|Has Thunderbird been installed and working for a while? Is this a sudden change and can you tie it to any other recent problems with the system?|
Try running Malwarebytes to double check for infections. If any, remove them and post the log back here for a look at. Note, when installing it, uncheck the free trial box so you get the free version.
I have in the past had a problem once with Thunderbird on outgoing mail and the only way that fixed it for me was to delete the email account in Thunderbird and reenter the information. As many times as I 'fixed' the information, nothing worked but as soon as I deleted the account and reentered it with the same information, all was fine. Maybe that will work for you if other things do not.
You have to be a little bit crazy to keep you from going insane.