This can be achieved by scoping the host value of registry variable SYSADMIN. For configuring email addresses to alert for OS metrics , please navigate to the following dashboard:
Administration | Rules and Notifications | Manage Registry Variable | SYSADMIN | View and Edit Details | Registry Values | Click on the Green Icon in Registry Values | Search for "Host" | Select the desired hosts.

This method cannot be used for alerts from agents on other cartridges.
For more customisations, please contact Professional Services.