To see mail queue, enter:
# mailq
# postqueue -p

To remove all mail from the queue, enter:
# postsuper -d ALL

To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred

Grafik arabirim:
# pfqueue

Kaynak: http://www.cyberciti.biz/tips/howto-postfix-flush-mail-queue.html