Discard local users submitting mail to MX server.

My MX servers are run on machines separate from the main mail system which
authenticated users use to send out their mail.

So ideally, there would be no reason to see mail with the From address set
to a valid local address at the MX servers and I'm evaluating blocking such
mail.

Has anyone done that before? What were your experiences? Worth the chance
of potentially blocking legit mail?