Email probs- Mail delivery failed,

I use Outlook Express and WinXP Pro. Now and then I receive emails from "Mail Delivery System" saying an email could not be sent, and the email is being returned to me. Here's part of the message (with email address removed): "This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: xxxxxx@xxxx.com host lookup did not complete: retry timeout exceeded". Before this I get similar emails with this message: "This message was created automatically by mail delivery software. A message that you sent has not yet been delivered to one or more of its recipients after more than 72 hours on the queue on smtp1.global.net.uk. The message identifier is: xxxxxxx The subject of the message is: Re: xxxx The date of the message is: Tue, 4 Sep 2007 23:32:17 +0100 The address to which the message has not yet been delivered is: xxxxxx@xxxx.com Delay reason: host lookup did not complete No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you." Can anyone tell me what these messages mean please? Thanks.