zerohero,
Don't go to ntlworld (unless they are hosting the site, which I guess they are not).
Go to the ISP hosting your site and tell them that you are using AspMail to send mail items from a forum and need to know which smtp server entry you need to use...
Some ISP's use 127.0.0.1 or localhost. Some use the SMTP entry for normal mailboxes (ie. smtp.ntlword.com), or they may have a different smtp server altogether.....
I'm with oneandone and although the smtp server for mail via a normal mail client was auth.smtp.oneandone.co.uk, the smtp server for this type of mail was totally different...
Best of luck, as I know how annoying it is!! 