In order to receive an email when a machine state changes you can set this up in your 'Notifications' section.
Go to the 'More' drop-down menu (to the right of 'Settings' tab) and select 'Notifications'.
Add and name a new group under 'Notification Groups' - notification group description is optional.
Select the tab 'Enable Alerts' for alert options.
Under the section All Alerts | Protection | Machines, enable the checkbox 'Status of Agent has been changed'.
Select the tab 'Notification Options'.
Enable the checkbox 'Notify by email' and add a recipient email address.
Once the group and notification has been configured, select the 'Change' option under 'Email Settings' and enable the checkbox 'Enable email notifications'.
Ensure that SMTP Server settings are configured.
Once all of these settings are complete, an email will be sent to the designated address when an agent goes offline/online.