In order to clear the Qmail queue, initiate the following commands from shell :
# service qmail stop
# find /var/qmail/queue/mess -type f -exec rm {} \;
# find /var/qmail/queue/info -type f -exec rm {} \;
# find /var/qmail/queue/local -type f -exec rm {} \;
# find /var/qmail/queue/intd -type f -exec rm {} \;
# find /var/qmail/queue/todo -type f -exec rm {} \;
# find /var/qmail/queue/remote -type f -exec rm {} \;
# service qmail start
Another easy way to remove the mails from queue is by initiating :
# /usr/local/psa/admin/sbin/mailqueuemng -D