Spam temporarily jumped 445% last Thursday

The campaign was an example of the recent trend of junk email using PDF attachments as a way of getting around spam filters. But Postini said the surge marked the biggest increase in spam it had ever recorded, boosting total spam volumes 445 percent in a one-day period at its peak on Thursday morning.

Sophos Labs, which also detected the spam surge beginning on Tuesday, said it intercepted around 500 million PDF spams advertising shares in a particular target company.

The massive size of the campaign is unusual, according to Sophos Labs director Mark Harris. "To date, the trend has been for smaller campaigns that rapidly evolve and modify themselves to try to get round anti-spam products," he said on the Sophos blog.

The campaign had tailed off by Thursday but didn't disappear -- rather, it continued to mutate over the weekend, Sophos said.

In the meantime, the campaign appeared to have another notable effect, in that the company targeted a small Florida-based firm called Prime Time Group involved in wireless and retail interests, and as of Thursday afternoon actually saw its share price rise 84 percent from its Monday value, before plummeting again.