Sysadmin Guide: How to Diagnose and Fix Emails Landing in Spam

When emails land in spam, the problem typically stems from one of three root causes: authentication failures, sender reputation issues, or poor content and engagement — and most teams troubleshoot them in the wrong order. A structured triage approach recommends checking authentication first, as SPF, DKIM, and DMARC records are free, binary, and fixable within hours. A common but deceptive failure occurs when SPF and DKIM both pass yet DMARC still fails, which points to an alignment issue rather than an outright authentication breakdown. SPF records can silently fail due to exceeding the 10 DNS-lookup limit — often triggered by adding one too many third-party email vendors — or by accidentally publishing duplicate SPF records. Retrieving raw message headers, rather than forwarded copies, is essential for accurate diagnosis, as forwarding rewrites the authentication data needed for analysis.