To Limit Maximum Email Size do the following, Login to cPanel/WHM as root Under Service Configuration, Click “exim Configuration Manager” Click “Advanced Editor” Scroll down and Click “Add additional Configuration Setting” Find “message_size_limit” in pull-down list and choose Enter limit that you want to change to Scroll to bottom and click “save” Bandwidth monitoring The easiest way to achieve this in cPanel would be to modify the bandwidth processing time for cPanel's statistics software. This can be configured at WHM >> Home >> Server Configuration >> Statistics Software Configuration >> Schedule Configuration >> Bandwidth Processing Frequency. Note, that this might cause a performance degradation, so keep an eye on the server when these changes are made. As these websites are likely using the same IP, monitoring individual users with a third-party plugin like Munin will be very difficult. Most user bandwidth moni