Hi Matt,
Basically BT's email is now looked after by Yahoo (when you log into your BT email, you actually now access a Yahoo website). I think the problem you're experiencing is not because it's being blocked but due to ntl:'s proxy server. It's a problem that comes up every now and then with the Yahoo site.
Try changing your proxy server in Firefox (a list of proxy servers are available
here). Also see
here for instructions on how to configure Firefox if you're not sure
Ben