You can set a reverse DNS (RDNS/PTR) simply by submitting a support request with the IP address and record you would like changed.
Note: You must have an A record setup for the FQDN (hostname) pointing to the IP for which you want to setup a rDNS record, before you can set a rDNS record with that hostname. This restriction ensures that you can only set rDNS values for domains that are pointed to your VPS.
If you want to setup the rDNS entry "mail.example.com" for your IP 123.123.123.123 then you need to first create an A record for "mail.example.com" pointing to your VPS/IP 123.123.123.123.
(For example, we would not allow you to setup a rDNS record with "mail.google.com" since "mail.google.com" does not point to your VPS obviously and hence this restriction is in place to prevent potential abuse.)
