Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter.
Reverse DNS is separate from forward DNS.
Forward DNS for “xyz.com” pointing to IP address “126.96.36.199”, does not necessarily mean that reverse DNS for IP “188.8.131.52” also points to “xyz.com”. This comes from two separate sets of data.
A special PTR record type is used to store reverse DNS entries. The name of the PTR record is the IP address with the segments reversed + “.in-addr.arpa”. For example the reverse DNS entry for IP 184.108.40.206 would be stored as a PTR-record for “220.127.116.11.in-addr.arpa”.
Many e-mail servers and spam filtering solution on the Internet are configured to reject e-mails from IP address which does not have reverse DNS.
How to ceck rDNS
You can check the rDNS for the IP "18.104.22.168" using the following commands. $ host 22.214.171.124 126.96.36.199.in-addr.arpa domain name pointer google-public-dns-b.google.com.
$ dig PTR 188.8.131.52.in-addr.arpa +short google-public-dns-b.google.com.
Most of the case, you need to contact your Internet Service Provider ( ISP ) for updating the rDNS. Make sure the to set proper A record for the hostname to the proposed IP before contacting ISP for adding rDNs .