If you cannot connect to Exchange and you have SecureMac (OIT provided application package that contains Fetch and terminal shortcuts) installed on your computer, you need to delete a file on your computer. Here's how:
Open a new Finder window.
On the Finder sidebar under the Places heading select the User denoted by the house icon, or from the root of the site.
Scroll to locate the file called edu.mit.Keberos. Delete the file by dragging to trash or right click and Move to Trash.