predictions.

Key considerations

• Beyond SpamAssassin: While SpamAssassin can provide clues, do not treat its output as the definitive word on your DKIM health. Focus on actual inbox placement and DMARC reports for a more accurate assessment. For more on this, check troubleshooting emails landing in spam.
• In-depth diagnostics: For precise DKIM troubleshooting, use comprehensive online tools that analyze the email's headers and provide detailed validation results, as suggested by experts (e.g., About My Email). This helps pinpoint the exact nature of the DKIM signature issue.
• Check email headers: Always obtain and review the raw email headers. They contain the definitive authentication results from receiving mail servers, providing far more actionable data than SpamAssassin scores alone. You can also review how to check authentication results in headers.
• Read the rules directly: If you have access to a SpamAssassin installation, examine the rule definitions directly in the configuration files (e.g., in /usr/share/spamassassin) for the most accurate understanding of how specific rules are triggered.

Key findings

• Rule descriptions: SpamAssassin documentation provides descriptions for specific rules, including DKIM-related tests, outlining the conditions or patterns that trigger these rules in an email header or body.
• Technical details: The documentation often includes technical details such as regular expressions for header or body clauses that define what a rule must match to fire, useful for administrators setting up or debugging their own SpamAssassin instances.
• DKIM's role: Documentation clarifies that DKIM provides validation results for interpretation by spam filters. DKIM itself is not explicitly a spam prevention tool, but a mechanism for verifying email authenticity.
• Scoring system: SpamAssassin is described as a scoring-based engine, where the firing of a rule adds to a cumulative score rather than immediately flagging an email as spam. Individual rule scores contribute to the overall likelihood of an email being categorized as spam.

Key considerations

• Reference authoritative sources: For the most accurate understanding of SpamAssassin's standard tests, always refer to the official Apache SpamAssassin Project documentation. This is the primary source for technical definitions. See the Apache SpamAssassin test definitions.
• Combine with DMARC: While SpamAssassin provides signals, a robust email authentication strategy relies on SPF, DKIM, and DMARC. Documentation for these standards, such as those from SIDN on DMARC implementation, offers a more complete picture of email validation. Consider an advanced guide to email authentication for deeper insights.
• Beyond technicality: Understand that documentation primarily focuses on how a rule works, not necessarily its precise impact on deliverability. For real-world impact, observe DMARC reports and inbox placement rates.