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A comprehensive audit of your email program covering authentication, reputation, content, infrastructure, and monitoring.
Send test emails from every platform that sends as your domain. Check Authentication-Results headers for "spf=pass". Fix any failures.
Each sending source should produce a valid DKIM signature. Check for "dkim=pass" in headers. Misconfigurations here are common after ESP changes.
DMARC requires either SPF or DKIM to pass AND align with the From domain. Check your DMARC reports for alignment failures.
Sending IPs should have a PTR record that resolves back to the IP. Without it, some mailbox providers reject connections outright.