This article refers to the Barracuda ArchiveOne versions 6.5 and higher, and Microsoft Exchange 2010 and higher.
You can install Microsoft Exchange 2010 and 2013 in a cross-forest topology where Exchange is installed in each forest with synchronization between the forests. For details, see the Microsoft TechNet article Deploy Exchange 2010 in a Cross-Forest Topology or Deploy Exchange 2013 in a cross-forest topology.
To deploy ArchiveOne in an Exchange cross-forest topology, you should install ArchiveOne in each forest. ArchiveOne should only process mailboxes in that forest.
For instructions on installing ArchiveOne, refer to Step 3 - Install the ArchiveOne Service and Components.