Consider Your Elevator Pitch
While subject lines are already small on desktop and webmail, with mobile you have considerably shorter real estate to work with. Having a great ending to your subject line doesn't mean anything if it gets cut off. Make your subject lines short and sweet. This situation also applies to your email copy. Your copy should be concise and to the point. Your calls to action should easy to spot, rather than requiring users to search for them. As mentioned above, subject line length and the display of pre-headers is dependent on the device whiere an email is read. A good guideline to follow is to keep subject line length around 40 characters, and pre-header length less than 100 characters. While the shift to larger phones allows for more characters, measure how many of your subscribers are consuming emails on smartphones with larger real estate to determine the optimal length for your audience. As wearable devices become more popular in 2015, specifically devices like smart watches, brevity will be even more important than ever.
What's the View Like?
Dimensions for each platform differ, checking to see how your emails render on each platform and device your customers are using. Studies indicate that tablet users prefer consuming content and emails in landscape mode, while smartphone users prefer portrait mode. Designing a mobile-friendly email that looks great regardless of the view or device can be tricky, but responsive design can help your emails look great in most use cases.
Design Your Buttons With Form and Function
Unlike a desktop computer's mouse, mobile users use their fingers to click on links. Consider using buttons with compelling copy for your calls to action instead of links, and providing tap targets large enough for the average finger (Apple suggests 44x44 points). If your emails contain text links, space them far enough apart from other links to provide your subscribers a positive experience. Too small links requiring mobile users to zoom in to click the link, or links too close together that users end up clicking the wrong button, can affect their decision to interact with your email on mobile in the future.