To set up Microsoft Outlook Express 6 to use SMTP Authentication, follow these steps:
Open Outlook Express. Click Tools > Accounts.
On the Mail tab, select the email account you would like to edit and click the Properties button.
From the Account Properties form, on the Servers tab, check the My server requires authentication checkbox.
Click OK, then Close. It might be necessary to close and restart Outlook Express for these changes to take effect.