I'm agree with this reply. Also you should establish a trusted relationship between both domains, and give the same permissions to BESAdmin account. Remember, you will use the same besadmin account to manage both domains.
Posted via BlackBerryForums.com Mobile
Originally Posted by knottyrope
as long as it is in the same forest it will work, but users can not send from both domains as the device is only tied to one based on the primary SMTP address.