Hi,
Yes, that is correct what is happening.
According to what you have posted, you are trying to send email to
www.channel4.com@ntlworl.com which is not a valid email address.
www.channel4.com is the website for channel4, and the @ntlworld.com bit is the "domain" part of people who use ntl for their email.
If you are trying to send email to someone at channel4, their email would be something like
firstaname.lastname@channel4.com (or along those lines)