CANNOT SEND EMAILS

What prevents you from sending messages?

Is there any error message?

What is your anti-virus software?

Note no config editor in tools menu in version 91.

Config Editor to do what?

Looks like a duplicate of this question, which has a possible workaround :

https://support.mozilla.org/en-US/questions/1347196

For those who upgraded to version 91 and can't send email. Disclaimer: I'm no expert, just an average Joe user of Thunderbird.

I had this same problem and nothing worked. But in the course of trying to solve the issue, this is what I discovered. My OS is Windows 7 Ultimate and it is 32/64 bit. Thunderbird installed on the 32bit folder (86programs). When I upgraded, I eliminated that folder and contents. I wanted TB to be 64bit. That's where the problem began. In the end, I desperately removed and reinstalled TB again and let it do what it wanted. Which meant it went the 32bit, 86folder again. My send problem disappeared. BUT, the TB icon also disappeared. I copied the icon from the TB site, opened the TB folder, used the mouse to move the icon atop the thunderbird.exe. The icon returned and sine then, all has been well. As I said, I'm no expert and acted out of desperation when all else failed. Hopefully, this helps someone else.