Have the option for a condensed view with less information density in order to see more emails on screen at once.
Great news! This is literally the most basic of features in an email application.
This is the number one feature missing that's preventing internal adoption, particularly at the executive level. We need the ability to view more than 8-10 conversations in a single pane of the app. Most people want to view up to 50 at a time, like in gmail. This is likely to be the number one reason we'd struggle to get internal buy-in to renew our subscription.