/var/spool/clientmqueue contains number of files

Check the contents of /var/spool/clientmqueue:

# ls -ltr /var/spool/clientmqueue | more
# more /var/spool/clientmqueue/filename

If it is all unwanted email, the below procedure will remove the files:

# cd /var/spool/clientmqueue
# find . -type f -exec rm {} \;

Note : If the email is important, then configure sendmail correctly to deliver this mail.