PTR records are mainly used to check if the server name is actually associated with the IP address from where the connection was initiated.
IP addresses of all Intermedia mail servers already have PTR records created.
If you are using both Intermedia mail servers and external mail servers (e.g. Dedicated Web Server or Cloud Server) and the external server does not belong to Inter media infrastructure, you need to create PTR record because it will help your server pass some security tests when connecting to other mail servers. To do that you will need to contact the company which owns the IP address of the server. Usually it is your Internet Service Provider (ISP).