If autosave is enabled for message composition, a copy of the incomplete message is stored in the Drafts folder. Check the Copies & Folders section of Tools/Account Settings to see if folder assignments for drafts are correct. There might be a problem with your setup that is giving the wrong folder type to some folders, e.g. Inbox is being treated as a Drafts folder.

"sfhowes," thanks for the reply. Oddly, I have more than one Thunderbird account and the "Copies/Folders" setting was not the same for each account. I have no idea how that happened. I turns out that I was getting a copy of the incomplete (draft) message in my inbox but the draft was not actually being sent to the intended recipient. From your reply I was able to get my settings correct. Thanks for the answer, I appreciate your time and effort. Thanks for volunteering.