by lunarg on August 20th 2014, at 10:12

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:

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