In the link above "configure postfix for smtp relay host" the instructions are;
Howto configure postfix to use a remote SMTP relay host
This is a public service message, because when I queried google "configure postfix remote smtphost", I couldn't find anything useful to the question I was trying to answer.
So you're running Linux, and your ISP is filtering traffic on port 25, thus blocking you from sending outgoing mail, and so you want to configure your mailserver to relay all your mail out through your ISP's server, like they want you to. If you're running postfix, this turns out to be very easy -- in /etc/postfix/main.cf, just set the variable:
relayhost = smtp.yourisp.com
then restart postfix (/etc/init.d/postfix restart, as root).
so in your case you set the line to;
relayhost=smtp.ntlworld.com
Try that. I used another howto and setup mine this way;
relayhost = [smtp.ntlworld.com]
The mail is sent fine using this method but I do get comments from postfix about not being able to get DNS validation from my isp mail server. This does not affect the mail transfer though. Your users will be able to pickup their mail in the usual way from pop.ntlworld.com with their mail client.
Edit: I think you need to add some lines to /etc/postfix/main.cf
see the
postfix relay relay clients
I think you have to specify the hosts you want to relay mail for in the form;
mynetworks = 168.100.189.0/28, 127.0.0.0/8 specifying the range in the first section