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spam is most often considered to be electronic junk mail or junk newsgroup postings. Some people define spam even more generally as any unsolicited email.
However, if a long-lost brother finds your email address and sends you a message, this could hardly be called spam, even though it is unsolicited.
Real spam is generally email advertising for some product sent to a mailing list or newsgroup.
In addition to wasting people's time with unwanted e-mail, spam also eats up a lot of network bandwidth. Consequently, there are many organizations, as well as individuals, who have taken it upon themselves to fight spam with a variety of techniques. But because the Internet is public, there is really little that can be done to prevent spam, just as it is impossible to prevent junk mail.
However, some online services have instituted policies to prevent spammers from spamming their subscribers.