Licenses on the Email Security - Server appliance can be purchased in two ways.
Subscription-based appliance license
This appliance license has an expiry date. When the license expires, the appliance stops inspecting as well as the passing of traffic. The user can only access the license screens and cannot use the appliance.
Perpetual appliance license
This appliance license does not expire. The customer owns the appliance and has unlimited access to all the features. The appliance will continue to inspect with the latest updated OS and SC.