Why might rDNS be empty?

Question

Domain Name System

When checked in mail-tester, X-Spam-Last-External-rDNS: - empty, does not contain any data. This is when sending emails from your server to confirm registration, via Joomla.
Previously, when I added domains, this problem did not exist. Why can this be so? I probably forgot to register something in the DNS records?

Answer 1

rDNS is registered with your ISP, which provides the IP.

Answer 2

rDNS should not only not be empty, but should also match HELO
. I've only met a couple of hosters who could set rDNS in the hosting control panel. Basically, just write to the hosting support with a request to register rDNS for the domain, then check it through the mail tester to match with
HELO ten