The basics of spam, “phishing” and other e-mail pests, how they got your address in the first place — and what to do now that you’re inundated.
“Spam”, according to various anti-spam groups (see below), is defined as “Unsolicited Commercial E-mail”. That is, e-mail you didn’t ask to be sent to you that is commercial in nature (e.g., an advertisement) — even if it’s not bulk mail sent to millions of people — and you don’t otherwise have a “prior business relationship” with the sender. “A Great Income Opportunity!” from someone you’ve never heard of is spam; a special offer from barnesand noble.com, where you have an account, is probably not spam, even if you aren’t interested in the offer. Of course, any legitimate online retailer will stop sending you advertising mail if you ask not to receive it anymore (unlike a spammer — more on that below), and no company should send you commercial mail unless you’ve gone through a “verified opt-in” process where you not only request the mail, but respond to a message sent to your address to confirm you really do want mail from them — and that you actually own that address. That’s what legitimate bulk mailers do. It’s a reasonable and simple process for all parties.
Plenty more information here