/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.