The message pre-load feature of Outlook.com, allows the downloading of new un-opened messages (in the background). To disable this feature:
- Sign-in to your Outlook.com account.
- From your Outlook.com home-screen, click on Settings > More Mail settings.
- Scroll to the Reading email section > Group by conversation and pre-load messages.
- In the "Message pre-loading settings" section, select "Don't pre-load messages"
- Click on "Save" to validate your settings.