Gmail team has recently declared a new lab feature in gmail, on the official gmail blog – the new lab feature in gmail is named Message Sneak Peek. Once you turn this lab feature on you can right-click on a email in your inbox shows a preview pane with the message in it. This way you can preview a message before opening it this can be a very useful feature for many people who end up getting so many mails daily in their gmail inbox.
In order to enable this feature, login to your gmail account and then click Settings Link or the green colored Lab icon.
Among the all he lab features, look for the feature Message Sneak Peek feature, enable it and click Save Changes button at the bottom.
Once you click the button Save Changes, your gmail inbox will load and right clicking on any email message will show the preview pane.