PTR records are used for the Reverse DNS (Domain Name System) lookup. Using the IP address you can get the associated domain/hostname. An A record should exist for every PTR record. The usage of a reverse DNS setup for a mail server is a good solution.
Why you need a PTR records?
PTR records provides trust that the given hostname or a domain are connected to the IP address. The PTR records are must have for outgoing mail servers, because most of the mail providers reject or mark as spam messages received by mail servers without valid reverse dns configuration (missing PTR or missmatch A record for the hostname).
If you have any issues relating to this topic please submit a ticket to firstname.lastname@example.org.
Source: Cloud DNS