If the spam get rejected they know that you don't accept the mail. It is up to the sending server to handle the rejection.
OTOH, since spam detection mechanisms are not perfect (and black lists based ones are evil), rejecting means you can lose good emails, while with filtering you give yourself (and your users) an option to look at the spam folder from time to time to see if a good message has been flagged as a false positive.
Bye
--
Que les importa a las viudas, a los huérfanos, a los desvalidos
si las masacres se hacen en nombre del totalitarismo o en el
sagrado nombre de la libertad y la democracia.
Mahatma Gandhi (1869 - 1948)
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