Sure, but how would allowing bounces lead to being used for spam? If it bounces, that means the mail was not delivered. Why are these errors bad for a reputatio

Sure, but how would allowing bounces lead to being used for spam? If it bounces, that means the mail was not delivered. Why are these errors bad for a reputation? It doesn't even matter whether the mail was spam or not, since it wasn't delivered anyway. That's what I don't get, of course I know they're trying to keep their reputation good.