RDNS_NONE - Delivered to internal network by a host with no rDNS

I’m trying to increase the score of outgoing emails from one of the sites on my CyberPanel and I’m following the article Achieve 10/10 Email score with CyberPanel! - CyberPanel

I have corrected almost all the points but there remains an annoying penalty that reports this error:

-1.274 RDNS_NONE Delivered to internal network by a host with no rDNS
This may indicate you do not have a rDNS configured for your hostname or the rDNS does not match your sending IP

What’s wrong with my CyberPanel?