FCrDNS
Forward-Confirmed Reverse DNS: a check that a hostname's PTR record points at an IP that itself resolves back to the same hostname, the standard sanity test for mail servers.
FCrDNS (Forward-Confirmed Reverse DNS) is a two-step check. Given an IP, look up its PTR record to get a hostname; then look up the A or AAAA records for that hostname and confirm the original IP appears in the answer. If both directions agree, the IP is FCrDNS-validated. Almost every major mail receiver (Gmail, Outlook, Yahoo) requires sending IPs to have valid FCrDNS, and many will downgrade or reject mail that fails the check. Setting up FCrDNS requires control over both forward DNS (A/AAAA) and reverse DNS (PTR), and PTR is usually managed by the IP's hosting provider.