Browsing a collection of information on a mobile device is a common task, yet it can be difficult due to the small size of mobile displays. A common trade-off offered by many current mobile interfaces is to allow users to switch between an overview and detailed views of particular items. An open question is how much preview of each item to include in the overview. Using a mobile email processing task, we attempted to answer that question. We investigated participants' email processing behaviors under differing preview conditions in a semi-controlled, naturalistic study. We collected log data of participants' actual behaviors as well as their subjective impressions of different conditions. Our results suggest that a moderate level of two to three lines of preview should be the default. The overall benefit of a moderate amount of preview was supported by both positive subjective ratings and fewer transitions between the overview and individual items.