Ensure that SPF and DKIM are properly configured for your domain. Create a DMARC policy in your DNS records. The policy will look something like this: _dmarc.yourdomain.com IN TXT "v=DMARC1; p=none; rua=mailto:dmarc-reports@yourdomain.com" Monitor the reports and adjust the policy as needed, moving from monitoring (none) to quarantine and eventually to reject.