Reverse DNS is typically the opposite of DNS. It is the process of resolving an IP Address into a domain name. They are generally used by email servers since spammers use invalid IPs. Hence email servers use reverse DNS to check if the incoming message is coming from a valid source before it can come onto their network. If the IP address and domain name do not match the server blocks that particular message.
How does reverse DNS work?
Reverse DNS lookups query DNS servers for a PTR (pointer) record, if the server does not have a PTR record, it cannot resolve a reverse lookup. PTR records store IP addresses with their segments reversed and they append ‘.in-addr.arpa’ to that. For example if a domain has an IP address of 184.108.40.206, the PTR record will store that information as 220.127.116.11.in-addr.arpa.