Jeanne Jennings published an interesting bit of research on open rates and inbox rates at ClickZ recently. Essentially she looked at two different industry studies and compared their results.
The first study was the Return Path Global Delivery Survey and the second was the Epsilon North American Trend Results. What Jeanne found is that while Return Path shows a decrease in inbox placement, Epsilon is seeing an increase in average open rate.

There are any number of reasons this could be happening, including simply different ways the numbers are calculated. I am not sure it’s just a numbers issue, though. Many of Epsilon’s clients are very big companies with a very experienced marketing team. The Return Path data is across their whole user base, which is a much broader range of marketers at different levels of sophistication.
I expect that the Epsilon data is a subset of the Return Path data, and a subset at the high end at that. It does hint, though, that when the inbox is less cluttered, recipients are more likely to open the commercial mail that does get in there.

Delivery Improvement • inbox • open rate • Spam