As root, simply type: postfix reload. Actually there are two ways to do this. The one we just mentioned requires root privileges to execute. If you need to flush the queue as a user there is a symlink to sendmail in /usr/sbin you can use to do the same thing. One example of when this might be needed is if you want to flush the queue in your dial-up script as a regular user. It works the same way it always did with sendmail: /usr/sbin/sendmail -q.