Called by the mailet container to indicate to a mailet that the mailet is being taken out of service. This method is only called once all threads within the mailet's service method have exited or after a timeout period has passed. After the mailet container calls this method, it will not call the service method again on this mailet.
This method gives the mailet an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the mailet's current state in memory.