FCrDNS

A check where the PTR name is resolved forward and the result includes the original IP address.

In plain words

FCrDNS verifies that an IP's PTR record and the name's A/AAAA record match — forward and reverse DNS are consistent.

Practical example

If the PTR of 1.2.3.4 returns mail.example.com and the A record of mail.example.com returns 1.2.3.4, FCrDNS passes.

Where you encounter it

When diagnosing email delivery problems and configuring mail servers to reduce spam classification.