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1·A destination unreachable error PDU was received.

收到了一个目的地不可达的PD U协议数据单元错误。

2·You can probably guess what a destination unreachable attack will be like.

您可能会猜想“目的地不可到达攻击”会是什么情形。

3·Or an destination unreachable message, when there is no route available such as when a line goes down.

或者，当没有可以使用的路径时，比如某条线路被关闭了，ICMP会发出无法送达目的地的信息。

4·The destination port is unreachable.

无法连线到目的连接埠。

5·The destination address is unreachable.

目标地址不可达到。

6·This message was undeliverable due to the following reason: Your message could not be delivered because the destination server was unreachable within the allowed queue period.

以下理由导致您的信件无法寄达：由于无法在允许的伫列周期之内与目标伺服器取得连线，导致这封邮件无法寄达。