Spam Filter ISP can check any user-specified RBL blacklist to see if the sender's IP address is being blacklisted. Reliability can be improved by requiring an IP to be blacklisted by two or more RBL servers for it to be marked as spam.
Spam Filter will analyze all URLs specified in the email body itself, and will check any user-specified SURBL blacklist server to see if the URL in the email is being used to host spam-related websites.