• According to ReturnPath, by the end of the year more email will be opened on mobile devices than on the desktop.
• Advertisers now have the ability to use a mobile decision program to determine what device the email is being opened on, and serve the optimized creative for that device.  This PDF gives a (very) technical explanation.
• Timing/day matters - During the week and in the morning/evening, more emails are read on the desktop than on the weekend or during the middle of the day.
• Calls to action can change - click to call, download an app, etc.

From the article:

According to a study by ReturnPath, email opens on mobile devices (smartphones and tablets) grew 82.4 percent in the past year, and that figure is currently on track to surpass both webmail and desktop email views by as early as this summer. With half of all Americans now owning a smartphone and tablet ownership skyrocketing, it is imperative that advertisers adapt their email marketing campaigns and strategies to accommodate these new habits.

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