How Gmail Changed the Delivery Game
Upcoming Gmail Inbox: This is in beta at the moment. Google will aggregate emails on a topic and aggregate them together. A simple example would be that I'm going to Italy and all my travel plans and itineraries will be displayed together.
Email delivery continues to change. My advice is to think about the issues raised and put a proactive plan in place. First off, analyze results by ISP and see if you have huge swings.