Methods and apparatus, including computer program products, for communication with an external source application. In one aspect, the methods include determining in an independent reader application residing on a client computing system whether a property of an electronic package received and displayed from a source application residing on a server computing system is enabled, determining an authentication of the source application, and enabling a user interface-like communications channel between elements of the electronic package and the source application only if the property is enabled and the source application authenticated, wherein elements of the displayed electronic package are sent to and incorporated by the source application upon authentication of the property of the electronic package and the property of the source application.