More than 85 per cent of spam currently plaguing businesses has come from the US.

Analysis of spam sent to companies worldwide by messaging security company CipherTrust found that the overwhelming majority of spammers are using computers based in the US to send junk email.

Since May 2004 nearly 86 per cent of spam originated in the US. But only 28 per cent of the total number of computers that sent unsolicited commercial mail during this time were located in the US, indicating that it is only a small number of US computers sending a large amount of spam.

"When it comes to spam the US-based spammers are the driving force, and assertions that the majority of spam comes from overseas simply aren't true," commented Paul Judge, chief technology officer at CipherTrust, in a statement.

"Our research shows that while spammers do in fact utilise IP addresses outside of the US to send unwanted email, they are not scaling this practice to the degree they do in the US."

Judge added that spammers sending those messages are relatively few in number, likely to total around 200.

The study found that many spammers are bending the rules of the US Can-Spam Act to appear to comply with the legislation, specifically with unsubscribe link requirements. Although these spammers provide unsubscribe links, they make the process far more complicated by demanding that requests to unsubscribe are sent via postal mail.

Requiring such effort from email users greatly lessens the chances that they will unsubscribe, enabling the spammer to continue sending messages to the same recipients, the research found.