A PTR (Pointer) record does the opposite of a normal DNS lookup — instead of resolving a domain name to an IP address, it resolves an IP address back to a hostname. This is called reverse DNS (rDNS), and it's a common check performed by mail servers and network diagnostic tools.
PTR records live in a special reverse DNS zone: IPv4 addresses use the in-addr.arpa domain (with the IP octets reversed), and IPv6 addresses use ip6.arpa. Unlike A/AAAA records, which anyone can set for a domain they control, PTR records can typically only be set by whoever controls the IP address block — usually your hosting provider or ISP, not you directly.
Many receiving mail servers perform what's called Forward-Confirmed reverse DNS (FCrDNS): they do a PTR lookup on the sending IP, then a forward lookup on the resulting hostname, and check that it resolves back to the same IP. A missing or mismatched PTR record is a strong spam signal to many mail providers — servers sending mail from an IP with no PTR record, or one that doesn't match the sending domain, are more likely to be flagged or rejected.
Use our Bulk PTR / Reverse DNS Lookup to check the reverse DNS hostname for many IP addresses at once — useful when auditing a pool of sending IPs for email deliverability, or diagnosing mail servers before they go live.