For those who haven't heard, the Direct Marketing Association recently released some pretty astounding numbers surrounding email. In its latest Power of Direct economic-impact study, the DMA reports that in 2005, email marketing returned an astonishing $57.25 for every dollar spent. The same study found that the ROI of non-email-related online marketing was $22.52, or less than half.

All told, advertisers spent $12 billion for non-email-related online marketing, but just $300 million for email. Or put another way, email provides more than twice the ROI of other Internet marketing channels, but gets only 2.5 percent of the investment. I suspect we'll see that percentage grow significantly over time as more companies begin to understand email's unique ability to cultivate customer loyalty and high ROI over the long-term. Thanks to Email Data Source CEO Bill McCloskey, who ponders this conundrum in his blog.