mdadm does not send mails after reboot

Question

I have updated my ubuntu-server from 16.04 to 18.04 and mdadm in general runs fine, but (as said in the headline) it doesn't send any mail after reboot.

mdadm --monitor --scan --test --oneshot 

works just fine and I get a mail.
I have edited the etc/default/mdadm like this:

# mdadm Debian configuration
#
# You can run 'dpkg-reconfigure mdadm' to modify the values in this file, if
# you want. You can also change the values here and changes will be preserved.
# Do note that only the values are preserved; the rest of the file is
# rewritten.
#

# AUTOCHECK:
#   should mdadm run periodic redundancy checks over your arrays? See
#   /etc/cron.d/mdadm.
AUTOCHECK=true

# START_DAEMON:
#   should mdadm start the MD monitoring daemon during boot?
START_DAEMON=true

# DAEMON_OPTIONS:
#   additional options to pass to the daemon.
DAEMON_OPTIONS="--syslog --test"

# VERBOSE:
#   if this variable is set to true, mdadm will be a little more verbose e.g.
#   when creating the initramfs.
VERBOSE=false

I use postfix (it should send the mails via a gmx.com - adderess) and the testmails are received.

The mail-address (the same that is used by postfix for sending) is added correctly to the mdadm.conf.
I marked one harddrive as faulty (manually) and got a mail, but I have no idea why I don't get any mail after rebooting.

My /etc/init.d/mdadm looks like this:

#!/bin/sh
#
# Start the MD monitor daemon for all active MD arrays if desired.
# This script is not used under systemd.
#
# Copyright © 2001-2005 Mario Jou/3en <[e-mail]>
# Copyright © 2005-2009 Martin F. Krafft <[e-mail]>
# Distributable under the terms of the GNU GPL version 2.
#
### BEGIN INIT INFO
# Provides:          mdadm
# Required-Start:    $local_fs $syslog
# Required-Stop:     $local_fs $syslog sendsigs 
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: MD monitoring daemon
# Description:       mdadm provides a monitor mode, in which it will scan for
#                    problems with the MD devices. If a problem is found, the
#                    administrator is alerted via email, or a custom script is
#                    run.
### END INIT INFO
#
set -eu

MDADM=/sbin/mdadm
MDMON=/sbin/mdmon
RUNDIR=/run/mdadm
PIDFILE=$RUNDIR/monitor.pid
DEBIANCONFIG=/etc/default/mdadm

test -x "$MDADM" || exit 0

test -f /proc/mdstat || exit 0

START_DAEMON=true
test -f $DEBIANCONFIG && . $DEBIANCONFIG

. /lib/lsb/init-functions

is_true()
{
  case "${1:-}" in
    [Yy]es|[Yy]|1|[Tt]|[Tt]rue) return 0;;
    *) return 1;
  esac
}

case "${1:-}" in
  start)
    if [ -x /usr/bin/systemd-detect-virt ] && /usr/bin/systemd-detect-virt --quiet --container; then
      log_daemon_msg "Not starting MD monitoring service in container"
      log_end_msg 0
      exit 0
    fi

    if is_true $START_DAEMON; then
      log_daemon_msg "Starting MD monitoring service" "mdadm --monitor"
      mkdir -p $RUNDIR
      set +e
      start-stop-daemon -S -p $PIDFILE -x $MDADM -- \
        --monitor --pid-file $PIDFILE --daemonise --scan ${DAEMON_OPTIONS:-}
      log_end_msg $?
      set -e
    fi
    if [ "$(echo $RUNDIR/md[0-9]*.pid)" != "$RUNDIR/md[0-9]*.pid" ]; then
      log_daemon_msg "Restarting MD external metadata monitor" "mdmon --takeover --all"
      set +e
      $MDMON --takeover --all
      log_end_msg $?
      set -e
    fi
    ;;
  stop)
    if [ -f $PIDFILE ] ; then
      log_daemon_msg "Stopping MD monitoring service" "mdadm --monitor"
      set +e
      start-stop-daemon -K -p $PIDFILE -x $MDADM
      rm -f $PIDFILE
      log_end_msg $?
      set -e
    fi
    for file in $RUNDIR/md[0-9]*.pid ; do
      [ ! -f "$file" ] && continue
      ln -sf $file /run/sendsigs.omit.d/mdmon-${file##*/}
    done
    ;;
  status)
    status_of_proc -p $PIDFILE "$MDADM" "mdadm" && exit 0 || exit $?
    ;;
  restart|reload|force-reload)
    ${0:-} stop
    ${0:-} start
    ;;
  *)
    echo "Usage: ${0:-} {start|stop|status|restart|reload|force-reload}" >&2
    exit 1
    ;;
esac

exit 0