First, PLEASE do not post the same topic twice. It causes confusion and duplicate work. Second, writing to gmail works fine with latest version. If you are having difficulties, you are encouraged to post complete error message so someone can assist. Exit Thunderbird if running. Third, here are steps to revert

   click windows key and 'r' key
   Enter: thunderbird.exe -P --allow-downgrade
   click on 'OK'

You are wrong. Others are also reporting this same very frustrating and time wasting problem as you should be aware. The new version is dated 2 days ago, 22 October, and that is when this serous problem began.

So simply answer the questions I asked.

1) How do I return to a previous version of Thunderbird from say a week ago when all was working OK?

2) How do I prevent Thunderbird from auto-updating?

I answered question number 1 in my response. for non updating, go to settings>general and locate the setting. I use the latest release and it works properly with gmail accounts. I have no idea what problems you are experiencing, but no matter, as you are reverting to a prior release.

You are right. The problem is not just with the latest Thunderbird version.

I went to my backup computer. It has a verson of Thunderbird from Sept and says it will update to the latest version when the computer is restarted, which it has not been in some time.

And sure enough. If I try to send to a gmail email it will not go through. If I send to non gmail emails it does go through. My gmail is sending and receiving normally but will not receive from thunderbird. I get the same error message when sending to others with gmail emails that just a few days ago were no problem

here is error message received: This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

 markbruzonsky@gmail.com
   host eig-west.smtp.a.cloudfilter.net [34.223.136.48]
   SMTP error from remote mail server after end of data:
   550 417KtL1DKCgT6417KtHd94 - <mark@bruzonsky.com> sender rejected AUP#CDRBL

Reporting-MTA: dns; galaxie.websitewelcome.com

Action: failed Final-Recipient: rfc822;markbruzonsky@gmail.com Status: 5.0.0 Remote-MTA: dns; eig-west.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 417KtL1DKCgT6417KtHd94 - <mark@bruzonsky.com> sender rejected AUP#CDRBL

You are right. the problem whatever it is is not the new version.

Now I realize many emails are not going through, not just to gmail addresses.

Here for instance I just tried to send to my Yahoo email and it did not go through with this error message received:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

 markbruzonsky@yahoo.com
   host eig-west.smtp.a.cloudfilter.net [34.217.196.71]
   SMTP error from remote mail server after end of data:
   550 42zjtjFtM2Zy042zktTFQt - <mark@bruzonsky.com> sender rejected AUP#CDRBL

Reporting-MTA: dns; galaxie.websitewelcome.com

Action: failed Final-Recipient: rfc822;markbruzonsky@yahoo.com Status: 5.0.0 Remote-MTA: dns; eig-west.smtp.a.cloudfilter.net Diagnostic-Code: smtp; 550 42zjtjFtM2Zy042zktTFQt - <mark@bruzonsky.com> sender rejected AUP#CDRBL

Your error indicates that gmail is rejecting you. There are many possibilities, but my first guess is validation of domain. you have a personal domain and there is the possibility that the email host may not have DKIM and SPF security settings in place. Or maybe gmail sees your email as spam. Hard to know. A web search for AUP#CDRBL will open many websites with suggestions. You are receiving a generic error code of 550, which has several possibilities, none of which are related to Thunderbird.

I have been using this same email and domain for 20 years!

Now today I have learned it is not only gmail emails that are not going through but also emails to other places.

So whtever the problem is it must have something to do with Thunderbird.

This has never happened before and everything else is working normally on my desktops and portable computer. So all the troubleshooting leads to Thunderbird.

I have been using this same email and domain for 20 years!

Irrelevant in the extreme. Email is an evolving ecosystem and the requirements for receiving and sending mail as imposed by the major corporations involved changes often. As your issue is a blocklist, what is and is not acceptable can be changing by the hour.

Now today I have learned it is not only gmail emails that are not going through but also emails to other places. So whtever the problem is it must have something to do with Thunderbird.

That is actually highly unlikely and is not the sort of thinking that will determine what the issue really is because your issue is being on a blocklist.

This has never happened before and everything else is working normally on my desktops and portable computer. So all the troubleshooting leads to Thunderbird.

No! What troubleshooting you have done leads to mail being rejected by the recipient server (The MTA). The text you posted is not an SMTP submission rejection, something that would involve Thunderbird as it submits the email for delivery to your designated SMTP server.

What that message is is a rejection from the recipient server and the error contains AUP#CDRBL

AUP usually means Acceptable Use Policy. CDRBL I am not sure what the CD in this instance is about, but the RBL is a Reputation Block List.

So I checked against blocklists and your domain is registered. https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3abruzonsky.com&run=toolpage

Salesforge have a good discussion on the blocklist you are listed in here https://www.salesforge.ai/blog/uceprotectl3-blacklist-how-to-remove-it

Your issues have nothing at all to do with your use of Thunderbird and everything to do with your domain being listed on a blacklist, probably as a spammer or potential spammer. You might want to consult your hosting company or hire a web consultant to manage your website and online properties to work towards avoiding these issues going forward.