A system and method for delivering asynchronous callbacks from a network server to clients that are unreachable by the network server. The system provides a callback server that receives callback registration requests from clients. The callback server generates a web service based on the description of the service that is being invoked by the client and an associated URL address based on its own address. The network server uses the dynamically generated service accessible at this URL address to deliver the asynchronous callbacks. The callbacks are either delivered to the clients by the callback server, if they are reachable inside the network domain, or made available on the callback server for the clients to poll. In addition, the callback server provides functionality that may be too expensive for the clients to provide locally.