A PTR record is an assertion
Whoever leases an IP address controls its reverse zone and can put whatever they like in it — including mail.well-known-bank.example. The PTR alone therefore proves nothing. Only the reverse check turns it into evidence: if the hostname found resolves forward back to exactly this address, two independent parties have committed to the same statement — the holder of the address block and the holder of the domain. That is FCrDNS, forward-confirmed reverse DNS.
Major providers treat this as a hard criterion. Google and Microsoft name it explicitly in their requirements for sending systems; some servers refuse the connection outright without valid FCrDNS.
What this tool checks
- PTR record per address, for IPv4 and IPv6.
- Forward resolution of the name found, with the matching query type — A against IPv4, AAAA against IPv6.
- FCrDNS verdict per address: confirmed, no PTR set, or PTR present but with no way back.
- AS number and operator — whose network the address sits in.
For a domain with MX records, its hosts are checked rather than the domain itself. Someone asking whether "their mail server" has reverse DNS means those addresses — not the web server answering under the same name. A bare IP address or a hostname without MX is checked directly.
The four failure patterns
- No PTR set. With many cloud providers that is the state on delivery.
- PTR points at the provider's default name such as
ip-203-0-113-10.hoster.example— for which no matching A record exists under your control. - PTR set, A record forgotten. The reverse direction is right, the forward one missing. This is the most common half-failure.
- Only IPv4 maintained. The server also sends over IPv6, where the PTR is missing entirely — which shows up here immediately, because both address families are listed separately.
The PTR is set with the operator of the address block, that is your hoster — not your domain's DNS provider. That is why it is so often simply forgotten.
What this check cannot see
Whether the name your server announces in HELO matches the PTR is not visible in DNS — that requires a connection to be made. The full analysis of a real email performs that comparison.