The width of the email content is capped at 800px wide and shrinks to fit at smaller sizes. Under 480px wide, the left and right outer grey padding is removed and the white content area becomes full width. The logo and the headers shrink at smaller sizes.
The full size width of the email fits within the max width of most email clients. Percentage widths are used so that the content shrinks down appropriately at smaller viewpoints. For email clients that accept media queries, additional instructions are included to optimize the rendering at smaller screen sizes.