Stop postfix

["Andrea Bencini" <andrea bencini tin it>]

I installed F8.
I updated F8 (yum update)
I removed sendmail (yum remove sendmail).
I installed postfix (yum install postfix).
I installed dovecot (yum install dovecot).
I installed fetchmail (yum install fetchmail).
I installed samba (yum install samba).
I installed dhcp (yum install dhcp).
If I shutdown and restart ( shutdown -r now) the messages are regular ( all 
OK).
If I shutdown (shutdown -h now) I receive this message:
"Start killall: Stop postfix [Failed]
                                     [Failed] "

Can you help me?
Thanks
Andrea