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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