While there has been much debate about what was truly the first spam, these unsolicited messages became well known in the mid 1990's. When people realized that this was the most cost-efficient way to advertise on the web, spam quickly became a serious issue - the RBL (Real-Time Black List) released in 1997 was solid evidence.
Over the years, spamming techniques have evolved in order to penetrate several filtering programs designed to stop the problem. As soon as new blockers and filters are developed, spammers quickly find a way around them. This had led to a vicious cycle that is spiraling out of control.
Direct Spam Mailing
Early types of spam were directly sent to internet users. In these days, spammers had no need to disguise themselves. While it was a terrible annoyance, early spam was relatively easy to block; all it took was black listing a specific sender or the IP address from which the mail originated. This resulted in spammers spoofing their email addresses and falsifying contents of the messages.
Open Relay Spam
Most mail servers were open relay in the mid 1990's. This allowed individuals to send email to anyone they desired. The growing rate of spam and other security problems caused administrators to reconfigure email servers across the globe. Completing this took more time than many spam recipients would have liked. Aside from that, many owners and administrators of servers were not willing to follow along. As the process initiated, security analysts worldwide began searching for all remaining open relay servers. Black lists became available, giving administrators the ability to block incoming spam from all servers found on the lists. While this drastically reduced the amount of spam for some, many spammers still target open relay servers for mass mailing campaigns.
Modem Pool Spam
Once the act of flooding open relay servers became less effective, spammers started using dial-up connections as a source of distribution. They became innovative by exploiting vulnerabilities in the structure of ISP dial-up services. Since dial-up modems utilize a dynamic IP address, spammers were able to spam users from a different IP address per session. To combat the problem, internet service providers placed a limit on the number of messages an individual could send out per session. Anything over the set limit was sent back to the user and categorized as a spam message.
Robot Networks
Robots, more commonly known as zombies, were responsible for the majority of the spam being sent in 2003 and 2004. This form was much more than an inconvenience for the user. Spammers used Trojan horses to download malware and crippling viruses onto several machines, allowing them to be controlled from a remote location.
Industry analysts have estimated that Trojans are actively operating on millions of computers throughout the world. Some are advanced in ways that allow them to install other Trojans, initiate DDOS attacks and much more - all this from opening a single spam message.
Conclusion
Spammers currently use a combination of various methods to be more effective. This is because many anti-spam programs are incapable of detecting them all. As spam filters improve to protect recipients, spamming techniques will continue to evolve and keep our mailbox clogged with unwanted messages.
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