Simple Mail Transfer Protocol. The protocol widely implemented on the Internet for exchaning e-mail messages. The protocols provide conventions for connecting networks and routing traffic between them. It supports local area networks, as well as interconnections between local area networks. Electronic junk mail or junk newsgroup postings.
Some people define spam even more generally as any unsolicited e-mail.
Real spam is generally e-mail 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 private online service, such as America Online, have instituted policies to prevent spammers from spamming their subscribers.