The reason is you cannot send an email from gmail using ntlworld smtp mail server.
If you could do that then there would be all kinds of spoofing going on thats simply not how email works. You can change the reply to address(different thing) but the email will be sent from
username@ntlworld.com.
The only 2 ways to send from gmail is on the web mail interface or with the gmail smtp server.
To send mail you have to authenticate to the server etc. Ntl authentication I believe is just needing the user to match to a valid ip range and be recently have logged into pop. Gmail will need a valid user/pass and of course this is authenticated on the actual gmail mail server.