/var/log/syslog has been populated with this error message for quite a while :
exim4: ALERT: exim paniclog /var/log/exim4/paniclog has non-zero size, mail system possibly broken
To stop filling syslog log file, make sure to delete /var/log/exim4/paniclog log file and restart the exim daemon as follows :
root@localhost:~#service exim4 restart
Stopping MTA for restart: exim4_listener.
Restarting MTA: exim4.