On a shared computer, if a previous user forgot to logout of SquirrelMail, and the session is still alive (i.e. the browser is still running), the next user that logs in will have their e-mail address overwritten with the previous user's address.
The user's preference file must be edited to rewrite the address to the correct one.
To prevent this problem, new users should always quit the browser completely or reset the browser cache/history when logging in from a shared computer.