RedHat/CentOS 5.8 默认就有sendmail，而6.4默认没有。
手动清理方法：
find /var/spool/clientmqueue/ -type f|xargs rm -f
定时清理方法：
echo "find /var/spool/clientmqueue/ -type f|xargs rm -f" >/server/scripts/del_sys_file.sh
echo "00 00 * * 0 /bin/sh /server/scripts/del_sys_file.sh >/dev/null 2&1" >>/var/spool/cron/root