Red Hat Enterprise Linux 5 and its derivative CentOS 5 includes a graphical tool called Security Level Configuration for configuring the firewall options. This guide describes how to use this tool to open up the ports necessary for providing an email server.
1. Click System, select Administration and click Security Level and Firewall. This will launch the Security Level Configuration window.
2. For a mail server with webmail, check Mail (SMTP), Secure WWW (HTTPS) and WWW (HTTP) in the Trusted services list.
3. We also need to open the ports for the POP3 and IMAP services. In the Other ports field, add the port numbers below.
You can see more service and port numbers by typing in getent services from the Terminal window.