When I worked for CNet Training I was responsible for designing and constructing HTML emails and sending them out to their entire client database using email marketing software (ExactTarget). During my final months at the company I started experimenting with responsive layouts.
These emails were inspired from a tutorial that I found online. The author of the tutorial ensures that the design is bulletproof and works in all email clients, smartphone mail clients and apps. It uses minimal media queries and a fluid width approach to ensure maximum compatibility.