In her article "Email Marketing Design for Mobile Devices" Kristin Hersant shares somes tips and guidelines for designing email campaigns and newsletters that render well on a mobile device.

Conventional wisdom says that the standard layout width for an email is 600 pixels wide. In order for your email creative to render properly on a smart phone, you will need to design your layouts at approximately 480 pixels wide, or 80% of your original layout size. In order to be readable on a regular cell phone screen, your email will need to scale down to 50% of its original size, which is a tall order. According to the panel, 85% of the email delivered today is not readable when it’s scaled down to 50% of its size.

Here are some of the tips she shares:

1. Make sure that you’re designing your emails using a grid system. This means that you need to layout your content in vertically and horizontally aligned blocks, with “streets and alleyways” in-between them. This will enable your design to shrink down without losing its integrity. 
2. Make sure that your text is readable on a mobile device. Think about larger headlines and body size copy and use short, direct calls to action. A
3. Consider a Single Column Layout.
4. Always test rendering on mobile devices when creating master templates to ensure they accomplish your readability goals. 
5. use background colors to visually separate topics instead of horizontal rule
6. Instead of designing at 600 pixels wide, try reducing your layout to 450, 500 or 525 pixels. 
7. Make sure your point is conveyed with or without images enabled.
8. Use a Viewport Meta Tag. Using this piece of code will make email render properly on the iPhone, rather than shrinking the full email. It also makes the email render more quickly. View a code example here.

Source: StrongMail