According to a recent article in Marketing Sherpa, 69% of Outlook users employ some type of email preview before opening it. They discovered that many heavy emailers (themselves included) do not take this feature under consideration when designing an email. Basically, the first 3 lines of text in your email are displayed in Outlook’s auto-preview as well as many portable electronic devices that read email.

The trick is to make those first 3 lines compelling copy. Try to avoid putting administration text first like: “Click here to view this email with images” or your issue date and issue number. Also, try and make this text as intriguing as possible. It will be competing with other email to get read, so consider standing out with CAPITAL LETTERS or text symbols to catch the eye:-)

Try viewing some of your own designs in auto-preview and see how compelled you are to open them. Here is a link to good and bad samples to help model your next design.
http://www.marketingsherpa.com/cs/epreview/study.html