Linux Babe published a tutorial about setting up a Postfix SMTP server on CentOS

Run Your Own Email Server on CentOS - Postfix SMTP Server

This is the best tutorial series about building email server on CentOS from scratch on the Internet. Not only will you have a working email server, but also you will have a much better understanding about how email works.

Run Your Own Email Server on CentOS - Postfix SMTP Server