Postfix: Blocking inbound messages with fake addresses

[Solomoriah]

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.)

[jledhead]

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

[Solomoriah]

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.

[Solomoriah]

Evidently, nobody knows how to do it?