Answer: All qmail releases are announced at cr.yp.to/qmail.html. You can pick up the qmail package there.
As of November 1998, the latest qmail package is qmail-1.03.tar.gz, with MD5 checksum 622f65f982e380dbe86e6574f3abcb7c. This package has been mirrored at hundreds of sites around the Internet. If you want to find a mirror close to you, try ftpsearch.
Answer: Follow the step-by-step instructions in INSTALL in the qmail package. If you are upgrading from a previous version, use UPGRADE instead of INSTALL.
Answer: Find a host in your network, say pc.heaven.af.mil, that isn't running an SMTP server. (If people are using mail addresses @pc.heaven.af.mil, you should already have an MX pointing pc.heaven.af.mil to your mail hub.)
Set up a new MX record pointing lists.heaven.af.mil to pc.heaven.af.mil. Install qmail on pc.heaven.af.mil. Replace pc with lists in /var/qmail/control/*. Make the qmail man pages available on all your machines.
Now tell your users about qmail. A user can forward email@example.com to firstname.lastname@example.org to have his mail delivered through qmail. He can set up .qmail files; he can start running his own mailing lists @lists.heaven.af.mil.
When you're ready to turn sendmail off, you can set up pc.heaven.af.mil as your new mail hub. Add heaven.af.mil to /var/qmail/control/locals, and change the heaven.af.mil MX to point to pc.heaven.af.mil. Make sure you leave lists.heaven.af.mil in /var/qmail/control/locals so that transition addresses will continue to work.
Answer: Some sites have clusters of machines that NFS-mount a single /usr disk. Files can't go in /usr unless they are the same on every machine. (This used to be much more common than it is now.) The qmail programs depend on uids that are often different on every machine; for example, the qmail-queue program is setuid qmailq.
/etc is also a machine-specific directory. Unfortunately, it is very small on some machines. (This, too, used to be much more common than it is now.) The qmail configuration often takes quite a bit of disk space.
If I had put the qmail programs into /usr/bin or /usr/local/bin or /usr/qmail/bin, or if I had put the qmail configuration into /etc, some installations would have failed horribly. In contrast, /var/qmail works.
You can set up symbolic links under /var/qmail if you want to move the qmail files to different disks.