Chris

I've found in the past that Mail can remember and default back to the wrong SMTP settings for no good reason, retaining incorrect ones even after you've tried to change them.

Go in to Mail Preferences, and on the Accounts tab, find and click the Outgoing Mail Settings dropdown. At the bottom of the list, click on Edit Server List, then go through the list that pops up and delete all of them one at a time (make sure you have a handwritten note of the details for any you know are correct and wish to re-instate later).

Once you've got a clean slate, click Done, then go back to the drop-down and choose Add Server. Start adding the ones you need, taking care to get all the details correct for each one. Then select the NTLWorld SMTP server for your NTL mailbox. That should do it...