New to thunderbird. When I try to send a message I get an error - 'login to server smtp.supanet.com has failed'. I enter the correct password but the login stil

If you know for sure that your username and password are good then you might have a SMTP server setting wrong. Confirm those settings with your email provider. The settings in Thunderbird can be found by going to the Menu Bar and selecting Tools-Account Settings-Outbound Server(SMTP) Use the edit button to make changes.

No menu bar. Press the alt key.