Bill Nussey reports new findings of the Direct Marketing Association’s Power of Direct economic-impact study; the key point is that the ROI of email marketing is the double of other online marketing activity.

Here is a quote:

"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.
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."

I’ve read in the past other reports saying that email marketing has the best ROI.
And probably the ROI is increasing (compared to other media) because marketers are learning how to use effectively the enormous opportunities of email marketing.

Based on my experience, the main limit is … fantasy!