/etc/init.d/postfix stop does not terminate "master"

The "master" daemon does not exit when issuing "/etc/init.d/postfix stop" command.

$ sudo netstat -tapn|grep \:25
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 5179/master
$ pgrep master
5179
$ sudo /etc/init.d/postfix stop
 * Stopping Postfix Mail Transport Agent postfix [ ok ]
mache@mmitu:/tmp$ pgrep master
5179

No related messages are thrown to syslog.

To stop postfix I have to run: "sudo pkill -9 master".

Output of sudo sh -x /etc/init.d/postfix stop:

+ PATH=/bin:/usr/bin:/sbin:/usr/sbin
+ DAEMON=/usr/sbin/postfix
+ NAME=Postfix
+ TZ=
+ unset TZ
+ SYNC_CHROOT=y
+ test -f /etc/default/postfix
+ test -x /usr/sbin/postfix
+ test -f /etc/postfix/main.cf
+ . /lib/lsb/init-functions
++ '[' -e /etc/lsb-base-logging.sh ']'
++ . /etc/lsb-base-logging.sh
++ lsb_release -is
+ DISTRO=Ubuntu
+ case "$1" in
+ RUNNING=
+ '[' -f /var/spool/postfix/pid/master.pid ']'
++ sed 's/ //g' /var/spool/postfix/pid/master.pid
+ pid=5179
++ ls -l /proc/5179/exe
++ sed 's/.* //'
+ exe=/data/backup/usr/lib/postfix/master
+ '[' X/data/backup/usr/lib/postfix/master = X/usr/lib/postfix/master ']'
+ log_daemon_msg 'Stopping Postfix Mail Transport Agent' postfix
+ '[' -z 'Stopping Postfix Mail Transport Agent' ']'
+ log_use_usplash
+ type usplash_write
+ usplash_write 'TEXT Stopping Postfix Mail Transport Agent postfix'
+ log_use_fancy_output
+ TPUT=/usr/bin/tput
+ EXPR=/usr/bin/expr
+ '[' -x /usr/bin/tput ']'
+ '[' -x /usr/bin/expr ']'
+ /usr/bin/tput hpa 60
+ FANCYTTY=1
+ true
+ /usr/bin/tput xenl
++ /usr/bin/tput cols
+ COLS=127
+ '[' 127 ']'
++ /usr/bin/expr 127 - 7
+ COL=120
+ printf ' * Stopping Postfix Mail Transport Agent postfix '
 * Stopping Postfix Mail Transport Agent postfix ++ /usr/bin/expr 127 - 1
+ /usr/bin/tput hpa 126
                                                                                                                              + printf ' '
 + '[' -n '' ']'
+ log_end_msg 0
+ '[' -z 0 ']'
+ log_use_usplash
+ type usplash_write
+ '[' 0 -eq 0 ']'
+ usplash_write 'SUCCESS ok'
+ '[' 120 ']'
+ printf '\r'
+ /usr/bin/tput hpa 120
                                                                                                                        + '[' 0 -eq 0 ']'
+ echo '[ ok ]'
[ ok ]
+ return 0
+ exit 0