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Reverse DNS
IP address (v4 or v6) → PTR hostname lookup. Useful for tracing the rDNS of a mail server or checking that your IP resolves to the hostname you think it does.
/tools/reverse-dns does a PTR lookup on any IPv4 or IPv6 address. Most consumer IPs have a generic ISP rDNS (abc.def.adsl.example-isp.com); server IPs should have one matching the hostname the operator uses. A mismatch is the first thing Gmail / Outlook look at when scoring a sending IP — your MX must resolve back to itself.
Pure DNS, no caching, runs in well under a second.