Outlook.com comes with the instant action feature, a small pane that will be displayed, upon hovering your mouse over a sender name or subject of a message.
A list of possible action will be displayed and you can enforce them, without opening your mail messages. In most cases this new feature will be quite useful, but however, if you want to disable it, just follow the below procedure:
From your Outlook.com home-screen, click on Settings > More Mail settings.
Go to Customizing Outlook > Instant actions.
Uncheck "Show instant actions".
Click on the "Save" button to validate your settings.