If so, you need to change some default settings.
In Thunderbird goto Tools > Account Settings. In the new window, highlight Outgoing Server (SMTP).
Highlight your default SMTP and click Edit. In the new window ensure that under "Security and Authentication" the Connection security: is set to None.
You may also have to make other changes. EG on mine, I had to ensure the Port was 25, and that the "Use name and password" was not selected.
my Thunderbird comes up with this message
An error occurred while sending mail. The mail server responded. 5.7.0 Must issue a STARTTLS command first. 14sm2176134ewy.3. Please verify that your email address is correct in your Mail preferences and try again.
AND another one:
Sending of message failed. The message could not be sent because connecting to SMTP server smtp.gmail.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your SMTP server setting is correct and try again, or else contact your network administrator.
I have changed nothing whatosever. It worked yesterday, but today it won't. Doesn't make sense.