How did you set your transport rule?
I would try something like this:
If:the header "to" contains "@yourdomain.com", or you can test if "the recipient domain is..." or "the recipient address includes specific words " would work.
Do: Reject the message with explaination...
Except: the recipient is a member of this group (this groups contains all existing users) or the recipient is this person(ctrl+A to select all users)
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