A PTR Record, is the reverse of an A Record in DNS. Being as it tells the "IP" what domain is associated to it. You cannot manage your PTR Record, because you do not own the IP Block. If you have a VPS, there should be a "HOSTNAME" option, or PTR option under your VPS control panel. Otherwise, contact your host to set the PTR record for you.
BTW, if you name your server srv1.example.com, your PTR should be srv1, and postfix should use srv1 as well.