You will need to modify the config files for IMP to use a different IMAP server than localhost.
Look at /home/httpd/html/horde/imp/config/defaults.php3
I can't remember what changes were made, but there is a section
$default->server =
This should point to the IP address of your exchange server.
If this works, you will want to create a custom-template to make sure the change sticks.
John can probably give you more info than me!!!