By default, HostBill outputs number of useful informations about it work with each cron run to console.
In some cases this cause email to be sent to server administrator. This can be prevented by disabling output by adding
to the end of HostBill cron job line
So entire cron job task should look like:
php -q /path/to/your/hostbill/admin/cron.php > /dev/null 2>&1