The only type of SMTP server apart from the ntl one that you can use from an ntl connection is one which has an open relay... which is bad news. These are frowned upon as they are commonly abused by spammers.... see
here for a bit more information.
I am quite confused about what you are trying to achieve... if you setup ntlworld email accounts for people, surely they are on ntl connections? if so, why do you need to need an smtp other than smtp.ntlworld.com. If they are not on ntlworld connections, why are these people using ntlworld email accounts?