undefined reference to 'errno'

This is due to the fact that qmail does not have a errno.h file. This causes a problem with glibc 2.3.1 and later.

To fix this you can find the errno.h patches for all of Dan Bernstein's software including qmail, daemontools, and ucspi-tcp. These patches are available from ftp://moni.csi.hu/pub/glibc-2.3.1/

or use Netqmail, which you can download from:

qmail.antivirus.ie/netqmail/

Charles Cazabon, Dave Sill, Henning Brauer, Peter Samuel, and Russell Nelson have put together the netqmail distribution of qmail. It is comprised of qmail-1.03 plus a patch file, some documentation, and a shell script which prepares the files for compilation.