If you're connecting to the net via t-mobile then you cannot send via ntl. This is pretty much the case with every ISP and is there to stop people spamming.
POP-3 will allow you to collect your mail from your home ISP (ntl) via another ISP (t-mobile) but SMTP will not let you send.
The way around this is to set your identity as
xxxx@ntlworld.com but use the t-mobile SMTP server to send. Continue with your settings as ntl as your POP server.
If you're having problems sending via t-mobile, you might want to check that they haven enabled you on their system. If it still doesn't work then it's their problem and the line he fed you about ntl blocking t-mobile on your mobile is utter rubbish - it's their network!
Good luck.