Why does Email do this?

We have Exchange 2000 and Outlook 2000 that when we get emails with Attachments the email stays as an unread message. So when you open the email and read it and then close the message it stays as an unread message instead of an read message. The question I have is why does it do this. The Exchange and Outlook did not use to do this but does now. So I would like to know how to remedy this problem. Please let me know how to fix this and why does this issue occur? Please email me with this solution at wroming@ci.killeen.tx.us as well as submit the answer in Tech Republic so that I may rate your answer. Thank you for your time on this matter and have a great day.