Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

christ1 said

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode Does the problem go away?

yes, thank you!

Windows safe mode disables anti-virus software, which is most likely the culprit.

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. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

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

ref zorro's other topics which I have just closed: - https://support.mozilla.org/en-US/questions/1152671 - https://support.mozilla.org/en-US/questions/1137700 - https://support.mozilla.org/en-US/questions/1146016

https://support.mozilla.org/questions/1152670 - not https://support.mozilla.org/en-US/questions/1152671