Which is why you don't bounce spam. What you want to do is do your filtering during the SMTP dialog, so you don't even need to accept spam (and bounce it later, file it into the recipient's spam folder, or throw it away). You reject apparent spam before your MTA has accepted responsibility for it, and if there is a false positive, the sender gets a nice SMTP error message, from their own MTA, saying their message couldn't be delivered.
(People who won't look at error messages from their own MTA are beyond help.)
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