Qmail, clearing the queue
From FreeBSDwiki
To delete all messages from the queue, shut down qmail, then:
server# cd /var/qmail/queue && find intd todo local remote mess info bounce -type f -print |xargs rm
Start qmail back up, and the queue should be empty. Note that if, for example, you just want to clear bounce messages, you could do:
server# cd /var/qmail/queue && find bounce -type f -print |xargs rm