One of the simplest and most effective ways of controlling the flow of spam on your server is to enable RBLs for your mail program. RBL stands for Remote Black List.
These lists contain the IPs of many know spammers.
On cPanel servers enabling RBLs is as simple as checking a box.
First log into WHM for your account. Once logged into WHM, use the find box up in the upper left to search for ‘exim’.
One of the results this will return is Exim Configuration Editor, this is the option you should choose.
Toward the bottom of this editor there will be a RBL section. Simply click the checkbox next to each of the options, bl.spamcop.net and zen.spamhaus.org, and then click on save to force the changes to take effect.
Enabling these RBLs will not completely stop the flow of spam, but should be able to put a sizable dent in it.