One of the most used --and thus, abused-- services granted by the Internet is the electronic mail. Nowadays, anybody involved with technology has at least one electronic mail (E-Mail) address, and E-Mail is beginning to replace the role of the Postal Service in communicating people far apart.
E-Mail was first conceived more than 25 years ago, and since the mid-eighties its use has been widespread in the academic community. Several different mechanisms have been conceived to write, deliver and read the mail, but the most commonly used today is based on SMTP (Simple Mail Transfer Protocol), defined by RFC 822. This protocol is very lightweight, easy to understand and implement, and robust -- but it is beginning to show its age.
In 1989, the commercial use of Internet was approved. Slowly but steadily, users all over the world --most of them with very little technical understanding-- started using it. This has led to the Internet boom we are still living today -- a highly successful arena with a place for every single topic that can be imagined. One of the activities which has flourished most is the electronic commerce. However, due to the large amount of online stores and businesses and the lack of high-quality sites, attracting customers has become a priority that must be fulfilled at any cost. The easiest and cheapest way for promotion is by E-Mail -- An E-Mail message needs to be sent only once, but it can be sent with hundreds or even thousands of destination addresses at a time. This is practically cost-free for the originating user, but it is not cost-free at all for the network operators and end users at all: The message is delivered to a server, which must process it and send it to all of the specified addresses, taking huge amounts of processing power and bandwidth. Hundreds or thousands of servers, when receiving the spam, must allocate their resources to it, storing useless and often large mails in their hard drives. Thousands of users paying an Internet connection have to download their mail --bloated by spam-- to their personal computers.
These are the main reasons why server administrators all over the world are getting more and more concerned about spam.