There are several books on web design and usability but when it comes to email there is a bit less information.
From my experience, the layout (just the high level design / position of text and buttons) must match the intent. In addition, a responsive design helps multi-device compatibility.
A guide used in previous experience was from MailChimp which outlines the number of columns layout and discuss purposes. Smashing Magazine has also been a source of inspiration with many example-driven guidelines.
What are your thoughts, what layouts/design worked for you, good click through rates or the goals of the emails achieved?
Or what other authoritative source of email design do you use?
