Alternatively, if your installation has the official Configserver Firewall GUI..
![Roll Eyes ::)](https://forum.centos-webpanel.com/Smileys/default/rolleyes.gif)
Go to Firewall Configuration..
SMTP_ALLOWGROUP = mail,exim,postfix,dovecot
(not all will be required)
You can also find this in /etc/csf/csf.conf <-- may as well
read the file whilst you're there.