1. Select File| Account Settings
  2. Highlight the Microsoft Exchange account.
  3. Click the Change icon.
  4. Click the More Settings button.
  5. Select the Security tab.
  6. Under "User identification", check the box in front of "Always prompt for logon credentials ".
  7. Click OK.
  8. Click Next.
  9. Click Finish.
  10. Close the Account Settings window.