Perform the following steps:
Choose Tools -> Settings.
Click Delegation for Exchange in the left pane.
Click on the Acting as a Delegate tab. The table in the tabbed section displays the list of users for whom you are a delegate.
If you want to send email messages on behalf of the user, select the check box next to the name of the user. When you compose a message, you can select the email address of the user from the From drop-down list.
Click Close to close the Settings dialog.