It bogs down mail servers and LANs, impacts user productivity, and raises ISP's blood pressure. The first runs on a dedicated box, while the other products run on a firewall server. Filtering software inspects sections of each incoming message using rules.
Filters can check message headers against a list of known or suspected spammers.
ISPs and spam-watching Internet users compile these lists. Some spam-catcher outsourcers like Bright Light go father, maintaining decoy user accounts on Web sites and Usenet postings, gathering new spam from which new source names can be harvested. Of course, this means choosing from and managing multiple rules and lists. To help their customers, Seattle Lab offers free software that reads many of these lists and consolidates them into rules for Seattle Labs's SLMail filter. And BSDI's MailFilter includes it sContinuous Update Subscription Service, automatically adding new filter rules to sites automatically.