Just to be clear, in Outlook have you checked the tick-box in the box in the E-Mail Accounts dialogue under the Outgoing Mail Server section that says: 'My server requires authentication'? Not: 'Log on using Secure Password Authentication (SPA)' - don't check that. Confusing huh?
(If you are setting up a POP3 mail client on your work PC to connect to your NTL mail account then you will *have* to authenticate on the outgoing mail sever i.e. to send mail.)
Can you post the exact error message? And what version of Outlook are you using? Unlikely cause but what firewall are you using?