Reverse DNS + FCrDNS Checker

What Reverse DNS Is

Normal DNS maps names to IP addresses via A and AAAA records. Reverse DNS does the opposite: it maps an IP back to a hostname via a PTR record in the in-addr.arpa zone (for IPv4) or the ip6.arpa zone (for IPv6). PTR records are controlled by whoever manages the IP block — typically your hosting provider or ISP, not you — so setting or changing one often requires opening a support ticket.

What FCrDNS Is and Why It Matters

Forward-Confirmed Reverse DNS is a round-trip check: look up the PTR for an IP, then look up the A or AAAA record for the hostname the PTR returned, and verify that the forward lookup contains the original IP. If the round-trip matches, the hostname is considered legitimate — nobody but the operator of both the IP and the hostname could have configured it that way. Mail servers (Gmail, Outlook, most enterprise mail platforms) reject or deeply penalize mail from sending IPs with no PTR or with broken FCrDNS, because it's one of the cheapest and most reliable spam signals available.

Common Issues

The most common problems are: no PTR at all (hosting provider never set one), PTR pointing at a generic hostname like 123.45.67.89.pool.provider.net that doesn't resolve back, PTR pointing at a hostname with no A/AAAA record (the forward lookup fails), or PTR pointing at a hostname whose A record was changed without updating the PTR. All of these break FCrDNS.

Fixing Mail Deliverability

If you're sending mail from your own infrastructure and finding that receivers reject or spam your messages, broken reverse DNS is almost always one of the first things to check — alongside SPF, DKIM, and DMARC (covered by the Email Auth Checker). I help clients audit and fix their mail deliverability end-to-end. Get in touch or see integration services.