Find the hostname behind any IP address via its PTR record.
Reverse DNS queries the special in-addr.arpa / ip6.arpa zones for a PTR record. Mail servers check it for spam filtering. Many IPs (especially client and cloud ranges) have no PTR — that 404 is a normal answer, not an error.
8.8.8.8 → dns.googleNo. Only the IP owner can set PTR, and most IPs have none. That is normal.
Receiving servers may reject mail from IPs without matching forward/reverse DNS.
Yes. Enter any IPv4 or IPv6 address.