service postfix stop
postsuper -d ALL 
mailq 
service postfix start

Remove some mails:

service sendmail stop
rm /var/spool/mqueue/*
rm /var/spool/mqueue-client/*
mailq 
service sendmail start

Remove alot of mails:

service sendmail stop
cd /var/spool/mqueue-client/
for i in * ; do rm $i ; done 
cd /var/spool/mqueue/
for i in * ; do rm $i ; done 
mailq 
service sendmail start

http://askubuntu.com/questions/403847/who-is-sm-msp-queue-and-how-does-he-she-it-flood-my-var-log-syslog