When composing a new email message, Outlook Web App 2010 will show the Global Address List first by default. If you would like OWA to default to a different contact list, follow the steps below:
1. You will need to log in to the OWA portal at https://na01.msexchangeoutlook.com/
2. In the upper right hand corner, select Options.
3. Once the menu appears, Select "See All Options"
4. Along the left side, click Settings
5. In the middle pane, select General
6. Set the "E-Mail Name Resolution" option to the contact list you would like to use
7. Click Save in the bottom right corner.