A synchronization system establishes a session between a client end point and a server end point via an intermediary server of a server pool. When messages sent from the client end point to the server end point are received at the server pool and the intermediary server is unavailable, the messages are routed to a new intermediary server. The end points, however, may be unsynchronized as a result of the unavailability. The new intermediary server can detect that it is a new intermediary server for the session and assumes that the server end point and the client end point may be unsynchronized because of the unavailability. The new intermediary server then notifies the client end point that it may need to re-synchronize with the server end point.