SpamBlocker uses various public blocklists on a reputation basis.
While you might think this is dangerous, it's actually quite good, as most spam comes from severs that don't send any ham (legitimate email).
Because SpamBlocker blocks rather than replies, the sending server gets a message (which you define inside your exim.conf file) that the email has been blocked. If the email was legitimte the sending server will send it back to the sender, who will follow your instructions on getting whitelisted.
Lots of SpamBlocker specific threads in these forums.
Jeff