This could be ntl email generated. Depends on how much is stored on the email servers for your account. It could even be a large email that has been sent to you waiting for you to download.
I'm not sure of the exact capacity of the ntl webspace but it's not huge. You can have a look at you webspace contents, rather than what is in your email client (i.e. outlook) by loggin in through the ntlwold site.
edit : doh, must type faster
