A Pointer record (PTR) resolves an IP address to a fully-qualified domain name (FQDN) as an opposite to what A record does. PTR records are also called Reverse DNS records.
Here is what it looks like when performing external DNS lookup:
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 Smarsh Hosted Services mail servers already have PTR records created.
If you are using both Smarsh Hosted Services mail servers and external mail servers (e.g. Dedicated Web Server or Cloud Server) and the external server does not belong to Smarsh Hosted Services 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).
You can check your PTR record in external DNS lookup solutions like MX Toolbox.