We receive a a lot of spam addressed to (random) non-existing users. This causes postfix to generate "unknown user" bounces to the fake sender.
As a workaround I use a catch-all to redirect any mail to myself and manually forward mail in cause of typos in the local_part.
I want postfix to drop spams to non-existing users and only bounce them when their spam score is below a threshold. That way people trying to send ham to non-existent users will get a notice and there would be less backscatter.