In our small environment, we had the following:
Windows Server 2012 R2 with Exchange 2013 SP1
Windows Server 2008 R2 with Exchange 2010
The Windows Server 2008 R2 hard drive died, and I have rebuilt the server as a new Windows Server 2012 R2. I followed the instructions in ADSI EDIT to remove the old Exchange 2010 entry. My problem are the mailboxes that were hosted on the Exchange 2010 database which no longer exists. I have disabled the mailboxes in Exchange 2013 but I need to remove the mailbox entries associated with the users so I can create new mailboxes on Exchange 2013. Help....I don't even know if this is the correct approach.