You can make Outlook open automatically when you click a link to send e-mail (sometimes called a "mailto" link) in a document or on a Web page. Similarly, you can make events and contacts open in Outlook when you open them from the Finder or the desktop.
On the Outlook menu, click Tools > Accounts.
Select the account you want to make the default.
Click the Settings icon at the bottom of the pane, and then click Set as default