That means that the smtp server you're trying to reach blocks your ip-address, because it's either listed as a spam source or as a part of a dynamic ip-block that shouldn't send mail.
My first guess would be that you're trying to reach the wrong smtp server. Your ISP's smtp server should be configured to accept mail from you and if you're using a third party as mail provider you aren't authenticating yourself.
Use your ISP's mail server.
This problem isn't TB related it should also happen with Outlook Express when you're trying to use the same smtp server.