Junk e-mail promotions often link to cleverly designed shopping web sites that offer pirated software disguised as the real thing, but for a fraction of the cost. Their numbers have doubled in the last month, according to Clearswift's latest Spam Index.
While this type of scam accounts for only 6.6% of the millions of spam e-mails forwarded to Clearswift, it is the highest percentage recorded since the index began and has overtaken spam that includes pornography or profanity, at 5.4%.
Alyn Hockey, Clearswift's Director of Research, warned: "Even though the software prices may look incredibly tempting, recipients are strongly advised not to click on links contained within any unsolicited e-mails".
"While the software may be illegal, the links may also contain viruses and leave your PC and network wide open to unwanted intrusion," he added.
Spam promoting healthcare products is still the most common form of spam, accounting for 54.4% of all spam, while financial spam is also a dominant force, at 24.6% of the total.
Junk e-mail offering tips for penny share investments makes up a significant proportion of this figure, said Clearswift. First seen in March, these mails are becoming more sophisticated and now often include background on the market and examples of how previously advertised shares have shot up in value.