Two developers of open-source anti-spam software, Jonathan Zdziarski and William Yerazunis, claim their products can block 99.97 percent of incoming spam thanks to new techniques.
Commercial spam filtering software is expensive, and is at best only 99 percent effective, meaning that they miss 30 messages out of every 3,000.
"Cost is measured in two ways when you're dealing with spam filtering - cost of implementation, and the cost of inaccuracy," said Zdziarski. "Most commercial filters generally suffer in both areas - they're too expensive and not nearly as accurate." Source