I am trying to change an agent's email address in the admin interface, and I get this error: "The email address is already in use". Why is this happening?
The email address you're trying to change to is already associated with a user on the helpdesk.
Go to the agent interface and find the user's profile (the quickest way is to enter the email address in the search bar).
You can either change the user's email address, or delete the profile (doing the latter will also delete all the user's tickets).