What is your anti-virus software?

I use MS Defender, Avast Premium and Malwarebytes, all paid. These have not changed, except for updates, etc., and Thunderbird WAS working before the update to 115.6.1. I also have three other computers which are all running Windows 11, and those were automatically updated to 115.6.1 and have the same issue. I managed to get 115.6.1 working on the W10 machine after changing permission as noted on https://support.mozilla.org/en-US/questions/1147014.

Please don't blame the anti-virus software I use as it has never caused me a problem. The machine that did NOT have this problem runs W11 and was NOT set up to auto update Thunderbird, and is still working fine.

All webmail including my hosted email provider's webmail, works fine.

Thanks, Jim Darrough

Please don't blame the anti-virus software I use as it has never caused me a problem.

I'm not blaming anti-virus software, but it is certainly a suspect. If you don't want to follow-up on this that's fine with me. Good luck.

Wait, what? I didn't say I would not follow up. But here's some more information that may be be helpful. On one machine, I installed Outlook email, and it works fine.

WHat do you think I should try now?

Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?

Among other things Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Was there some sort of change in the latest version (115.6.1)? Outlook can download messages with the same antivirus programs running. This is confusing.

I will reinstall TBird on my W11 box and try the safe mode trick, though. Just unsure what changed between 115.6.0 (which worked fine) and 115.6.1 is all.

Thanks for the help.

Must be Avast Premium causing the issue, I guess. Wonder why it didn't cause any problems with Outlook mail?

I will reinstall TBird on my W11 box

You'll almost certainly waste your time. Your profile folder is stored in a separate place from the Thunderbird program, so that if something ever goes wrong with Thunderbird your information will still be there. It also means that you can uninstall Thunderbird and reinstall without losing your settings and data. The problem arises when anti-virus software is messing with Thunderbird mail files. Those are the files in your profile.

Just unsure what changed between 115.6.0 (which worked fine) and 115.6.1 is all.

Probably a lot:

• your anti-virus signatures - at least daily
• your mail files which anti-virus s/w is constantly scanning when you receive and send messages

It has got nothing to do with the version of Thunderbird. If your anti-virus s/w declares just one email message as suspicious, it potentially will quarantine the entire Inbox file.

Must be Avast Premium causing the issue

I wouldn't be surprised. Avast has been problematic in connection with Thunderbird in the past. https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

How did you get to that conclusion anyway?

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

Thunderbird did an update this morning. I am thankful for all your help, Chris, especially the links.

I will see if the problem is gone on my three other machines and report back.

Thanks again, James Darrough