The message means that the NTL SMTP server could not contact the gawab SMTP server to pass the message on.
I have tried it from my home connection and get the following;
558 Your network address is blacklisted, this means that your network range or your server is in our blacklist. If you think you were added by mistake, please send us just an empty email from your blocked email address to blocked@gawab.biz and we will investigate this issue and remove your domain from our blacklist immediately if we found no spam reports associated with your SMTP IP. If you have an email account within our service and getting this message while trying to send from our SMTP service , please make sure your outgoing server is smtp.gawab.com
So it *may* be that NTL's server is blacklisted as well.
