I'm having trouble configuring outlook express to work properly.

the error is:

The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was '*****'. Subject 'sdf', Account: '******', Server: 'mail.*private*.com', Protocol: SMTP, Server Response: '501 <*****>: sender address must contain a domain', Port: 587, Secure(SSL): No, Server Error: 501, Error Number: 0x800CCC78

I have already configured my email through cpanel (pop3).  but when I open outlook express, it doesn't work.  can't send or receive.

Might check out this page here:

https://support.lunarpages.com/knowledge_bases/article/36

Also searching Google for the error code number also brings up several things you can try to troubleshoot this issue.