Exchange 2007 Problem.

Hi,

I am trying to add NewUser entry in Exchange. I have written a code to execute command Enable-MailUser.
I have set all commands parameter as Identity, ExternalMail.
But it is giving me exception when I execiute Invoke method on pipeline. The exception is :
System.Management.Automation.CmdletInvocationException: Method not found: 'System.String Microsoft.Exchange.Configuration.Tasks.RecipientObjectActionTask`2.get_RusServer()'. ---> System.MissingMethodException: Method not found: 'System.String Microsoft.Exchange.Configuration.Tasks.RecipientObjectActionTask`2.get_RusServer()'.
   at Microsoft.Exchange.Management.RecipientTasks.EnableRecipientObjectTask`2.get_UseRusServer()
   at Microsoft.Exchange.Management.RecipientTasks.EnableRecipientObjectTask`2.PrepareDataObject()
   at Microsoft.Exchange.Configuration.Tasks.SetTaskBase`1.InternalValidate()
   at Microsoft.Exchange.Configuration.Tasks.ObjectActionTask`2.InternalValidate()
   at Microsoft.Exchange.Configuration.Tasks.RecipientObjectActionTask`2.InternalValidate()
   at Microsoft.Exchange.Configuration.Tasks.Task.ProcessRecord()
   at System.Management.Automation.Cmdlet.DoProcessRecord()
   at System.Management.Automation.CommandProcessor.ProcessRecord()
   --- End of inner exception stack trace ---
   at System.Management.Automation.Internal.PipelineProcessor.SynchronousExecuteEnumerate(Object input, Hashtable errorResults, Boolean enumerate)
   at System.Management.Automation.Internal.PipelineProcessor.Execute(Array input)
   at System.Management.Automation.Runspaces.LocalPipeline.InvokeHelper()
   at System.Management.Automation.Runspaces.LocalPipeline.InvokeThreadProc()


Can you please suggest me the way to proceed?