PTR record
A PTR (Pointer) record is the reverse of an ordinary DNS lookup: instead of resolving a name to an IP address, it resolves an IP address back to a hostname, stored in the special in-addr.arpa (IPv4) or ip6.arpa (IPv6) DNS zones. Mail servers commonly check that an incoming connection's IP has a PTR record, and that the PTR's hostname in turn resolves forward back to the same IP (forward-confirmed reverse DNS) -- a mismatch or missing PTR record is a common reason legitimate mail gets flagged as spam. Residential and dynamic IP ranges often have generic, ISP-assigned PTR records rather than ones controlled by the account holder.
By AdminKit team, Netbase JSC · Updated