If your ISP implemented IMAP (or, more generally, some mail system where you could have more folders than just an inbox on the mail server, and of which IMAP is the only nonproprietary one), it could deliver the ones it deemed to be spam into a different folder, so that you didn't have to deal with it until you felt like it. That way, legitimate emails that got misidentifed wouldn't be bounced.

Of course, it might be better to bounce them. That way the sender would know immediately that you didn't get it instead of having it languish in that spam folder for months, only to be automatically deleted.

So nevermind.