Delivery error on Outlook

When sending to a mail account outside my organisation, and then only to certain accounts, I firstly receive a Delivery Delay notification, then a undeliverable message:
“Could not deliver the message in the time limit specified. Please retry or contact your administrator.”

I’m running Exchange 2000 with SP3 on Windows 2000 server SP3.

When sending to those specific accounts via hotmail, the mail is sent.