With each passing year, the number of applications and devices that people use to check their email grows. As a result, an email marketer’s job gets a bit more complicated with each passing year, as the areas in which to focus testing efforts reach a tipping point, preventing many email marketers from identifying the most efficient way to test. Where are other email marketers and designers focusing their testing efforts? We analyzed over 360 million Litmus previews tests to find out. As you plan your design and testing efforts in 2016, we hope this list will serve as a good starting point.