Qmail probably doesn't come preinstalled on your machine. It probably isn't even shipped in source form with your machine. You must go to the FTP server, download it, configure it, compile it, test it, and install it. If this sounds like a huge amount of work, it's not—some of these steps can be a single command.
The official place to get qmail is through Dan Bernstein's web and FTP server at http://cr.yp.to. (The .to domain is actually the island nation of Tonga, but they'll sell a "vanity" address to anyone willing to pay, and Dan's professional interests center around cryptography.) An alternate address is http://pobox.com/~djb/qmail.html.
Both URLs are currently redirected to Dan's FTP server, koobera.math.uic.edu, at the Math department of University of Illinois at Chicago. For the rest of this book, we'll nickname that site koobera. The actual name of the site is subject to change at any time, which is the whole point behind using cr.yp.to and pobox.com.
If you use a web browser or a graphical FTP program to open an FTP connection to koobera, the list of files you receive may be scrambled. Dan uses an FTP server of his own creation, publicfile, which is good and bad. It's good because it's a typical Dan Bernstein program: small, secure, and fast. It's bad because most web browsers and visual FTP programs don't know how to parse the server's listing format.
Visual FTP programs without special support for anonftpd's file format (EPLF, ...