There’s a slight bug with the Exchange 2010 server, in that it automatically remaps any shared mailboxes you’ve used when the client is either Outlook 2007 or 2010.

If you’re mapped a mailbox in the past, an automapping feature kicks in and puts the mailbox back on your profile, even if you’re starting out with a brand new profile. To disable this, you have to log into your exchange server and issue some commands from the Exchange Powershell:

Add-MailboxPermission "domain\user" -User "domain\user2" -AccessRights FullAccess -AutoMapping:$false

domain\user = user mailbox you no longer want to view
domain\user2 = the primary user, usually the one you’re setting up

This has been tested on Exchange 2010 SP2 and SP1 successfully