Reason
Email from your email server has been rejected because the PTR resource record (PTR-RR) of your IP address does not comply with our policy.
Solution
Make sure that you do not use a dynamically assigned IP address, but one that is permanently assigned to your e-mail server.
The reverse DNS record may not match the default entry of your hoster/provider such as "123-123-123-123-static.yourprovider.tld".Default entries usually result in a rejection.
Assign your email server its own full domain name (FQDN), recommended format:"mail.yourdomain.tld"