Validate your SPF record, check DNS lookups, and ensure your email authentication is properly configured.
SPF (Sender Policy Framework) is an email authentication protocol that helps prevent email spoofing by specifying which mail servers are authorized to send email on behalf of your domain.
Check if your SPF record is properly formatted and valid.
Ensure you don't exceed the 10 DNS lookup limit.
Verify your DMARC policy for complete protection.