Reverse DNS is a key component of the configuration of your mail server.
Not having Reverse DNS can mean not sending emails! Without well-configured Reverse DNS zone and PTR records, the rest of the email servers can’t check your domain’s IP address and discard your messages or throw them into the spam box.
Everything you need to know about Reverse DNS
A Reverse DNS is a service that provides Reverse DNS zones for your domain. The Reverse DNS zones serve to host PTR records that can be used for verification purposes, to check the IP addresses and if they lead to the correct hostnames.