Definition
PTR record (reverse DNS): A DNS record under in-addr.arpa or ip6.arpa that maps an IP address back to a hostname. Receivers expect a sending IP to have a PTR whose hostname resolves forward to the same IP, and Gmail requires it of all senders.
How reverse DNS works
For the IP 192.0.2.25, the PTR record lives at 25.2.0.192.in-addr.arpa, with the octets reversed. IPv6 addresses use the ip6.arpa zone the same way, one hex digit at a time.
The zone belongs to whoever owns the IP block, such as your hosting provider or ISP, not to your domain's DNS host. That is why you set a PTR in the provider's console or by asking them.
Why mail servers check PTR records
Spam from infected home computers comes from IPs with no PTR or with generic names such as 192-0-2-25.dyn.isp.example. A real mail server has a stable, specific hostname like mail.example.com.
Receivers check that the PTR exists, that its hostname resolves back to the same IP, and often that it matches the name the server gave in its HELO or EHLO greeting. A mismatch in any of the three counts against you.
How to set a PTR record for a sending IP
On a cloud server, set the reverse DNS name in the provider's networking settings. On a dedicated IP from an email platform, the platform sets it for you.
Use a hostname on a domain you control, create the matching A record first, and configure your mail server to greet with the same name.
$ dig +short -x 192.0.2.25
mail.example.com.
$ dig +short A mail.example.com
192.0.2.25How RocketVerifier handles it
Our free reverse DNS lookup finds the PTR record for any sending IP and confirms forward-confirmed reverse DNS. Our own verification IPs self-test before they carry traffic: port 25 open, forward-confirmed reverse DNS and a HELO name that matches. The email deliverability checker scores SPF, DKIM and DMARC for your domain, and Monitors recheck those records and blocklist status on a schedule with alerts.