You probably have specified to use the Sent folder for saving draft messages. Check under Tools > Account Settings > [account] > Copies & Folders > Drafts & Templates > Keep draft messages in: to see the current name of the folder and change it to Drafts.

That was checked and double checked - and checked again now - before positng the comment. That is not the issue.

I have an idea what may be causing you to send messages. This I have observed with a friend who also had exactly the same issue as you explained. They did not realise what they were doing, but the error was actually themselves, they had developed a habit and were not aware of it. Please read on for explanation.

When composing the email, I suspect that every now and again you will press the 'Return' / 'Enter' key to go to another new line.

If your hand is slightly leaning on the keyboard and you inadvertantly also press the 'Ctrl' key which is just below the 'Return' / 'Enter' key, then this is the shortcut key to invoke a 'Send Now' instruction.

One possible way to protect against this is to set Thunderbird to spell check your message before sending it. Usually there's something in a message that is considered to be misspelled and if not, you can type some nonsense to ensure that the send will be interrupted by the spell check.

This is the dialog with that setting:

On Windows, the menu bar is hidden by default. You can tap the Alt key or the F10 function key to display it temporarily, then Tools > Options should call up that dialog and you can click Composition on the top row.

spell check was already on. Can't say for sure about the Ctrl button. Very carefully watched my fingers, so Ctrl not touched, and saved as a draft - it went to the Sent folder. Obviously all those prev msgs that looked like they were sent, were actually the auto save as draft, but going to the Sent folder. Created a new folder called NEW drafts and changed settings to save drafts there. After first save as draft it was momentarily in the new folder, but then even before it could be opened it was in the sent folder. Typed a new msg, saved as draft - saw it go to NEW drafts, then go to Sent folder - before the send msg button pushed. When a msg is saved it goes to drafts until sent [it it remains open till then] and then goes away. In my case the saved msg which is actually a save as draft or autosave does not go away because it went to the sent folder. So, how do I fix my draft folders?

Please add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?
• Who is the email provider?

In Safe Mode the same thing happens Just have Windows Firewall and Windows Defender. Mine is an older Windows 7 laptop that was updated to Windows 10 - all updates regularly done. Email provider is Telus

Problem has gone away. Seems to have been an issue with Message Filters, though I don't know why, nor why it started now [filter has been used for years]. Messages sent from my devices were put into a different folder than those sent from my pc; the filter was to move them to the pc folder. When THAT filter was removed, the problem went away. I still have other filters that move messages, but no issues. Thanks to all those who assisted.