Please note that you cannot set rDNS [Reverse DNS] or PTR record on your VPS IP address at your end. To set rDNS record for your VPS IP address, you will need to submit the support ticket from client area. To set up rDNS record, we will need hostname (FQDN) and IP address. An ideal rDNS record should look like following.
mail.yourdomain.com ==> 127.0.0.1 // Replace 127.0.0.1 with your actual VPS IP address.
Once we add rDNS record against your VPS IP address, it may take 24 to 48 hours to propagate across the globe.