Responsive = Gmail App. No Excuses.
Ok a little rant here. When I hear people talk about responsive emails I start to cry a little inside. They think of 4-col layouts collapsing in to one, reverse stacking, sidebars. Basically it's a widely held view that a responsive email can be whatever you want. As most of us know The Gmail app challenges that idea, yet it's conviently missed out of many of the presentations i've witnessed on the subject.
Gmail now has the ability to add third-party accounts. It's also far more prevelant now that it's the default mail app for many android phones. In 2015 responsive means Gmail-first. It means designs that are built with the limitations in mind.