smtpin.ntlworld.com is correct for external mail - smtp.ntlworld.com is used for sending mail from ntl.
ntlworld.com MX preference = 5, mail exchanger = smtpin.ntlworld.com
Error 452 is ntl's standard rejection message, normally due to the remote MTA being blacklisted.
The server specified above, mx6.daemonmail.net, is currently on two different blacklists:
http://www.senderbase.org/search?sea...216.104.160.36