Before you can install and use chainmail, you, of course, need to get it, in the source or binary form. Chainmail is available for download from its download page at http://sourceforge.net/projects/chainmail. The distribution consists of two packages: chainmail proper and its accompanying library milteroo. You need to download both packages.
The chainmail download page contains precompiled packages for some specific RPM-based Linux® systems. If your system is one of those, you can download the binary RPM packages for your system. If binary packages for your system are not available, but your system is RPM-based, you are advised to download RPM Source (.src.rpm) packages and compile from them. Finally, if your system is not RPM-based you can download traditional .tar.gz source packages and compile from them.