You can use the mailq command sendmail -bp command to display a summary of the mail messages queued for future delivery.
OR
Remove all queued mails from: /var/spool/mqueue
-- Source http://www.cyberciti.biz/faq/linux-unix-bsd-clear-sendmail-queue/
mailqOR
sendmail -bpRemove all queued mails from: /var/spool/mqueue
cd /var/spool/mqueue/
ls
rm *-- Source http://www.cyberciti.biz/faq/linux-unix-bsd-clear-sendmail-queue/