554 Nemesis ESMTP Service not available No SMTP service Invalid DNS PTR resource record

Reason

The IP address of the email server you are using is not assigned to a domain name in the domain name system (DNS).

Solution

Please add the missing PTR resource record (PTR RR) DNS entry.

The PTR-RR 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"

If you have already configured a PTR resource record but the connection is still rejected, another possible cause may be an error in the DNSSEC signing of your record. Please therefore check your DNSSEC configuration.