Hi,
If you have already created DNS zone efg.com on abc.com domain controllers and there are another domain named efg.com , you will face DNS zone conflict , and user from abc.com domain can't resolve DNS entry on the target domain efg.com.
You have to consolidate DNS zone of efg.com by move all DNS entry in the DNS zone efg.com hosted on abc.com domain controller to DNS zone hosted on domain controllers of efg.com , then remove the efg.com DNS zone on all abc.com DCs , then you can add efg.com DCs as additionals forwarders, to help users from abc.com domain to resolve efg.com DNS zone.
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