One important factor in the increase has been the rise of so-called "image spam." Whereas traditional spam simply contains text that spam filters can learn to filter out, image spam places that text in images that can't be read by a regular spam filter. Some spam filters use optical character recognition to get around that problem, but spammers have been quick to litter image spam with dots and lines that confuse OCR software. Ironport says image spam now accounts for up to 45% of all spam.
Another factor that has spurred spam growth is malware that uses infected home PCs to propagate spam, the New York Times says. Not only does this malware help spammers avoid detection, it also makes image spam possible since the bandwidth required to send thousands of images every day is spread out among zombie machines and costs nothing to the spammer. The situation is so bad that Ironport Technology VP Patrick Peterson has gone so far as to say, "As an industry I think we [anti-spam companies] are losing. The bad guys are simply outrunning most of the technology out there today."