http://itknowledgeexchange.techtarget.com/itanswers/connect-users-on-child-domain-to-exchange-on-root-domain/
Question:
My organization have AD domain on one machine including windows server 2003 and exchange 2003. The company has bought another company with difference entity, so I created a child domain with the exiting windows 2000 server machine from the new company. Is it possible to have users on a child domain to connect to exchange server on a root domain? Thank you in advance.
Answer:
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.
References:
– KB 275294: Creating Exchange Recipient Update Service instances for remote domainshttps://support.microsoft.com/en-us/kb/275294