From http://www.networketiquette.net/core_rules_do_not_spam.html :
“It is proper netiquette to refrain from sending unsolicited messages through the internet or responding to them. Unsolicited sales messages are spam. Spam email messages are illegal in the United States.”
and
“Proper internet etiquette prevents sending unsolicited messages of any kind. It is not appropriate to waste a users time with unwanted information.”