This is a simple one, by default postfix will accept only mail originating locally, and all it takes is updating a config file to change that.
- Install the necessary packages
- yum install postfix
- Edit the /etc/postfix/main.cf file
- Find the line inet_interfaces = localhost and change it to inet_interfaces = all
- Restart the service
- service postfix restart
- Open the firewall
- iptables -I INPUT -p tcp –dport 25 -j ACCEPT
You should be able to test this by telnetting from a remote computer. If you receive a connection, your good to go.