qmail is a mail transfer agent (MTA) that runs on many operating systems. The qmail software was written by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program. qmail is the only broadly deployed MTA in the public domain.
The qmail wiki discusses qmail, how to install and maintain qmail, and the many patches people have written to add new features to qmail.
It's also a centralized place for people to post links to translations of the qmail documentation in many langauges, currently: Chinese, German, Korean, Polish, Russian, Spanish, French, Italian, Turkish, Swedish, Indonesian, and Portuguese.
(The cyberdesk wiki also occasionally discusses qmail).