Hi,
Do you complete mailbox migration?
If not, we need configure connector between forest to ensure mail flow:
1. Modify the accept domain from Authoritative domain to Internal Relay domain in source forest.
2. Add source domain as Internal Relay domain in target forest.
3. New send connector with smart host on source forest.
4. Modify the receive connector and ensure "Source Domain Exchange Server's IP address" in Remote server scope.
More details about it, for your reference:
http://www.careexchange.in/how-to-configure-cross-forest-connectors-mail-flow-exchange-2010-to-exchange-2010/
Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
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Allen Wang
TechNet Community Support
