A pump and dump spam attack hit email users with over five billion emails yesterday.

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

The spam blast, which IronPort says was one of the ten largest so far this year and represents 8-percent of all spam sent in the past day - roughly 5 billion messages - was designed to get people to buy a German stock. That stock rose in trading volume by 20 percent, 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 said, using a professional looking PDF attached to the email message that looks like an investment newsletter. This was the first time IronPort has seen spammers using PDFs to attempt to fool recipients.

Lately spammers have been using images, such as JPG files embedded in the body of an email that can readily thwart spam 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 email wallpaper.