Howtoforge published a guide about installing Qmailtoaster on CentOS 5.3

Qmailtoaster is a project that aims to make the installation of Qmail onto RPM based systems a snap. All of the packages are distributed in source RPMs so building the packages for your particular distro and architecture is as easy as running a script or a simple command for each package. The RPMs have all of the needed and commonly asked for patches included so you can have a mail server up and running in about an hour. When it's all complete, you'll have a full Qmail mail server installation ready for just about anything. I personally run Qmailtoaster servers for other companies and ISPs who have tens of thousands of users on their systems.

How To Install Qmailtoaster (CentOS 5.3)