An interesting case study in the evolution of spam is the attack known as “Stock Spam” or “Pump and Dump”. Initially the format of choice was text or html as it provided a small message size reducing bandwidth requirements. However, this content was easily filtered so was not successful and resulted in a shift to image attachments. By the time content filtering companies developed a solution for image spam it had already evolved to use Pdf documents. Most recently the shift has been towards mp3 audio but even spam using Word and Excel documents have been seen.
The spammer is making a trade off between bandwidth and delivery success rate. The increase in messages size with advanced media is worth it since it's more likely to avoid content filters. In the future we can predict seeing any format that is easily viewable by applications on the average internet users PC. It's quite possible that we would see powerpoint spam or even video spam using a common format such as mpeg or avi. Similar to the audio clips where low sample rates were used to reduce file size, it's possible that low sampling and basic frame structure would be used to minimize bandwidth requirements. As spam is becoming increasingly targeted to individuals, will we see a day when a project manager receives Microsoft Project spam?