Reverse DNS Lookup
PTR record for an IP, checked against forward DNS
Look up the hostname an IP address points back to (its PTR record), then check that the hostname resolves to the same address again. Mail servers run this check on every connection.
Frequently asked questions
What is reverse DNS?
Normal DNS turns a name into an IP address. Reverse DNS goes the other way: a PTR record, stored under a special in-addr.arpa (IPv4) or ip6.arpa (IPv6) name, turns an IP address back into a hostname.
What is forward-confirmed reverse DNS (FCrDNS)?
The PTR name must itself resolve back to the same IP. Anyone can make their PTR claim any name, so receivers only trust it when the forward lookup agrees. Many mail servers reject or spam-folder mail from IPs that fail this check.
Who can change a PTR record?
Whoever controls the IP address, not whoever owns the domain. Ask your hosting or network provider; for a CDNShark VPS you can set it yourself under the VPS's Network tab.