Failure to relayEMAIL using a wireless router and a desktop PC

I've just installed a BELKIN Wireless Router G+MIMO, to allow my wife to access the internet using her new ADVENT 4211C netbook, I suppose 'relaying' through my desktop PC. After the usual aggravation, this now works fine. But now she'd like to try email send and receive. I have an account 'anidonu@ntlworld.com. I've 'sort of'' installed Outlook Express, in that I completed the POP3 and SMTP window addresses, and gave her netbook address a name 'anita@ntlworld.com together with the password which I used to establish my email with NTLWORLD, and a 'success' message arrived. But attempts to send email are rejected, with the message '550 you are not allowed to send mail to ..." What is the operator error here? What should I do to correct it?