How can headers help troubleshoot deliverability problems?

When emails land in spam or disappear, headers contain the evidence explaining why. Here's what to look for:

Authentication failures: Check Authentication-Results for SPF, DKIM, DMARC verdicts. Failed authentication often causes spam placement. The header shows exactly what failed and why.

Spam filter decisions: Look for X-Spam-Status, X-Spam-Score, or provider-specific headers. These reveal which rules triggered and the cumulative score.

Routing issues: Trace Received headers to see where the message went. Unexpected servers suggest forwarding that might break authentication. Long gaps between timestamps indicate queuing or delays.

Rejection reasons: Bounce messages include headers from the rejecting server. Error codes and explanatory text explain the rejection: unknown user, policy block, content filtering, etc.

Practical approach:

1. Get headers from a problem message

2. Check Authentication-Results first (most common issue)

3. Look for spam-related X-headers

4. Trace the Received path for anomalies

5. Use header analysis tools to visualize the data

Headers are the diagnostic readout. They tell you what the receiving server saw and how it responded.