PTR or Pointer Record is almost the opposite of "A" record in DNS settings. The A record resolves a hostname such as "mail.xxx.com" to an IP address such as "192.168.1.224". Conversely, a PTR record resolves an IP address back to a hostname.

PTR records are mostly used to determine whether a mail server is actually associated with the IP address from which the connection was initiated. Many spammers spoof the address the email came from to bypass anti-spam measures. Receiving mail servers checking a PTR record can dynamically check whether the IP is empty or exactly matches the name of the mail server.