Spam

Spam filter or spam reporting service:

To identify the sender of an offending message, you may be able to identify the originating IP address through one of the following web sites:

Sendmail & other MTA (Mail Transfer Agents) support facilities like R-DNS-Lookup & black holing etc.

Spammers Are Liars Who Say Spam Helps Business.

Spam @ work wastes employee time = wages = lost profits.
Spam @ home wastes leisure time.
Spam comprises 60% (*) of internet mail bandwidth, not paid by the polluter, but by other innocents. (*): SZ reported Oct 2003 from Yahoo & MSoft.
Spam costs net servers extra bandwidth & disc & CPU costs to move the traffic.
Spam filtering is CPU intensive
Spam consumes yet more CPU time to filter & discard.
Spam filtering delays mail delivery.
Extra net servers cost acquisition capital.
Extra net servers incur running costs: electricity, wasted energy, more load on air conditioned server rooms, more global warming, etc.
Extra net servers need more computer administrators: more wages wasted, raised costs to consumers, & skills wasted.
Spam wastes money spent purchasing commercial spam reduction software.
Spam wastes volunteer programmers time, who write free spam reduction software, when they could be writing something more beneficial (if we weren't drowning in Spam).
Spam drowns individually written genuine business correspondence & proposals, individually written items to 1 or 2 specific customers, can take hours to write, but get deleted with tens or hundreds of spams by one click.

The Spam brand of compressed tinned pork food product was being sold, long before unconnected spammers started sending unsolicited commercial bulk emails.
Monty Python later did the Spam sketch (which also pre-dated unsolicited commercial bulk email).
Usage of the word is believed to derive from the above.
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Spamming from Germany is illegal, probably in UK too ? & now also from California - (SZ wrote 27.10.2003 Paul Willis & Claudia Griffin owners of PW Marketing LLC got fined $2 million, + an )
Spammers are scum, so disrupt both their business & personal(*) life, as they intrude into both our personal & business lives.
Report them to Internet providers, black hole mail list, the police, or anyone else appropriate.
Do make sure you've found a guilty spammer though, & not just a hijacked or masquerade 3rd party innocent etc. Most people using email aren't currently capable of identifying who's really spamming them, as the spammers masquerade as others. So get help from someone knowledgeable, EG your internet provider. Always include the full unchanged mail header, which is vital to chasing & prosecuting spammers.

There are lots of pages on this, This is a growing index of which abuse@ responders recommend what:

Berklix: How to get headers
Hotmail pointed to http://www.stopspam.org/ email/ headers/ headers.html Which is now (with a horrible black background & white print) http://www.stopspam.org/ index.php? option=com_content &view=article &id=45&Itemid=56
http://www.high-speed-internet-access-guide.com/ articles/ email-headers.html was reccomended by Suzy M. 2009.09 who wrote It contains some great information on how to read email headers and properly report them to ISPs. Hopefully you and your visitors will find the page as helpful as I did.

Complain to: USA Federal Trade Commission via Email.
Perhaps if enough people tell FTC they use a filter to auto delete all [unknown sender] USA email, FTC might persuade the idiots in the USA congress to repeal their obnoxious world polluting spam promotion law [Bill S.1618 Title iii ?], because it'll be bad for real American (except spammer) business, if too many in the rest of the world auto delete USA email.
If you need help, try

They virus & flood the net too often.
During the 24.09.2003 Microsoft Virus excrement flood, with 40M of unsorted mail, rising at over 12M/hour, I decided to later block mail from all people using Microsoft based mailers & operating systems.
Use free software instead - It's better quality than much common commercial rubbish !