Although sometimes not classed as true 'spam', this unwanted bulk email (UBE) has become a lot worse over the few years. You may have seen this type of email offering lower mortgages, cheaper airline tickets, and even advertising for very legitimate companies. Normally the main reason you get these is that you signed up for something online or bought something. The problem is once they get that 'permission' from you, very often the flyers get out of hand; now you are getting 30-40 flyers a day.
MagicSpam uses the MIPSpace database which tracks this activity at hundreds of ISP's across North America. If your ISP uses, or allows you to use such a database it can block over 15% of inbound connections, but more importantly, since other forms of Spam Protection do not stop them, for people who accidentally got on such lists, this protection can reduce the amount of flyers in your inbox more than any other tool.