Unable to write email to mailbox

Were things working OK for a while until this cropped up? How long ago did it start?

This is a tough one because the error message could be a server response indicating problems there, or it could be Thunderbird reporting difficulty transactioning with the server, or it could be a problem with Thunderbird itself. They can all trigger similar error messages, and the messages can be misleading.

Please check out this article and see if you think any of it applies to you. I suspect item #7, a malformed message on the server that Thunderbird is having trouble downloading. The solution is to use the email provider's web mail interface to delete that one problem message.

https://kb.mozillazine.org/Unable_to_write_the_email_to_the_mailbox

And here's another person with a similar issue who fixed it in a similar way as I described:

https://support.mozilla.org/en-US/questions/1463022

Also, Avast isn't the only AV that can lock up a file. Microsoft Defender can potentially do it, too, You can test that by creating an exception for your INBOX folder or whole Profile folder, or by restarting Windows in Safe Mode with Networking, which will temporarily disable the AV for that session.

Thanks for th information, I will check it all out. I basicaly had a program I was installing on my laptop blow it up. It was an older program that I used to use on WIn 10....something about one or more of the drivers it tried to install caused my laptop to stop basically bluescreen. I had managed to get a backup of most of my personal stuff but was more than a month out of date on my emails. Anyway - I restored what I could, then reinstalled Thunderbird - all looked pretty good, but then I started getting that error message. After I sent the info to you guys, I went to gmail online and cleaned up all my old email....needed doing anyway, then started working on getting my laptop reconfigured and set up. I got your email, then fired up Thunderbird and it downloaded all my outstanding email (163 messages in the last two days) without any error at all. It now seems to be working just fine. I have no idea what was happening or why but it seems to have cleared up. Nonetheless I will be looking into what you sent me. Appreciate your help with this.