This is slightly outside of the scope of this discussion. Nevertheless, I believe I need to be more clear, since the comment posted was factually inaccurate. qmail itself does not come with a license. This is because of djb's philosophy. Given just this, you would be allowed to modify your local copy, but not make copies of the software for others. In addition, he grants rights of redistribution for specific, unmodified versions that he released. They must have the same MD5 checksum.
As a result, it is possible to, for instance, distribute the basic qmail distribution, a minor patch, and some method of combining them, in the way that netqmail does. Nevertheless, it is illegal to distibute binary builds of netqmail, or sources in a format other than the one described above. This practically limits the scope of modifications you can make -- while it is possible to write a better version of diff/patch, at the moment, you cannot rename a file, since then the text of the file would be in the patch. You cannot combine two files, or split a file into two for similar reasons. We can get around some of this with a better version of diff/patch, but there will almost certainly always be limitations to the types of modifications you can make. This also limits how the software may be packaged. It is still non-trivial to patch binaries, so every user must make a local build. You cannot make a standard .deb or .rpm of netqmail (you can do the installer thing some .debs of proprietery software do, that grabs it and compiles at, at the addition of some amount of build time). If you have 50 servers in your location, you cannot install the same hard drive image on all of them if the hard drive image contains netqmail. You must compile each copy of netqmail locally. There are many other similar limitations.
If djb goes off the deep end, or dies, users of his software are SOL. They'll get patches with security fixes for some time, but the software will be fundamentally unmaintainable in the long term.
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