how to reject mail based on destination addresses IF the mail was not sent from local network ?

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Hello,

I would like to configure Postfix to reject mail whose destination
match a few addresses IF the mail was not sent from our internal
network.

Basically i would like to do a header_checks on 2 lines (Received: and
To:) but the documentation says that it's not possible.

What is the easiest way to do it ? (Using a spam filter is not possible)

Thanks in advance,

Anonymous Coward