Delay in e-mail notification to Yahoo

I can send e-mails internally and externally, I just cannot send e-mails to aol.com and yahoo.com. When I try to send e-mails to these hosts I get this message in return.

This is an automatically generated Delivery Status Notification.

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

Delivery to the following recipients has been delayed

I’m running Exchange 2000 and have no other problems sending to outside e-mail addresses.