Automatic LiveUpdate fails to update virus definitions. In order to update virus definitions you have to run LiveUpdate manually.
Symantec Mail Security for Microsoft Exchange (SMSMSE) creates a task in Scheduled Tasks (windows 2003) or Task Scheduler (Windows 2008) to automatically run LiveUpdate. The System account is assigned to the task. In some cases the System account is unable to run LiveUpdate.
Disable automatic virus definitions updates in SMSMSE and create a task using Scheduled Tasks (Server 2003) or Task Scheduler (Windows 2008) to run LiveUpdate. Make sure the user assigned to the task can successfully run LiveUpdate.