On your PC, open Outlook Express. On the top menu bar, click Tools and then click Accounts.
Click on your RTC Online account and then click Properties on the right. (If you have more than one account, you will have to do this for each account.)
Click the Servers tab at the top of the window and click the checkbox next to "My server requires authentication" near the bottom. Click Apply and close out of all windows opened during this setup.