Hi,

Thank you for contacting Microsoft community. We are glad to assist you with your concern.

As per the description provided, I understand that messages in Outlook 2013 open in full screen mode.

To resolve the issue, let us refer to the following thread which discuss a similar issue and check if it helps:

http://answers.microsoft.com/en-us/office/forum/office_home-outlook/force-all-new-messages-to-open-in-a-window-in/25569695-fe0e-4414-9a57-29140251b74e?page=1&tm=1364043595424

I hope this fixes the issue. If you still have issues, you may reply and we’ll be happy to help you.