FCrDNS check

Forward-confirmed reverse DNS (FCrDNS) checks that the sending IP's PTR (reverse DNS) record points to a hostname, and that hostname's own A/AAAA record resolves back to the same IP — a two-way match. Many receiving mail servers treat a mismatch or missing PTR record as an automatic red flag, independent of SPF/DKIM/DMARC, since it's cheap to fake a From: address but harder to fake a consistent forward/reverse DNS chain.