Understanding and Preventing Email Authentication Failures

Authentication failures are a leading cause of deliverability problems. Here is how to understand and prevent them.

Types of authentication failures

SPF failure: The sending server's IP is not listed in your SPF record. Common cause: sending through a service you forgot to include in SPF. DKIM failure: The DKIM signature does not validate. Common cause: DNS record missing or incorrect, DKIM not enabled in your email provider, or the email was modified in transit. DMARC failure: Neither SPF nor DKIM passes and aligns with the From domain. Common cause: misconfigured SPF or DKIM, or alignment issues when sending through third-party services.

Detecting failures

Check email headers. When you send a test email, view the full headers (in Gmail: Show Original; in Outlook: View Source). Look for "spf=pass," "dkim=pass," and "dmarc=pass." Any "fail" or "softfail" indicates a problem. Use the free checkers at Warm Inboxes — SPF Checker, DMARC Checker, and DNS Checker — to verify records are correct.

Common failure scenarios and fixes

New domain, forgot DKIM: Go to Google Admin or Microsoft 365 and enable DKIM signing. Add the DKIM DNS record. Added a new sending service, forgot to update SPF: Add the new service's include statement to your existing SPF record (do not create a second SPF record). DMARC alignment failure: Ensure the domain in your From address matches the domain validated by SPF or DKIM. SPF exceeding 10-lookup limit: Consolidate or flatten your SPF record. Remove any unused include statements.

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