So you have the root domain in one site and the child domain in another site?
And users in the root (parent) domain cannot send email to recipients in the child domain?
What about users in the child domain? Can they send email to the parent domain?
Are you familiar with the dcdiag and repadmin tools?
If so, I would run those (dcdiag is easy enough, in its most basic use, you just type "dcdiag" at the command line - without quotes - and press enter).
Can you tell if the domain controller(s) in the parent domain can resolve the names of the domain controllers in the child domain?
Are there any other errors in the Event Logs concerning DNS?
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