Exchange stores plenty of things in Active Directory, including mailbox information for each account. In case of a deinstallation of Exchange, this information usually remains behind, making it impossible to perform a new clean installation of Exchange and assigning a new mailbox to this user.

I found a solution here: http://www.telnetport25.com/2009/09/forcing-the-removal-of-exchange-attributes-on-a-directory-user-the-removeexchangeattributes-tool%E2%80%A6/

This command-line tool allows you to query and/or remove Exchange-attributes on AD objects (such as users and groups).