What do you think this has to do with Thunderbird email client?

The block is on a higher level, probably not even at your ISP-level. First your DNS cant translate the url to an ip-adress. Then all calls to any banned ip-adress will be (logged? and) directed elseware.

Way's around it is probably illegal in your country. But it's letting someone outside connect to the site and then you connecting to him (if your allowed).