4.11 Where is qmail/ezmlm/djbdns?

Dan J. Bernstein distributes all software he has written with a restrictive license, consequently, it's in the non-free area. Since the license he uses does not allow modified binaries to be distributed, you have to compile it yourself from the source and the Debian patches to obtain a binary package you can install in your Debian GNU/Linux system.

The source package names are qmail-src, ezmlm-src and djbdns-installer, respectively.

For qmail you need to install qmail-src first and then run build-qmail to build the Debian package. You also need to do install the ucspi-tcp-src package to get ucspi-tcp, which qmail depends on.

Dan J. Bernstein maintains a FAQ from distributors page if you are interested in reading his reasons (one of which is Cross-platform compatibility)