Many email users keep their preview pane feature turned on, allowing them to view at least the top two inches or so of your email before deciding whether or not to open the message. Use those top two inches to your advantage by emphasizing the call to action or key point of your email in that area.
Remember: the key is to compel your recipient to open the email, and then include your explanatory copy in the middle section of the message. Repeat your call to action at the bottom of your message for those 'scrolling' types who only read the top and bottom and simply scan in-between. Also remember that the preview pane counts as an 'open' even if the recipient doesn't actually click to open the message.
Keep this in mind when renting lists and reviewing the vendor's open rates, they can be falsely inflated by the preview pane.
Source: Go-To-Market Strategies