singing-milter is designed to be supervised. It does not detach the current shell.
The recommended way ist this script. In the simplest setup it's added to /etc/inittab
#! /bin/sh exec < /dev/null exec 2>&1 test -f /etc/default/signing-milter && . /etc/default/signing-milter cd ${WORKDIR:-/etc/signing-milter} exec env - /usr/sbin/signing-milter ${OPTIONS}