What is your anti-virus software?

Try to start Windows 10 in safe mode with networking enabled. Windows safe mode disables anti-virus software. Does the problem go away?

Christ1, thank you for your suggestion.

The anti-virus is windows defender. This strange behavior is a very recent phenomena not associated with any deliberate change.

Safe mode had no effect.

I will rephrase the issue since my previous email was not very clear.

I do not have problems either sending or receiving. The problem is that to see the contents of any folder I have to run the repair utility. Leaving a folder requires re-running repair when I return to it. In the absence of running repair, I see 3 or 4 old emails in orange/red.

Safe mode had no effect.

Can you confirm you did run Windows in safe mode?

The problem is that to see the contents of any folder I have to run the repair utility.

That sounds like serious mail file corruption. Anti-virus software is often the culprit. These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

The problem seems to have resolved itself spontaneously. Thank you for your attention.