A message with a high enough score is reported to be spam.
When calling of Mail:: Spam Assassin (SA) is enabled, it calls SA only once per message (regardless of the number of recipients), and tries very hard to correctly honour per-recipient preferences, such as pass/reject, and inserting spam-related mail header fields.
If you want to set up a spam-checking gateway for all recipients, local or not, you need a way to perform spam-checking as mail is received, before final delivery.
The filter checks the message and then either refuses it (which will cause Postfix to generate a bounce message), discards it, or reinjects the modified message into Postfix for further delivery.
Messages that pass the filter are reinjected so that Postfix can operate on them almost as if they were new messages; this allows Postfix to behave properly if the content filter rewrites message headers.
After-queue inspection allows you to use content filters of arbitrary complexity without causing timeouts while receiving mail, and without running out of memory resources under a peak load.