Odd Problem with a user receiving forwarded emails

We have a user here that receives some emails with emails attached. When he double-clicks on the attached email, the New Mail window opens. The problem follows his Outlook account–when he logs into another PC the problem is still there. I’ve done some testing and I can forward an email from
my Yahoo! account to him and a different user here–he has the problem and the other user can open the attached email fine. Also, not all attached emails he receives have this problem and other attachements (.jpgs, documents) work fine. When it does not work, the attachment size shows 64B.

We are using Outlook/Exchange 2003 with current service packs and patches.

Anyone have any ideas?