My question is very similar to this one, but for postfix.

We keep getting emails from addresses like "[email protected]" delivered to other "@ourdomain.com" addresses. From my google research, I understand it might not be practical to verify the email originated from our IP or VPN (Although this would be ideal, so if you can think of a way to do this, let me know), but in most of these cases the sender address (ex. "accounting") is not a valid account.

I imagine there must be a way to make sure that a local account exists before delivering the message.