Mail statistics are stored in a MySQL database that is located in the /var/lib/mysql/eximstats file. You will need to remove the entries from this database to clean it up.
Alternatively, you must clear the mail logs in the /var/log, exim_mainlog, exim_rejectlog, and exim_paniclog files daily.
A cronjob can easily execute a script that clears either of these daily.
Posted in: Cpanel