You could create an automation stitch using the "Outbound bandwidth rate exceeded" or "Inbound bandwidth rate exceeded" events. However, please take into account the requirements for this configuration to work:
1. For the alert email to be sent when the Automation stitch is triggered, an Email server needs to be configured under System -> Advanced.
2.The Outbound bandwidth and Inbound bandwidth rate exceeded event will get generated only when FIPS-CC is enabled as per design.
Only certain versions and models are FIPS-CC certified and the configuration has to be done while the unit is in factory default settings.