What you are experiencing is normal. You can only send mail through stmp.ntlworld.com when connected to an ntlworld/virgin internet connection (either dial up, ADSL, or Cable).]
I can use the Blueyonder SMTP engine with most ISPs....
For anyone using a 3g/umts (mobile) connection you might find that the mail sending is failing because you are being routed thro a proxy server- if this is the case then just untick it.
This is definately the case with my t-mobile acc on my windows mobile phone (as a modem/wifi gateway).