Typically, the most efficient (that is, the shortest or cheapest) route is used for message transfer, and additional routes stand by, in case the best route is temporarily unavailable.

• 通常,采用最高效(即最短或开销最低)的路由路径来传输邮件,而其他路由路径处于等待状态,以备在最佳路由路径暂时不可用时启用。