What is Reverse DNS?
DNS Ptr Records
Just like forward DNS allows an administrator to associate a domain name to an IP, the Reverse DNS (rDNS) makes it possible to map a random IP address to a hostname. In reality, if you type into your browser www.constellix.com, your browser will check the IP address for that specific domain to find the exact page being requested. But if you only have access to a certain IP address, you can find out the Hostname of that IP by executing a Reverse DNS lookup.
Within reverse zones, you will find what we call the PTR records, or Pointer Records. The PTR records are responsible for defining the reverse DNS for each host on the network. In other words, a PTR record is used to map a network interface (or IP) to a hostname. These records are most often used for reverse DNS.
For instance, an A record for mail.example.com points to the IP address 22.214.171.124. In the PTR of the reverse database, this IP address is stored as the domain name 126.96.36.199.in-addr.arpa, pointing back to its designated hostname "mail.example.com".
When a mail server receives an email, a three-way handshake takes place to verify the sending server.
Forward DNS Check> Reverse DNS Check> FQDN Check
During this process, the forward DNS must match the reverse DNS as defined in in-addr.arpa, which must match the fully qualified domain name in the message header.
When the three-way handshake passes, the email is delivered to the client Inbox without issue. If the check fails, the mail is either rejected outright or delivered to the clients' spam folder.
Properly configured Reverse DNS can help prevent your email from ending up in the recipient's spam folder.
Reverse DNS requires a special reverse DNS domain ending with .in-addr.arpa.
in-addr.arpa takes IP addresses and prepends them to their respective domain.