There are three types of alerts: system alerts, manager alerts, and user alerts.
System alerts are sent to defined SYSADMIN users' email addresses. You can specify system alert settings at the server level (Settings > Edit Server Settings > Alerts) or at the org level (Organizations > ORGNAME >Edit Organization). In addition, you can choose to customize email alert message text.
Available system alerts:
Manager alerts are sent to the manager(s) of the organization holding the affected client. Alerts can also be sent to parent org managers if you choose. An org manager is defined as a user with SYSADMIN, ADMIN or MANAGER permissions.
Available Manager Alerts:
The system level settings can be over-ridden on the Edit Organization page. You have to click the Customize option to view them.
User alerts (when enabled) are sent to a user's account email address when one of his computers has not backed up to any destinations in the number of days you specify. User alerts are disabled by default, but can be enabled or disabled for the organization.
You can enable/disable user alerts by following the instructions in the Manage User Settings and Defaults article.