Answer
If a system is capable of continuing to function after a component has failed, then that component is considered to be a "redundant" component.
From the perspective of an Internet messaging system full redundancy would require that if any one of the components within the system fail such as the mail store, user database, Internet connectivity, etc. the system would continue to function.
GMS can be configured to provide varying levels of redundancy dependant on customer requirements.
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Keywords:redundancy