PTR record (Reverse DNS) is the process of verifying a domain by querying the IP rather than the domain.

It is a reverse query method in which the receiving side searches the IP of the mail sending server and, if the domain matches the registered PTR value, judges it as a normal mail rather than classifying it as spam.

[PTR record query procedure]

Therefore, the PTR record must be registered with the ISP (KT, SK, U+) rather than DNS.

If you have built your own server, you must directly request it from your ISP. If you are using a separate mail solution or service, you can contact your mail service provider and request registration.

Reference) KT DNS Management Service: https://dms.kornet.net/reverse