Perhaps a more appropriate definition would be:
spam: A posting to multiple newsgroup/mailing lists/? which contains as its greater part an advertisement of a product and/or is with regards to a topic of no relevance to the forums in which it has been posted.
My suggestion: The electronic posts commonly called "spam" are posts to mailing lists, mailboxes, or newsgroups of a content inappropriate to the target destination's charter, or posts inappropriately mass-delivered without regard for their proper delivery. You can advertise for bovine sex slaves all you want, but I don't want to see the ad in comp.sys.next.sysadmin. You can write all the get-rich-quick schemes and chain letters you want--they're not illegal--but you'd better be sure you know where they're going. Therein lies the sum of my argument. Carl