check the content of the file /etc/sysconfig/network
Make sure you have in here what the external dns name is.
HOSTNAME=mail.johnson.com
This line SHOULD be in accordance of the /etc/hosts file. If your server has an ip address like 192.168.4.1 then your /etc/hosts file should look like
127.0.0.1 localhost.localdomain localhost
192.168.0.19 mail.johnson.com mail
Install DNSMASQ
yum -y install dnsmasq
modify the following line in /etc/dnsmasq.conf
mx-host=johnson.com,mail.johnson.com,50
modify the /etc/resolv.conf as follows:
nameserver 127.0.0.1
nameserver 8.8.8.8
restart dnsmasq
/etc/init.d/dnsmasq restart
check your work
dig johnson.com mx