PTR (or Pointer) records map an IP address to a domain or subdomain (Reverse DNS). For successful sending of mail, the domain or subdomain should also map to the IP address (Forward DNS).
FCrDNS Test
"FCrDNS, or Forward Confirmed Reverse DNS, is when an IP address has forward and reverse DNS entries that match each other. For FCrDNS verification, first a reverse DNS lookup is done to get a list of PTR. Then for each domain name mentioned in the PTR records, a regular DNS lookup is done to see if any of the A records match the original IP address. If there is a forward DNS lookup that confirms one of the names given by the reverse DNS lookup, then the FCrDNS check passes."
Example
FCrDNS test result:
57.128.189.44 resolved to ns3241178.stepcomms.com;
ns3241178.stepcomms.com resolved to 57.128.189.44;
rDNS is forward confirmed.
The PTR record is configured on the server. This might need to be done by the server hosting company unless they provide tools to do this. Better to use a subdomain for this so that you can configure multiple servers. Use the name of the server perhaps as the subdomain. Then set an A record for the subdomain to point to the IP address of the server. Finally, create an SPF record for the domain
In addition, set the default mail domain to the subdomain in the SMTP service:
Run the configuration tool to ensure all is correct:
