Right now the ntlworld.com SMTP server can be used only through a valid Virgin Media Internet connection. As you're using an alternative connection method, you can't send mail through the ntlworld.com SMTP server.
You'd need to use SMTP settings appropriate for your connection method, presumably those belonging to Three.
-- I have previously worked for one of Virgin Media's partners. My personal site: http://espirian.co.uk/
I've just posted this in another thread, but it sort of answers this one too (I hope).
I have got a solution for this.
I have a virgin mobile broadband dongle, as an add-on to my virgin media package. I was disappointed to discover that virgin mobile does not send mail through the ntlworld smtp server.
After hours of searching for settings, I received a phone-back call from a nice chap at Virgin Mobile tech support, who admitted that Virgin Mobile do not have ANY smtp server.
Meanwhile I had discovered that if you open a googlemail account, the gmail smtp server works fine with any internet connection. The problem then was that all my outgoing mail was stripped of the real return address and replaced with my googlemail address, which I had no intention of using - I'd only opened the account to use their smtp server. This was earlier this year.
However, Googlemail now allows you to enter any valid e-mail address as the "From" address and return address, and if you set that as a default in the Googlemail settings, then all your outgoing mail gives the appearance of having been sent from your ntlworld account (or whichever), and return messages will go where you want them to. Hooray.
A bonus side to this is that a copy of every outgoing mail is stored on Googlemail's server as it passes through their system, a feature which I've found very useful as you can browse through your "sent" emails from any computer by logging into the googlemail account.