If the size of your eximstats database is getting too large, you may have problems deleting the data using the normal WHM route. To clear it completely, you can do the following steps. This will clear it regardless of how big it’s gotten.

First login to shell as the root user. Then, login to mysql and run the following commands:

mysql> use eximstats
mysql> delete from sends;
mysql> delete from smtp;
mysql> delete from failures;
mysql> delete from defers;
mysql> exit;

That last line will get you out of MySQL. That’s it. You can also do a
service exim restart

Just to restart exim and makes sure things are ok. If your eximstats database has gotten pretty large, chances are you have a spam issue and may want to check your /var/log/exim_mainlog log to see if you have any kind of spam activity going on.