Are you by any chance pressing Ctrl+Enter (Send Now), intending to enter a hard line break?

And it could not be ctrl enter pressed by mistake. I tried it on a test and a dialogue box came up. Besides I have SEND LATER installed so I would have to go through that dialogue.

I was composing a very demanding reply requiring editing and it sent that out every five minutes without any indication to me that was happening.

The default autosave interval is 5 minutes, if it's enabled in Tools/Options/Composition/General, so I wonder if drafts are being mixed up with sent messages. Are the spurious sent messages copied to the Sent folder? Are drafts saved in an IMAP folder or in Local Folders? See if it still happens in safe mode (hold Shift when you launch TB), which will disable Send Later, among other things.

In response to your email, I do have the save every five minutes checked but also the inform before sending option, which does happen when I have tried to duplicate the problem. I now have an email to myself sitting for hours to see if it repeats. The messages did not get copied to sent. I will check the server after I get back from a meeting. Thanks for the help.

I was able to repeat the problem. Thunderbird is sending instead of saving email replies , both every five minutes while I am composing and when I click on save. I will see if it happens in safe mode.

The save function sends emails instead of saving them in safe mode as well as in regular mode. But the emails do not appear in the sent folder either locally or on the server. Now I am wondering if the emails ever did get sent at all! I will phone a recipient in the morning to check.

Check under Copies & Folders for the account to see what the targets are for sent and draft messages. If drafts are stored on the IMAP server, they are being 'sent' there every 5 minutes, not to the recipients, which explains why they don't appear in Sent. If drafts aren't needed on other devices, it's better to save them in Local Folders.

That may be the key although it will take some time to verify. As I mentioned, there were sensitive issues I was dealing with and I thought multiple drafts were being sent out without my knowledge. It was very upsetting. Now I realize the sending was only to the server and not to the intended recipients, so I am greatly relieved and only need to verify this reconfiguration of settings. MANY THANKS