This article explains how to migrate Active-Directory users of a so-called account forest domain to an Exchange resource forest
domain including the conversion of linked into regular mailboxes. Running an Exchange installation in a separate forest has the advantage that Exchange schema extensions are kept completely separate from the forest the account domain belongs to. It is also an administrative boundary for example in case Exchange is provided by a hosting company. On the other hand, it adds complexity and additional domain controllers to the environment that have to be taken care of.