In case you're wondering, qmail is now open source, and qualifies for use of the OSI-Certified trademark. Other programs which Dan Bernstein licenses similarly, including djbdns, ucspi-tcp, and daemontools, are not yet open source. For a program to be "open source", you must be able to, among other things, change the source and redistribute it. DJB used to prohibit distribution of modified code. DJB has promised that all his code will be placed into the public domain. Public domain code is unlicensed, and it qualifies for use of the OSI-certified trademark.