A PTR record is what lets someone do a "reverse" DNS lookup - that is, they have your IP address and want to know what your host/domain is.
Essentially, PTR records resolve IP addresses into hostnames.
One common myth about PTR records is that they are created for domain names and your domain has to have one to make sure your mail will not be rejected by other mail servers. The truth is that PTR records are created for IP addresses, not domain names. Which means that if you are using our servers to send mail, you do not need to worry about your PTR record. IP addresses of all our mail servers already have PTR records created.
If, however, you are using not only our mail server, but also some other server outside our network and that mail server IP does not have a PTR record created then, we have no way to change that and you need to contact the company which owns that mail server. For example, if you are sending mail through your ISP mail server, you will need to contact their support team and request to configure PTR records for all IP addresses their mail servers are using.
You can also always verify whether a particular IP address has a PTR record created by running the "nslookup" tool or by using sites like http://www.dnsreport.com.