Quoting from the home page of qmail, where you can find more information as well as the package itself:
qmail is a secure, reliable, efficient, simple message transfer agent. It is meant as a replacement for the entire sendmail-binmail system on typical Internet-connected UNIX hosts.
See also the now defunct qmail.org (latest snapshot on archive.org). For more information, search the net.
At the time of this update (November 2013), nothing at all. I haven't been involved in running any sort of mail service in years. So what you find here only has archival value at best; hence the faded look. If you do use some software found here, please read and understand all the code before using it, and proceed at your own risk. Thank you.
Well, I'm using it to replace sendmail. What else? (And no, you can't play the Towers of Hanoi using qmail. I use emacs for that, not sendmail.)
Below, you will find some scripts and small programs that have helped me to run qmail efficiently. Use them if you wish, but beware: Though I try to write carefully, I can not quarantee that this is either bug free or secure. Caveat emptor and all that.
I keep this page around only for historical reasons,
and in case someone still finds my ancient contributions useful.
But I am not interested in maintaining it beyond the barest minimum.
Update 2019-10-15: Fixed broken links (and added my default css).