Callback notifications when the callback server is unavailable

What happens when the server that is setup to receive callbacks isn't available due to a network issue or an issue with the server itself. Are any attempts made to retry the notification. Also, are there any suggestions on how to handle the integration situation if it isn't possible to receive callbacks due to firewall rules? Should the integrator create some sort of polling to check the status of open packages and their signers that it has sent or is there a better solution? Thanks, Jimmy