exchange server spam problem

["shadow floating" <nadengine () googlemail com>]

I ve been recently found many supicious emails sent from the internet
to the internal clients using the sender address as a legitimate email
address of one of the internal users, do you know how to configure
exchange server to stop such emails (by authenticating users before
sending for example),....I also suffer from internal email spoofing
that users send each other with spoofed internal emails....any help
would do.
thanks alot

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