When trying to send mail I get an error message ' error occurred while sending mail ' sender address rejected ..you are blocked sending mail

That is a message from your email provider, not from Thunderbird, and, unlike some server error messages, this one is pretty unambiguous: you are are indeed blocked by them. They may, rightly or wrongly, think you have been sending spam. There's not much you can do about it except to contact your email provider's tech support. It might turn out to be that they have been having technical problems on their end.

Before you do that, you should confirm that you are using the correct SMTP server settings by comparing them with the settings on your provider's help web page.