Now, as antispam filters have been updated to catch image spam, spammers have moved on.

According to Secure Computing, which reported the 17 percent jump in spam levels today over yesterday, spam currently accounts for 88 percent of all e-mail traffic and PDF spam makes up 11 percent of that figure. With current spam levels close to the all-time high of 90 percent, Secure Computing predicts that record will be matched or broken in the next 30 days, according to a company spokesperson.

Messaging security service provider Postini says it saw the biggest spam blast ever from Aug. 7 to 9. The blast, launched from a botnet, combined two popular spam tricks; attached to the e-mail message was a PDF file that many filters can't read to determine whether the content is spam, and the attached file was a "pump and dump" scheme that promoted buying the stock of a company called Prime Time Group.

During this spam blast, spam volume jumped 53 percent in one day, says Postini, and the value of Prime Time Group's stock climbed 20 percent, leaving the spammer -- who likely sold the stock once the price was adequately inflated -- a lot richer.

Meanwhile, spam watchers at McAfee have already spotted a trend within the PDF spam trend. On Aug. 10 the McAfee Avert Labs blog told of FDF spam, messages that use the file extension .FDF instead of .PDF, most likely in an attempt to fool antispam filters that have been updated to scan PDFs for signs of spam.

Proofpoint on Tuesday reported that PDF spam has grown from 5 percent of all spam when it was first detected in late spring to its current 25 percent of all spam. The company also announced that its antispam technology has been updated to catch PDF spam as well as other forms of "attachment spam," such as those with attached Excel files that contain text, as have a handful of other antispam vendors.

And so users of those upgraded products will be protected, until spammers come up with their next trick.