As a result, automatic filters tend either to misidentify spam as legitimate mail or, worse, brand real mail as junk. There are, however, steps that the Internet community could take, both on its own and in conjunction with governments, to control the problem. One simple measure is fixing a flaw in the design of the Internet. The Net was designed as a friendly, cooperative tool for academics, and it was set up so that if a University of California at Los Angeles mail server was down, a researcher there could send messages through a server at Stanford University. A sort of Net vigilantism is gradually shutting down relays.
Subscribers refuse to accept any mail coming from the listed servers.
Since this stops legitimate mail as well as spam, any ISP that ends up on the list comes under intense pressure from unhappy customers and will block the offender. It's rough justice, but it works. If every ISP, school, or business that runs a mail server would honor the blacklists, the problem of spam would be much reduced. Other approaches require government intervention. In the U.S., it is illegal to send an unsolicited ad by fax.