550 Administrative prohibition trying to setup rdns check


Mar 26, 2020
cPanel Access Level
DataCenter Provider
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
Last edited:


Product Owner II
Staff member
Nov 14, 2017
The host lookup on IPv6 is failing in this instance. We don't use this in the exim configuration for this reason, as well as general host lookup issues that can be found on many servers. What we use in place is SpamAssassin rules which perform rDNS lookups and score mail based on this.

The following threads go over this issue though: