A major spam outbreak on Wednesday flooded the internet with around 5bn messages aimed at inflating a German company's share price.
The security firm IronPort said the spam blast was one of the 10 largest so far this year and represented 8% of all spam sent that day.
The attack was designed to entice recipients into purchasing a German stock - which rose in trading volume by 20%, IronPort said.
The technique, called "stock pump-and-dump spam", allows the spammer to sell shares at a profit once the stock price goes up.
The German spam message was cleverly designed, using a professional looking PDF attached to an email message set out like an investment newsletter, IronPort said. The security company said it was the first time it had seen spammers using PDFs to attempt to fool recipients.
Spammers have recently been using images, such as JPG files embedded in the body of an email, to thwart anti-spam filters that are unable to search the images. Last week, security vendor Secure Computing reported that spammers had begun evading filters by presenting messages as part of an email wallpaper.