Perhaps scroll to the bottom of the account list where is says outgoing server (SMTP) and actually look at the outgoing server setting instead of the incoming ones. My guess they say STARTTLS.

Thanks. I changed it in this SECOND location back to SLS/TLS. However, now it hangs until it times out trying to send the message. I think I saw some notes about the new problem and will check them out. How do you suppose it got changed in the 2nd location to STARTTLS?

Looks like these guys have yet another lot of server settings. https://www.spectrum.net/support/internet/email-settings/

I just tested those settings. They are underwritten with Certificates issues by Symantec. Perhaps they missed the mail from Symantec advising they messed up and the whole world was going to distrust their certificate in a years time. But they obviously have no taken any action.

mail.twc.com does have a valid certificate from digicert however.

So at this point perhaps try; Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Settings for TWC/Spectrum outgoing server:

mobile.charter.net on port 587, SSL/TLS, normal password authentication, User name = email address.

https://support.mozilla.org/en-US/questions/1234783#answer-1157069v

I shutdown my laptop and went away for the afternoon. When I restarted, the send mail was working just fine.

Evidently, I needed to have the outgoing server set up as STARTTLS, but the incoming server needs to remain SSL/TLS.

Don't know why, just that it now works. Magic? Thanks for all the help.