Reading further online, I made an exception within Bitdefender (my antimalware program) to not scan my Thunderbird profile folder. I've been running Bitdefender for years and never had an issue, but you never know. I then rebooted.

It made no difference.

So only the pop account is affected? Starting immediately upon updating to version 141?

Also,

What AV software and extensions are used?

Here is a screenshot of my extensions, none of which are new for me.

Please see my second post for AV info.

The full name of my AV program is Bitdefender Total Security.

I though it was worth posting the question to see if anyone else had the same problem and because I also thought it was an interesting question.

Since I have backups of everything from the day before the problem, if I don't hear anything to fix the issue, I will definitely consider restoring my Thunderbird profile to what it was before the problem occurred and then just re-download the messages from the last two days.

It turns out that Bitdefender has an issue that they are aware of.

The short term solution has been to disable the Encrypted Web Scan feature located in the Online Threat Prevention module.

It worked for me and many others.

We still have these questions...

Only the pop account is affected? Starting immediately upon updating to version 141?

In Bitdefender -> Protection -> Online Threat Prevention -> toggle off the setting Encrypted web scan. Restart computer and Thunderbird.

Are new messages better?

If not, then Help > Troubleshoot Mode better?

Restore isn't likely to help you. And may make things worse.

To Wayne Mery:

Sorry.

Yes, only the POP account with Optimum is affected.

And yes, yesterday I spotted an announcement about the new features in version 141 so I assume that the update to that version was installed shortly before the commencement of the problem, but I cannot be absolutely sure about that.

I'm in the middle of something right now. After that finishes, I will toggle off the Encrypted Web Scan in Bitdefender and reboot.

I'll let you know what happens.

I toggled off the Encrypted Web Scan in Bitdefender, rebooted and relaunched Thunderbird—didn't solve the problem. New messages to my Optimum POP account are still coming in the same way.

BTW, I also have Antispam (email scanner) in Bitdefender turned off. Nothing new, but I figured I should mention it.

Some more background—I normally run as a Standard user in Windows. To change the Bitdefender settings, I have to log in to my Admin user account and do the Bitdefender changes from there. It's about the only time I ever go into the Admin account.

I checked with my son, who also uses Thunderbird version 141, but with no add-ons. His OS is Windows 10 Home version 22H2 and he uses Avast Antivirus. His emails are coming in normally.

I also tried Troubleshooter Mode in Thunderbird with all add-ons disabled. Didn't work.

I see on the Thunderbird site that version 141.0 was released on July 22 so maybe the update is not the problem.

It seems like it's narrowing down to Bitdefender.

Thanks.

Yes, it does seem to be bitdefender. On or two people have been helped by uninstalling bitdefender. A few by disabling Encrypted Web Scan.

But you are one of several still don't have a solution. Pleaese make your voice heard by giving details at https://community.bitdefender.com/fr/discussion/104823/mails-in-thunderbird-showing-as-source-code-with-bitdefender-installed, or making a totally new posting.

I see from Troubleshooting Information|Show Update History that I was updated to version 140.1.1esr on August 10 at 10:42:43 AM, although per Help|About Thunderbird I'm currently running version 141.0 due to my fresh install yesterday. That update to version 140.1.1esr happened right at the time when my email problems started.

Also—I see that several other people are having the same issue and the problem does indeed seem to be Bitdefender.

See forum post "Thunderbird 141.0 issue with incoming pop email today. CAUSE: bitdefender."

Anyway, I'll be gone for a couple of hours. There's somewhere I have to be.

Based upon what I'm seeing being posted in the various forums, I'm going to just wait and let Bitdefender figure this one out.

Fortunately, I can still monitor my email via Optimum webmail. I dislike the webmail interface, but it's better than nothing.

Solution update posted at 8:48 Eastern time (12:48 UTC) Thursday by Alexlandru_BD, an admin at bitdefender, "The developers reverted the changes made in the last signature update and this should do the trick until a permanent fix is deployed. Just click on update in systray and reboot once completed. But keep in mind this change is being rolled out gradually, so it may not reach everyone at the same time.." More details at https://www.reddit.com/r/Thunderbird/comments/1mo693n/comment/n8ammvq/

Thanks, Wayne. I am now going through the comments on other forums, etc. to try and recover the emails that came in, which are still messed up, before I ran the fix.