First, if you are using one of the free internet services like AltaVista or NetZero to connect to the internet you will not be able to send and receive mail through Outlook Express.  Most free services do not allow you to send mail through their service.  You will need to get a different internet connection or use the browser based WebMail.

Second, check your mail settings to make sure that they are correct.  To check settings go to Tools, then Accounts.  Single click on the problem account and click Properties.  On the General tab of the Properties window verify that the email address is correct.  Click the Servers tab to bring it to the top.  On the Servers tab verify that the incoming and outgoing mailservers are whateveryourdomainis.com for both. Verify that the Account Name is the username of the mailbox.  The username is the portion before the @ sign.  For a mailbox of joe@yourdomain.com the Account Name/username is joe.  Re-Type your password to make sure it is correct.  Click Apply to keep any changes and then close out of the Properties window.  Close Outlook Express and re-open it to make sure it is using the new settings.  Click Send and Receive to try again.