Is your ISP blocking port 25? Many are doing that now, and that prevents you from sending email, unless you change the port. This has nothing to do with Thunderbird or any other email client. In your case, you can test that by opening up a command prompt window and typing:
telnet smtp.server_host 25
if you receive a welcome message, then you should be able to send email, you just need to configure the authenication correctly. If it times out, then your ISP is blocking port 25, and you will need to use an alternate port for sending email.