Postfix: Blocking inbound messages with fake addresses

I'd like to block inbound email messages that appear to be from our domain, but which are not in our password file.

For instance, suppose a message arrives from spammer (at) ourdomain.com, and we have no such user... that message should be rejected without further processing, since we know they cannot exist.

Is there a Postfix guru in the house who knows how to do this? Is it even possible without writing a plugin of some sort?

(I had to mangle the imaginary address to get the forum to let me post this.)

how are you handling that now?
Rejecting Unknown Local Recipients with Postfix

We're blocking unknown local recipients now; it's unknown local senders I want to block.

The logic I'm wanting goes like this:

IF an email is from a user apparently local to the server, AND that user does not exist, REJECT the message.

Even better would be this (in addition to that):

IF the email is from a user apparently local to the server, AND the message arrived on the external interface/from the outside network, REJECT the message.

These two rules would block a mess of spam.

Evidently, nobody knows how to do it?