put your smtp server ip address on the spam database lookup at
www.dnsstuff.com and that will tell you where it is blacklisted
i believe that ntl uses spamhaus sorbs etc
you arent using your ntl connection to host an smtp server are you?
this is going to be blacklisted as ntl broadband connections are dynamic ip addresses this means that it will be blacklisted due to the fact it is a dynamic ip address