Outlook Prompting for Credentials on One Message

[rshendrix]

Using Outlook 2003 and a user has received an email and when they click to open it, a window pops up asking for username and password. You can cancel and the message will still open OR you can put your credentials in and the message will open. Actually, the message is displaying in the background while the pop-up window is asking for credentials. I had the user forward the message to me and the same thing happens.

This is only happening on this one message - none of the others.

Any ideas on what's happening?