Are all four accounts with same email provider? And you receive same error, regardless of account used? Do you have four separate SMTP servers defined? (You can check that at account settings pane, scroll down left side to 'Outgoing servers (SMTP) and click to view.) Are they all defined identically? Please click 'edit' for one of them and post a screenshot of the contents for review here. This may not be where the solution lies, but much detail is missing from your post. Problems such as you reported are often done where there are too few SMTP servers, or where SMTP settings are incorrect. Thank you.

I'll answer your questions to the best of my ability. My knowledge of how this all works is close to zero. I've been a user of Thunderbird for several years without trouble, Was formerly using the the MS mail program. By the way I'm using Windows 7.

I'm not sure how to answer the first question. Our internet and phone provider is via a local telephone coop, Pioneer Connect.

The internet email account and support for the Pioneer provided internet service is provided by a different company, Peak Internet. They do NOT provide any assistance or support for Thunderbird.

All 4 Outgoing smtp accounts are defined differently and I've attached a snip of each.

Does this help?

My questions were to increase available information. I see all four are different accounts. I did notice the @gmail account should be using Oauth2 as the other one does. And the Peak account appears to require port 587 and STARTTLS from Peak's own Thunderbird page:

https://www.peakinternet.com/residential/discover-peak/email-support/#Thunderbird  I suggest focusing on getting one of the accounts to work. 

On the missing messages from zimbra website: is that the peak account? You might try rightclicking the account on main pane in left side and clicking 'subscribe' to see if thunderbird is subscribed to the folders.

I made the changes you noted.

I opened the Subscribe window. When I clik on Folder List the attached Thunderbird cache notice window pops up in the lower corner. It prevents opening the Folder List.

Maybe it will help if you post your incoming server setting.

Peak.org IMAP Mail Server

    Server Name     mail.peak.org
    User Name        W7WLL
    Port                    993
    Conn Secur        SSL/TLS
    Auth Method      Normal Password

Gmail.com IMAP Mail Server

    Server Name      imap.gmail.com 
    User Name         [email protected]
    Port                     993
    Conn Sec           SSL/TLS
    Auth Method      OAuth2

YRFPD.org IMAP Mail Server

    Server Name     imap.dreamhost.com
    User Name        [email protected]
    Port                    993
    Conn Sec           SSL/TLS
    Auth Meth         Normal Password

SWLCWPUD.org IMAP Mail Server

    Server Name     imap.gmail.com
    User Name        [email protected]
    Port                    993
    Conn Sec           SSL/TLS
    Auth Method     Normal Password

I trust this is the information you are looking for.

Correction The User name for peak.org is [email protected], NOT W7WLL

Okay, inventorying the settings looks ok. Something seems corrupt, but I cannot see why. Fortunately, these are IMAP account, so there is no data loss if the account is recreated. And that is my suggestion, not having any further ideas of my own here. That is, you've spent a lot of time trying to fix this and it may be easier to remove everything and restart. I have no idea what happened during the upgrade, but something did.

What would be the proper way or sequence to do the 'fix'?

Do I uninstall the apparent corrupted Thunderbird program and then do a reinstall from the Thunderbird site? If so, will the mail folders repropagate or would I use Subscribe and reinstall folders from the Folders list for each account?

If not the above process what do you suggest as the proper sequence?

I apologize for what may seem basic questions, but a computer programming wizard I'm not.

Thank you for helping me through the processes you've outlined. I've learned more than I did when we started.

First, I am assuming all data folders are messages. If you have any local folders, they need to be protected. Here are my suggested steps: - if there are any local folder items, I suggest copying the elsewhere (exit thunderbird, locate the Mail\Local Folders folder and copy to a separate folder.) - Now, uninstall thunderbird - delete c:\users\<yourid>\appdata\roaming\thunderbird folder - that will give you a clean start - install thunderbird and exit - now copy the prior local folders content to the new Mail\Local Folders folder - IF you do not like threading, this is the opportunity to fix it prior to creating accounts

     - create a simple text file with name of user.js with these lines: 
          // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
          user_pref("mailnews.default_view_flags", 0);
      - copy the user.js file into the profile

- Now, start Thunderbird and install each of the accounts. - When done, be sure to do a full backup. My preferred backup that works is this:

  - exit thunderbird 
  - copy c:\users\<yourid>\appdata\roaming\thunderbird to a USB stick 

- Now, if you ever have to reinstall, all you have to do is

   - install thunderbird and exit 
   - copy the USB stick Thunderbird folder to c:\users\<yourid>\appdata\roaming   - it will prompt for overwrite - allow that

If you have difficulties, please post details. And, I'm pleased you're learning stuff... :)