Yes it is possible, with the right ACL in place, as the trust is transitive.
—- Added by Bharat Suneja —–
Yes, it is possible and quite common to have users in child domains have mailboxes in a parent domain or a different domain in the Active Directory Forest. The Organization container resides in the Configuration partition (aka “naming context”) in AD. As a result, an Exchange Organization spans the entire AD Forest. Recipients can reside in any domain in the Forest.
What needs to be done:
– Run Setup /DomainPrep in the domain where users or any Exchange servers reside (you’ve already done this for the parent domain where Exchange server resides. You will need to run /DomainPrep in the child domain).
– Create a Recipient Update Service (RUS) and point it to a Domain Controller in the child domain.
– KB 275294: Creating Exchange Recipient Update Service instances for remote domains