what anti virus?

Kaspersky Total Security. I have Thunderbird and Kaspersky on 2 Windows 10 machines, but the trash problem only exists on one of them.

Try disabling Kaspersky mail scanning. either for scams spam or malware. Historically it is the scam and spam scanners that anti virus products contain that usually place mail in the incorrect folder.This article appear to offer a how to https://www.digitalmastersmag.com/magazine/kaspersky-internet-security-2018-edit-security-settings-mail-connectivity-errors/

I find it concerning that they are transferring all mail via their servers. So all your mail is going to Moscow for examination before you see it.

Matt - Thank you for your response, but I don't get any invalid certificate warnings, so I don't see how the info in the link you sent will help me. I can't set an exclusion since that process needs the host name from the certificate details.

I did my best to explain how to use software I have never seen. So it is up to you to turn off the kaspersky stuff.

My testing solution is;

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Without me making an changes to Thunderbird or any other app my problem has just gone away on its own; no problem for over a week. I've waited to respond to make sure that the problem doesn't come back.