What Spam Is
An exact definition of "spam", should it exist would probably not be agreed upon by many. This is because and why any definition of junk email includes a subjective part — "I recognize a spam when I see it!".
- Emails you did not ask for that were
- sent in bulk
- from senders you do not know
are suspicious of being spam.
Newsletters you did sign up for, an email from a college friend and messages from people trying to contact you personally are, of course, not spam. A newsletter somebody signed you up for to annoy you is not spam but a different kind of email abuse. An email sent to you in bulk by an unknown sender that you do in fact welcome and find useful may not be spam either.
Every email you asked for is not spam but not every email you did not ask for is spam. For practical means, there is no point in sophistic examination of all the reasons that can, could or must not justify the junk status of any email (if you enjoy the sophistic exercise, do go ahead!).
I used to include an example of a junk email message in this space but I'll point to your Inbox or, if you are graced with a potent spam filter, Spam folder instead.
Why Spam Is
The reason spam is thriving is none other than it works (for it to be a pyramid scheme spam has been around for too long). People do buy products advertised in junk email. Don't (if you do not like spam, of course).
Spam works because it is so cheap to send. Using means ranging from spam-friendly ISPs to ordinary people's computers turned spam-machines, spammers can send their junk inexpensively. At the same time, the risk of getting caught is substantial and involves dear costs — apparently not enough to offset the profitability of sending spam.
Why Spam is Bad
Spam is bad because the cost is borne by the recipient. Spam costs time, money and resources, making email a significantly less attractive medium. Fortunately, spam has not ruined it completely, though, and neither will it.