You have to contact your VPS server provider to create rDNS record. As you will need hostname (FQDN) and IP address for your server. Your record should be like this:
mail.yourdomain.com ==> 23.54.1.0 // 23.54.1.0 IP address should be your VPS server.
After adding the record, it will take 4-48 hours to progate the record.
mail.yourdomain.com ==> 23.54.1.0 // 23.54.1.0 IP address should be your VPS server.
After adding the record, it will take 4-48 hours to progate the record.