Healthcare product advertisements or offers continue to top inboxes at 54.4 percent of spam e-mails, while a significant proportion of other spam e-mails flogged cheap software or bogus stock tips.
Spam advertising the sale of potentially illegal software represented 4.6 percent of all e-mails in May.
"The very low prices suggest products are counterfeit and can leave purchasers short if the goods are faulty, if they receive the software at all," said Lindsay Durbin, Clearswift senior security architect.
Last month, the Business Software Alliance (BSA) warned against buying from unknown Internet sources after it monitored a growth in spam-related complaints.
Spammers are still offering investment "tips". First seen in March, Clearswift said these e-mails were becoming more sophisticated and include background on the market and examples of how previously advertised shares have shot up in value. Clearswift added that these e-mails made up a significant proportion of all finance-related spam, which in turn accounts for 24.6 percent of the total spam received.