Clear postfix mail queue
Show Queue
Postqueue -p
Delete mail to/from mailbox
postqueue -p | tail -n +2 | awk 'BEGIN { RS = "" } / info@mailbox\.co\.uk/ { print $1 }' | tr -d '*!' | postsuper -d -
Clear Postfix Queue
postsuper -d ALL
or just defered mailpostsuper -d ALL deferred
Source: http://www.cyberciti.biz/tips/howto-postfix-flush-mail-queue.html