Spam is one of the most common types of undesired mail. It is sent in bulk to lots of people trying to sell some product or service. Many times, these products are not legal at all, as some drugs, but other times legal services are offered this way.

For an e-mail to be spam it must be sent without the consent of the recipient, that is, an e-mail with a commercial advertisement is not spam if you have asked for it. The legislation of each country is more specific as to what is spam and what is not.

The products which get more advertising in spam vary with time, but it is quite usual to receive spam about drugs like viagra or valium, about how to get fake college diplomas, how to get a mortgage or illegal software.

The problem of spam is economic. Sending spam is really cheap, so even if only a really small percentage of the receivers buy the product it’s still profitable. So, you must never buy products advertised this way, so spammers get the message that people don’t like to receive these kind of messages and won’t buy their products.

In the same way, the most expensive part of the spam is not payed by the spammer. He only has to find somewhere from where to send the spam and, once it has been send, he doesn’t have to pay anything more for it. But the message has to travel through other networks, has to be stored somewhere and has to be, finally, read or deleted. This has a cost in network bandwidth, in disk space occupied in, more importantly, in time spent by the final recipient having to classify and delete the e-mail.

For many people, the quantity of spam received is bigger than the quantity of legitimate mail, so they need some way to classify it automatically, as it almost gets impossible to do it by hand in a short time.