I can confirm that there is a limit on the number of emails that can be sent per hour through the ntl SMTP servers from one IP address - 50. However, I think this is 50 individual emails and not one email with over 50 recipients. If this protection has kicked in, you should get an email advising you to visit the AUP website for information.
However, I don't think that this should apply in this case. To be on the safe side though, I'd recommend you email
abuse@ntlworld.com - they're the people who can tell you for sure.