We have an Exchange 2000 Server within our company. Our users have Outlook 2000 on thier PCs. Someone tried to sending me an email with a file attachment. I was not able to receive the email with the attachment. The sender instead received a message in their email: Subject: "Delivery Status Notification (Failure)" with the following Comment: The following message to <email address> was undeliverable. The reason for this problem: 5.1.0 - Unknown address error 550 - '5.2.1 Message rejected.' Any ideas of what would cause this or where to start troubleshooting? Thanks, Joe