Exim is the standard mail sending program used with WHM. It is important to secure Exim, as your server could become compromised otherwise.
After logging into WHM as root, open the "Service Configuration" menu and open the "Exim Configuration Manager".
In the Basic Editor tab, there are various security options that can be enabled or disabled. These options include ACL Options, Access Lists, RBLs (Remote Blocking Lists), among others.
There is also an option for "Advanced Editor", through which you can manually edit the Exim configuration, but this is only recommended for expert users.