As explained above, most spam comes from criminals and shady businesses. Lists of millions of e-mail addresses are readily available for as little as $100. To send the spam to millions of addresses, they must use "cooperative" mail servers. Most legitimate mail servers are protected from spammers and most ISP prevent users from sending huge numbers of emails. Therefore spammers have to resort to the following methods:

* Using a spam mailing service. These are typically in China and third world countries that don't outlaw such practices or don't bother to prosecute them.

* Using an "open relay", which is described above.