Try it with the correct settings: mail.twc.com, 587, STARTTLS, normal password, email address.

https://www.spectrum.net/support/internet/email-settings/

Thanks, but I just continue to see "sender rejected" error messages, as if all of a sudden my email name and address has been rendered invalid. I'm starting to think somebody at Spectrum changed something and didn't tell anyone. And the Spectrum site and support people are useless/worthless.

And BTW, I AM able to send emails from the Spectrum webmail site -- using the same email name/address.

Do you have more than one Spectrum account?

Yes. I have one logon to pay my Spectrum bills and another to use their webmail.

So, is that two accounts in thunderbird? I ask because the error message is often tied to use of the wrong SMTP server. Please clarify.

Thanks for trying to help. I appreciate it.

I have multiple email addresses -- all with the same domain name. All are using the same SMTP server.

See https://support.mozilla.org/en-US/questions/1350593

You can have multiple Spectrum accounts on the same server, but there must be separate smtp that differ only in the User Name (email address.).