You wish to configure Symantec Messaging Gateway (SMG) for High Availability or Load Balancing.
The Symantec Messaging Gateway does not include any specific high availability features such as load balancing, fail over clustering, active/passive clustering. It is possible to use other third party solutions to help accomplish high availability.
(NOTE: Symantec recommends keeping a current backup of the Control Center box to minimize any Control Center downtime in case of a disaster. The scanners would continue to scan and process mail while the control center is down)