Encrypted MAPI Requirements:
The Branch peer must be joined to the same domain where the Exchange server is installed. The Exchange users can be part of any domains which have Parent/Child trust relationship within the same domain tree.
Encrypted MAPI Limitations:
Outlook users must belong to the same domain tree as the Exchange server and the ProxySG appliance. This release supports multi-domain in the context where all the domains are members of the same domain tree in one forest and there is a Parent/Child Two-way trust relationship exists between these domains. This release does not support multi-domains if the domains are in different domain trees in same forest or in different forests.