IPv6 has been enabled on CentOS 6 by default & if this causes
problem to emails on a cPanel server, following steps can be used to
disable IPv6 on a CentOS 6 cPanel server.
1) In WHM >> Exim Configuration Manager got to advance editor and add following line & save the changes:
disable_ipv6=true
2) To disable IPV6 on modprobe, create a file disable-ipv6.conf on
/etc/modprobe.d/ and add the following line which will force the system
to execute true instead of loading the IPv6 module:
install ipv6 /bin/true
3) Modify the /etc/sysconfig/network to look as following:
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=server.example.com
4) Stop & disable IPv6 firewall:
service ip6tables stop
chkconfig –del ip6tables
5) Make change to /etc/sysctl.conf by adding following line:
net.ipv6.conf.all.disable_ipv6 = 1
6) Reboot the server for changes to apply.