FCrDNS (forward-confirmed reverse DNS) is a consistency check applied to a sending mail server's IP address. The receiver looks up the PTR record for the connecting IP to get a hostname, then looks up the A or AAAA record for that hostname. If the resulting address matches the original IP, the IP is forward-confirmed.
It matters because legitimate mail servers are expected to have matching forward and reverse DNS, while many spam sources and compromised hosts do not. A failing or missing FCrDNS result is a common reason mail is throttled, greylisted, or sent to spam, so it directly affects email deliverability. FCrDNS is separate from DMARC, SPF, and DKIM; it concerns the transport-level IP rather than the message's From domain.
To pass, the operator of the IP, usually your hosting or email provider, must publish a PTR record for the sending IP, and the hostname in that PTR must have a forward record pointing back to the same IP. If you run your own outbound mail, ask your provider to set the reverse DNS for each sending IP and confirm the forward record exists. You can verify both directions with an FCrDNS or PTR lookup tool before relying on the server for production mail.
Check it on your domain
- Reverse DNS (FCrDNS) Checker: check forward-confirmed reverse DNS (FCrDNS) for a mail IP or domain.
- PTR Record Checker: reverse-DNS lookup for an IPv4 address.