550 Administrative prohibition trying to setup rdns check

So I was going through spam to see how we could improve the spam filter, and I realized that Cpanel default allow mail from servers regardless of whether they have valid RDNS or not. I don't think that makes any sense, I know even Gmail doesn't allow this, so I went to find out how to configure this. I found under Home "Service Configuration "Exim Configuration Manager, Advanced Editor, a setting "custom_begin_check_message_pre" which could be set to "require verify = reverse_host_lookup" which supposedly should check this. Problem is, when I set this, it is no longer possible to send mail. The server immediately returns an error "550 Administrative prohibition". The exim log file says "cpanel rejected after DATA: host lookup failed ( ::1 does not match any IP address for localhost)", but I have this line in /etc/hosts: ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6