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.
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