Step 1: Check that you have your 'incoming mail server' configured to be:  mail.your-domain.com.  If you just changed over to NetFronts servers, then you may need to use mail.hosting-advantage.com as the incoming mail server for a week, until mail.your-domain.com. starts to work

Step 2: Make sure the user ID (called Account Name in Outlook) is your full email address, such as john@your-domain.com.  Don't just use 'john'.

Step 3: Log in to your Control Panel at http://cp.netfronts.com and reset your email password.  You can do this by clicking on the Email button at the bottom.  Then click your email address and click the password button.

Also see Configuring Outlook Express