What is reverse DNS?
Reverse DNS maps an IP address back to a hostname using PTR records in the special in-addr.arpa (IPv4) and ip6.arpa (IPv6) zones. The PTR record is controlled by whoever owns the IP block — usually your ISP or hosting provider, not your domain registrar.
Mail servers check that the sending IP has a PTR record and that the hostname resolves back to the same IP (forward-confirmed reverse DNS). Missing or mismatched PTR records are a common reason for email rejection.
よくある質問
How do I change my PTR record?
Ask your hosting provider or ISP. Most cloud control panels have a reverse DNS setting for each IP.
What does forward-confirmed mean?
The PTR hostname has an A/AAAA record that points back to the same IP, which proves both sides are controlled by the same operator.