PTR Record (Reverse DNS)
A PTR (Pointer) record maps an [[ip-address|IP address]] back to a hostname, enabling reverse DNS lookups — the inverse of an [[a-record|A record]] or [[aaaa-record|AAAA record]] lookup. PTR records live in special reverse-lookup zones under the in-addr.arpa (IPv4) and ip6.arpa (IPv6) domains, where the IP octets or nibbles are written in reverse order. Mail servers use PTR records as a basic anti-spam signal: a sending IP without a matching PTR record (or whose PTR does not forward-confirm back to the same IP) is frequently rejected by spam filters. ISPs and hosting providers control delegation of these reverse zones.
Example
The IP 93.184.216.34 has a PTR record pointing to 93.184.216.34.in-addr.arpa that resolves to 'edgecastcdn.net'; mail servers verify this to confirm the sender IP is legitimately associated with the claimed domain.