Problem #1 can be fixed by replacing the password with a secure mail key:

https://support.mozilla.org/en-US/questions/1314522

I don't know why you can't setup the Charter account with the same settings as the others. Try adding the account offline (File/Offline/Work Offline) and enter the settings manually. In the Manual config. window, click Advanced Config. and check that the outgoing server on port 587 applies SSL/TLS, not STARTTLS:

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

After the account is set up, go back online from File/Offline/Work Offline and see if there are any errors.

sfhowes....thank you for your help!

Problem 1 has indeed been solved. All I had to do was create the secure mail keys. Thank you so much for linking to the directions for doing that.

Problem 2 though....I just am completely unable to get the new Charter email address set up in Thunderbird. In trying to somehow look at the server settings of a perfectly functioning Charter email account so as to duplicate them when setting up the new account, I'm just not seeing the settings I need to see when using the manual configuration option. In fact, the configuration box looks nothing like what I see when I look at the current settings box for the functioning account...especially when it comes to the outgoing server settings you mention. While I'm not completely computer illiterate, I just wish I understood the whole email server settings concepts better, as I'm betting this is something pretty simple that I'm overlooking.

Tools/Account Settings, look at Server Settings for a working account. It probably shows the incoming IMAP server as mobie.charter.net on port 993, SSL/TLS security, authentication = normal password, User Name = email address. In Outgoing Server (SMTP) at the bottom of the left pane of Account Settings, it should have mobile.charter.net on port 587, SSL/TLS security, authentication = normal password, User Name = email address. Each Charter account should have its own smtp, with the User Name the same as the incoming account. Select the incoming account in the left pane of Account Settings, and select the correct smtp server in the Outgoing Server (SMTP) drop-down in the right pane.

If you open Advanced config. in the setup window, while in offline mode, all of these settings can be entered for the new account.

What are the current smtp settings for the working account?

OK sfhowes...I'm thinking my settings must be very outdated, because they look nothing like what you have described. Perhaps they can be old and outdated and unsecure and still work? Anyway, I've attached 2 images below. The first is for my primary Charter account that works just fine in Thunderbird and the second is a screen shot of my SMTP settings. I hope this helps. Thank you again.

The old (non-secure) settings may still work, but they are not the ones currently recommended by Spectrum. IMAP is better than POP if you access the account from multiple devices, and secure connections enable access to the account when you're not on the Spectrum network.

Got it....thanks. So is it safe to say then that Thunderbird has not incorporated these server settings into its automatic configuration setup when adding a new Charter email account? If they have this latest info, shouldn't the auto-config work? While I'd really like to have the most secure setup I can, there doesn't seem to be anything at all that I have tried that will allow me to set things up with the settings Spectrum (Charter) is recommending....either automatically or manually. I'm just at a complete loss here as to why it doesn't work automatically, when after entering the email address and password, I see the message, "Configuration found in Mozilla ISP database." BUT....as soon as I click "Done", I get the yellow box with the message that says, "Unable to login at server. Probably wrong configuration, username or password."

I would never count on the setup wizard to apply the correct or best settings, which is why I add accounts in offline mode. I don't have a Spectrum account to test, but the settings mentioned in the 2nd link in my first reply did work at the time of posting, and I don't see anything on Spectrum's site to indicate that those settings have been superseded.

Thank you so much sfhowes. I'm going to try this again tomorrow on another system that I just built. It has pretty much only the Windows 10 operating system on it so far, so I will download and install Thunderbird and try to set up one of my Charter email accounts and see how it goes.

Thank you so much sfhowes. I'm going to try this again tomorrow on another system that I just built. It has pretty much only the Windows 10 operating system on it so far, so I will download and install Thunderbird and try to set up one of my Charter email accounts and see how it goes.

How did it go??

FWIW, Thunderbird settings that come from the autoconfig process are in largely obtained by what the service provider has defined in THEIR autoconfig definition.

if you've used SECURITY:"None" <- then that means your main PASSWORD or Secure-Mail-Key or App-Key/Password , etc are exposed in Internet , not-secure anymore, you should change that PASSWORD/KEY. and must change SECURITY from "None" , into "SSL/TLS" or into other that is not-"None".

I'm really sorry for not coming back to update the people that tried to help. I got sidetracked with other things and never attempted to set up the email account on my new Windows 10 machine. I'm going to try either tomorrow or Tuesday to get to it and will update you all with what happens then. Thank you all for your help and suggestions.

Well everyone, life has been hectic of late...but I've finally got everything sorted out with my new status of "retired" and I now have the time to do the things I want to do! My problems with Thunderbird have all been solved! The old SBC Global (AT&T) account email issue was indeed solved by simply creating the secure mail keys. So THANK YOU sfhowes for your solution. As for the issue with Spectrum (Charter) emails in Thunderbird, that problem was solved on Wednesday when I dumped Spectrum as my ISP....LOL. They had recently raised their monthly rate another $5.00 with absolutely no advance notice whatsoever on either previous bills, via email or by snail mail...and since then I had been looking into other options. I had been seeing all the ads lately for something called AT&T Fiber, so I went to the website to check for availability in my city and wouldn't you know....it was in my neighborhood. I now have 1GB download AND upload speeds (5 times faster download than Spectrum) for significantly less cost than Spectrum and all is good. I just wanted to come back to thank all who offered their help and suggestions. These support forums are a great place to both get help and offer help to others. Have a great weekend everybody!