qmail Backend
The qmail home directory is compiled into the program, and defaults
to /var/qmail, but may be overridden by $QMAILHOME.
Messages are delivered through the executable named in
$QMAILQUEUE, or $QMAILHOME/bin/qmail-queue if it is
not set.
The qmail-queue PID is logged in success messages for correlation
with the qmail logs.