When Advanced Threat Protection (ATP) module is enabled on a customer account, there is the option in the control panel to enable Real Time Alerts under Management >  Email > ATP:

The ATP module is powered by Hornet Security and when an ATP alert is generated the email address(es) list above will receive an alert such as this;

Sender:            Hornetsecurity ATP-Alert hornet-atp@hornetsecurity.com

Subject:           Hornetsecurity Realtime ATP + [classification of alert]