I've found when people discuss spam they really have no idea what they are talking about. There are as many different definitions of spam as there are people. In point of fact, this factor alone (not being able to define what spam is and what it consists of) makes it virtually impossible to control.
In order to control spam, a useful definition is necessary. Why? Simple. In order to control something, you must know what you are controlling. When you understand basic facts, then you can take whatever course of action is necessary. Until you achieve that understanding, you will be shooting blindly at an undefined target. This makes it very difficult to actually do anything useful.
So on that note, what kind of definitions for spam work and don't work?
Commonly spam is defined as unsolicited email. Unfortunately, this definition by itself is NOT spam and means absolutely nothing. What's wrong with it? This definition does not help you solve problem, and thus is incorrect. If this definition was true, then to prevent spam you would have to somehow contact a person to ask them if you could send them an email.
For example, I don't generally call someone on phone and ask them for an email message. That would be silly. In fact, by definition most email is unsolicited; I don't, for example, expect my wife or a friend to ask me if it's okay to send me a message.
Sometimes spam is defined as emails that are from unknown sources. Hmm. This really doesn't work well either. I'll get emails from my website from people I don't know - these are not spam. Also, sometimes my friends will pass my email address to their friends, who send me email. These are also not spam, even though they were from an unknown source and were unsolicited.
How about just plain annoying emails? That seems to be definition that most people have in mind when they mention spam. If email is annoying in some manner, and especially if it was unsolicited, it is spam. This definition probably gets a little closer to heart of matter, but it really doesn't define spam well.
What about unsolicited bulk email? This definition gets a little bit closer but it still doesn't really define spam well. I mean I give my email address to my bank and I really didn't ask them to send me emails (although I didn't ask them not to as well). Yet I would not call this spam as I do business with bank. Their emails might be annoying, but since I have a business relationship with bank I expect them to communicate with me occasionally.
Okay, so what is spam?
I like to think of spam as "unethical mass email". By this I mean emails which violate netiquette standards of majority of users of internet.
Note that by this definition, an individual email sent to a person is not spam. A commercial email, however, is another matter. Even a single commercial email might be unethical if it does not follow rules below.
Ethical emails are targeted well towards their audience. Unethical emails are mass mailings sent out blindly to a large number of people.
These are emails that are sent to thousands, tens of thousands, even millions of people, hoping against hope that a few dozen will be stupid or greedy enough to respond. These emails are untargeted and will not pertain to majority of recipients. Since majority of people reading message (usually upwards of 99%) will simply delete it immediately, this makes mailing unethical.
Ethical email messages include valid email header information. This information properly identifies sender of message. In addition, all of other header data in message is correct.