What is Spam?
From Merriam-Webster online:
unsolicited, usually commercial email sent to a large number of addresses
etymology: from a skit on "Monty Python's Flying Circus" (link goes to YouTube) in which chanting of the word "Spam" (referring to the Hormel meat product) overrides the other dialogue
Spam now makes up most of the world's email, often estimated at more than 80% of all email traffic.
By the way, "spam" is a word, not an acronym, so it's not "SPAM" or "S.P.A.M." It's just "spam" — it doesn't even get capitalized, unless it is at the beginning of a sentence or you are referring to the Hormel meat product.