Private Sub Application_Quit()
  OutOfOffice True
End Sub

Private Sub Application_Startup()
  OutOfOffice False
End Sub

Sub OutOfOffice(bolState As Boolean)
  Const PR_OOF_STATE = "http://schemas.microsoft.com/mapi/proptag/0x661D000B"
  Dim olkIS As Outlook.Store, olkPA As Outlook.PropertyAccessor
  For Each olkIS In Session.Stores
    If olkIS.ExchangeStoreType = olPrimaryExchangeMailbox Then
        Set olkPA = olkIS.PropertyAccessor
        olkPA.SetProperty PR_OOF_STATE, bolState
    End If
  Next
  Set olkIS = Nothing
  Set olkPA = Nothing
End Sub