host unreachable from outlook via exchan

When we sent out mails, I observe at MS Exchange Admin – Queues, outbound mail waiting for delievery, has this status said eg: abc@yahoo.com pending (host unreachable)
Note: (It’ll take about 15 minutes to have this and not all mail has this message..)

And after awhile a error message will sent back to the sender. One of my “friend” replied me saying ” I think you need to set your server for longer attempts” …