Type /usr/lib/sendmail -oQ/var/spool/omqueue -q and press Return.
The -oQ flag specifies an alternate queue directory and the -q flag says to run every job in the queue. Use the -v flag if you want to see the verbose output displayed on the screen.
When the queue is finally emptied type rmdir /var/spool/omqueue and press Return.
This removes the empty directory.