Your mail server has an IP address and a hostname. Reverse DNS is a way to look up a hostname from that address. It is often one of the first things to check when you are setting up an outgoing mail server.
Look up the sending IP
Open Reverse DNS Lookup and enter the IP address your server uses to send mail. Use the outgoing address, which may differ from the address of your website. The PTR answer shows the hostname published in the reverse zone.
For IPv4, the query name ends in in-addr.arpa. For IPv6, it ends in ip6.arpa. The tool shows this name so you can use the same question in another DNS client.
Check the name in the other direction
If the PTR points to mail.example.com, look up that hostname's A and AAAA records. When an answer includes the original address, the pair is forward confirmed. This is a DNS consistency check, not proof of who owns the server or a promise that messages will reach the inbox.
Ask the IP provider to change PTR
Reverse DNS is usually controlled by the organisation that manages the IP allocation. On a VPS, look for a reverse-DNS setting in the hosting control panel. If there is none, ask the provider. Adding a PTR record to your ordinary domain zone usually will not change the reverse answer.
Read failures carefully
No PTR answer and a failed query are different results. A timeout means the resolver did not answer in time. An unsuccessful forward confirmation can also be caused by a lookup failure, so retry before changing records. Once DNS agrees, review your sending hostname and SPF, DKIM and DMARC settings with your mail provider.