Forward-confirmed reverse DNS (FCrDNS) is also known as full-circle reverse DNS, double-reverse DNS. It shows the relationship Between IP address and domain name (Hostname).  Most mail transfer agents use FCrDNS verification and if in case, there is a valid domain name, put it in the “Received:” trace header field. With FCrDNS you can prove that your IP address is using a sending domain that is owned by you.

The IP address must have a valid pointer (PTR) record in DNS which resolves to a valid hostname.

The hostname must have an A record in the DNS which matches the sending IP address.

Let’s take an example

• Forward DNS  -> example.in (domain) → 1.1.1.1 (IP)
• Reverse DNS 1.1.1.1 (IP) → example.in (domain)

FCrDNS means that the Forward and Reverse DNS must match. In the above case, the forward and reverse DNS do match and therefore the FCrDNS is confirmed.