A major spam outbreak hit the Internet Wednesday causing the stock it was pumping to jump significantly in trading volume, according to communications security company IronPort.

The spam blast, which the company says was one of the ten largest so far this year and represents 8 % of all spam sent in the past day -- roughly 5 billion messages -- was designed to entice recipients into purchase a German stock. That stock rose in trading volume by 20%, IronPort says, presumably because of the spam.

This technique is called stock pump-and-dump spam, in which the spammer blasts messages persuading people to buy a penny stock, then once the stock price goes up the spammer sells his shares at a profit.

The German stock spam message was cleverly designed, officials at IronPort say, using a professional looking PDF attached to the e-mail message that looks like an investment newsletter. This was the first time IronPort has seen spammers using PDFs to attempt to fool recipients, it says.

Lately spammers have been using images, such as JPG files embedded in the body of an e-mail that can readily thwart antispam filters not capable of searching such images. Last week, security vendor Secure Computing reported spammers had begun evading spam filters by presenting messages as part of an e-mail wallpaper.