Marco... The answer is clearly stated on the page. This is a case of simply needing to read and observe. The mail archive HTML page says:
To access or search these archives, you must authenticate yourself with a user name and password. (This is a measure to discourage trolling of the archives by spammers.) Guest access may be obtained by entering the user name "unicode-ml" with the password "unicode" The archives are indeed public, and you do not need membership to access them. Just use the name and password as supplied. Rick