The legislation concerning whether or not spamming is in fact illegal
is unclear. There is also the problem with different laws in different
countries.
It is this uncertainty that the spammers are exploiting. The status today
is that most of the spamming goes without any form of legal action happing
against the spammers. From the spammer's standpoint there is very little
risk involved with spamming.
From the average persons grasp of what is right and wrong spam seems
as something illegal.
There have been some cases where the spammer have forged the header information
so that the spam appears to come from someone else. Yahoo.com and Flowers.com
have had such cases and won.
Some spammers include in the message something like this: This message complies with the proposed United States Federal requirements
for commercial e-mail bill, Section 301. Per Section 301, Paragraph (a)(2)(C)
of S.1618, further transmissions to you by the sender of this e-mail may
be stopped at no cost to you by sending a reply to this e-mail address
with the word "remove" in the subject line. For additional info,
see: https://www.senate.gov/~murkowski/commercialemail /EMailAmendText.html.
This may have you convinced that the spammer is acting within federal
law in sending you their message. This bill, commonly referred to as the
"Murkowski bill" would have legalized sending spam if certain
requirements where met. This bill passed the senate, but it did not make
it through conference committee and never passed the House. So this never
became a law.
CAN-SPAM Act of 2003:
CAN-SPAM Act was approved by the Senate in November 2003 and by the House
of Representatives in December 2003, and was signed into law by President
Bush on December 16, 2003. The Controlling the Assault of Non-Solicited
Pornography and Marketing Act requires unsolicited commercial e-mail messages
to be labeled (though not by a standard method) and to include opt-out
instructions and the sender's physical address. It prohibits the use of
deceptive subject lines and false headers in such messages. The FTC is
authorized (but not required) to establish a "do-not-email"
registry. State laws that require labels on unsolicited commercial e-mail
or prohibit such messages entirely are pre-empted, although provisions
merely addressing falsity and deception would remain in place. The CAN-SPAM
Act took effect on January 1, 2004. Apparently according to this law you
can send spam emails, as long as you mark them as spam in some undefined
way.
Other resources on spam law: https://www.spamlaws.com/