FCrDNS stands for Forward Confirmed Reverse DNS, is when an IP address has forward and reverse DNS entries that match each other. For FCrDNS verification, first a reverse DNS lookup is done to get a list of PTR. Then for each domain name mentioned in the PTR records, a regular DNS lookup is done to see if any of the A records match the original IP address. If there is a forward DNS lookup that confirms one of the names given by the reverse DNS lookup, then the FCrDNS check passes.
For example:
If IP address 50.116.84.38 resolves to mail.techjunction.co and host name mail.techjunction.co resolves to IP addresses 50.116.84.38, then, reverse DNS (rDNS) for IP address 50.116.84.38 is forward confirmed.
But if IP address 50.116.84.38 resolves to mail.techjunction.co but host name mail.techjunction.co resolves to IP addresses 50.116.84.40, then, reverse DNS (rDNS) for IP address 50.116.84.38 is NOT forward confirmed.