In case you're wondering, qmail is not open source, and does not qualify for use of the OSI-Certified trademark. Other programs which Dan Bernstein licenses similarly, including djbdns, ucspi-tcp, and daemontools, are also not open source. For a program to be "open source", you must be able to, among other things, change the source and redistribute it. DJB prohibits distribution of modified code and so programs which are so-licensed are not open source. Other code written by DJB has been placed into the public domain. Public domain code is unlicensed, and it qualifies for use of the OSI-certified trademark.