code: public static smtp_server[] get_filtered(nitro_service service, String filter) throws Exception
  	{
  		smtp_server obj = new smtp_server();
  		options option = new options();
  		option.set_filter(filter);
  		smtp_server[] response = (smtp_server[]) obj.getfiltered(service, option);
  		return response;
  	}
docs: Use this API to fetch filtered set of smtp_server resources.
  filter string should be in JSON format.eg: "vm_state:DOWN,name:[a-z]+"