Email spam is a subset of spam that involves sending nearly identical messages to thousands (or millions) of recipients. Perpetrators of such spam ("spammers" ) often harvest addresses of prospective recipients from Usenet postings or from web pages, obtain them from databases, or simply guess them by using common names and domains. By definition, spam occurs without the permission of the recipients.
As the recipient directly bears the cost of delivery, storage, and processing, one could regard spam as the electronic equivalent of "postage-due" junk mail. However, the Direct Marketing Association will point to the existence of "legitimate" e-mail marketing. Most commentators classify e-mail-based marketing campaigns where the recipient has "opted in" to receive the marketer's message as "legitimate".
Spammers frequently engage in deliberate fraud to send out their messages. Spammers often use false names, addresses, phone numbers, and other contact information to set up "disposable" accounts at various Internet service providers. They also often use falsified or stolen credit card numbers to pay for these accounts. This allows them to move quickly from one account to the next as the host ISPs discover and shut down each one.
Spammers frequently go to great lengths to conceal the origin of their messages. They do this by spoofing e-mail addresses (much easier than Internet protocol spoofing). The email protocol (SMTP) has no authentication by default, so the spammer can easily make a message appear to originate from any email address. To prevent this, some ISPs and domains require the use of SMTP-AUTH, allowing positive identification of the specific account from which an e-mail originates.
Spammers cannot completely spoof e-mail delivery chains (the 'Received' header), since the receiving mailserver records the actual connection from the last mailserver's IP address. To counter this, some spammers forge additional delivery headers to make it appear as if the e-mail had previously traversed many legitimate servers. But even when the fake headers are identified, tracing an email message's route is usually fruitless. Many ISPs have thousands of customers, and identifying spammers is tedious and generally not considered worth the effort.
Spammers frequently seek out and make use of vulnerable third-party systems such as open mail relays and open proxy servers. The SMTP system, used to send email across the Internet, forwards mail from one server to another; mail servers that ISPs run commonly require some form of authentication that the user is a customer of that ISP. Open relays, however, do not properly check who is using the mail server and pass all mail to the destination address, making it quite a bit harder to track down spammers.
Increasingly, spammers use networks of virus-infected Windows PCs (zombies) to send their spam. Zombie networks are also known as Botnets.
Spoofing can have serious consequences for legitimate email users. Not only can their email inboxes get clogged up with "undeliverable" emails in addition to volumes of spam, they can mistakenly be identified as a spammer. Not only may they receive irate email from spam victims, but (if spam victims report the email address owner to the ISP, for example) their ISP may terminate their service for spamming.
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