Though this article was written for FreeBSD, you can apply the same overall concepts to many other flavors of Open Source Operating Systems. For example, in SuSe Linux you would install Procmail from the YAST utility. Debian, use aptget. Basically each OS has its own package management system you can use. Your OS does not have Procmail listed in its package management utility? Do not dispair, you could always visit the site for the program itself and download the source code so you can build it fresh. In fact, compiling from code is the best way to ensure the resulting executible ocde is tuned specifically for the kernel of your OS and its particuliar settings.