Chances are your ISP's authentication requirements for sending mail are different from what you have Thunderbird set to. You need to know what type of authentication they have their SMTP server set up to expect, and you also need to know if they require authentication at all. (Some ISPs will protect their e-mail servers from being relay hosts by other means than via user authentication; if yours is one of these, then you have to configure Thunderbird not to try to authenticate, because anything it tries will fail.)
If you have another e-mail client that you have had working with this ISP's SMTP server, then open it up and find out exactly what it's set to as far as authentication.
In Thunderbird, go to Tools->Account Settings, then click on Outgoing Server (SMTP) on the left side of the dialog. Select the entry for your ISP's SMTP server, and see that your settings match exactly.
If you don't have another client working, you might try unchecking the "Use Name and Password" box. If your ISP doesn't require authentication, that'll be the only thing that works.
-BP