President George W Bush has signed the first US legislation to outlaw junk e-mails, otherwise known as spam.
Critics say the law will lead to more spam messages
The Can-Spam Act of 2003 will allow Americans to opt out of receiving unsolicited computer messages.
Spam is considered a nuisance by most internet users and now reportedly makes up 50% of all e-mail.
But critics of the measure say it will do little to stop spam and may even encourage some businesses to start sending unwanted e-mail messages.
Under the new legislation, law-breaking spammers can face multi-million dollar fines and jail sentences of up to five years.
The bill requires pornographic e-mails to be clearly labelled and says businesses must avoid fake return addresses and misleading subject lines.
It also asks that spam e-mail include a mechanism that lets people tell the sender that they do not want to receive any more messages.
But instead of banning all unsolicited messages, the law
means that businesses are free to send e-mail until people say they do not want it.
Critics of the bill would have preferred an opt-in scheme in which people who want spam are the only ones who get it.
Spammers, many of whom already operate on the margins of the law, are unlikely to abide by the new restrictions.
Many of those responsible for sending spam are also based outside the US and are beyond the reach of the new law.
Last month, the pressure group Spamhaus denounced the bill, saying it would have the effect of legalising spam throughout the US.
"Can-Spam says that 23 million US businesses can all begin spamming all US e-mail addresses as long as they give users a way to opt-out, which users can do by following the instructions of each spammer," said the group.
"Anyone with any sense would of course realise that if Can-Spam becomes law, opting out of spammers' lists will very likely become the main daytime activity for most US e-mail users in 2004."