First, select Tools and then E-mail Accounts from the dropdown menu:

By default, a wizard screen will pop up with “View or change existing e-mail accounts” selected as the default. Choose next at this screen.

You will see a screen with your E-mail Accounts here. Select the one that says “Microsoft Exchange Server”, and then click the Change button:

Click the More Settings button on the next page:

Click the Security tab on the next page and you will see this screen:

   

Make sure that you check the “Encrypt data” checkbox. This will ensure that all communications between the Outlook client and the Exchange server are encrypted.