Welcome to Outlook Express and it's incomprehensible error messages.
Are you sure you copied the message exactly? SMTP 550 is a "relaying not allow" error as is OE error 0x800CCC79. Yet, the text of the error message "550 you are not allowed to send mail to..." does not match the error numbers. Usually, these error mean that SMTP authentication has failed. See: Tools -> Accounts -> Mail -> NTL Mail -> Properties -> Servers that says: [x] My server requires authentication Check the box and give it the correct user login and password.
I'm assuming that your DSL connection is thru NTL World and not through some other ISP. If the DSL is through some other ISP, please change your OE outgoing mail server from smtp.ntlworld.com to whatever that ISP uses.