We migrating an entire domain and Exchange environment to a new datacenter. Two companies exist in this environment. Only one plans to stay at the new datacenter. Due to this we need to remove all instances of the old company in the Exchange 2013 environment. In general what steps do I need to take? Half of the users are with the old company and may be purged. I just need to make sure all Exchange references to the old company domain and respective routing are gone.