in-addr.arpa

`in-addr.arpa` is the special-purpose DNS zone that maps IPv4 addresses to PTR records for reverse lookups. The address octets are reversed and appended to the zone, so `8.8.8.8` becomes the DNS name `8.8.8.8.in-addr.arpa`, which holds a PTR record pointing at `dns.google`. Reverse zones are typically delegated by RIRs (for /24 and larger) or by ISPs (for smaller residential allocations). Properly maintained reverse DNS is required for SMTP servers to pass FCrDNS checks and avoid being treated as spammers.