To put it into perspective, spam  is the same as the junk mail you receive in your letter box with two major differences the first being cost and the second being ease of stopping delivery.

Arranging a mail drop to an average sized city can be quite expensive with pamphlet design and printing then having the thing delivered, the cost per hit can be quite high. As a result the organisation/business carrying out this exercise has to have a more than a feeling that costs will be covered.

Stopping the delivery is generally just a matter of displaying one of those "No Junk Mail" signs on your letter box. The distributors of this sort of mail know not to deliver, or there are penalties. As an absolute minimum you can phone the advertiser and complain, after all your have an address and phone number printed on the thing delivered. If it wasn't where would you go to purchase what was being advertised.

Email spam is a very low cost method of advertising anything you want. The cost per delivery is so low as to be almost non existent, and it is not proportional. The actual cost for 3000 deliveries is the same as 30,000,000 all the sender has to do is sent one email to a server list and the server does the rest. However do not for a minute think that there is no cost at all, its just that the spammer does not have to pay that cost.

And who does you ask? well you do actually.

Anyone with a connection to the internet is paying their share of the cost of running the internet. When you pay your ISP they pay for the traffic you have used and they pay for a connection to the next level and on and on it goes. All the spanner needs is a connection to the internet and you help pay for the enormous amount of traffic that spam emails, take up world wide. Second only to the spammers in using huge amounts of bandwidth are those idiots who insist on forwarding jokes, video clips, chain mail and last but not least not optimised digital camera photos to every one in their address books -  but that's another story.

Stopping the stuff is next to impossible. Yes, some ISPs are better than others at filtering this sort of mail than others, but filtering is not stopping. Just because you do not see it does not mean that network congestion is not occurring and costs are not being incurred in moving the stuff around.

How can you stop it?

You can't - you can help tho. If you are receiving large numbers of spam emails, change your email address. Never, never, never give your email address to a web site for anything, unless you would trust them with your first born. Large numbers of web sites offer something for nothing in return for your email address, that should be a clue (no points for guessing what the email address will be used for). Another favourite is electronic greeting cards, they get address's two at a time.

 It you are receiving spam never actually purchase anything advertised - it just encourages them.

The following text in Red is the header of an actual message received by by the mail server that collects unsolicited mail from supported web sites.
Under each line is an explanation of its meaning and if the information can be trusted, or has been added for its confusion value.