What leads you to say it is actually being sent? That's wrong, if it is happening, though you wouldn't be the first user to report this. A message being edited should go to Drafts.

I found this atypical behavior this morning Then I did a check before to complain.

Now I found in the "All Mail" folder 3 instances (evolving content) of the mail and in the "Sent" folder only 2  !?! I use Google mail account.

3 in all mail... one is probably a draft that did not get deleted by I am just guessing.

2 in sent folder. 1 Placed there by Thunderbird when you clicked send. 1 placed there by Google when the sent the email from their server to it's destination.

You might want to go into account settings > copies and folder and disable Thunderbirds copy to sent folder is it is causing you troubles.

Matt, thank you for your suggestion. I have encountered another anomaly: please look at the attached image.

Probably, in the end, I have to re-install the account or even the client.

As far as I can see, the two sets of settings have the same effect. Why worry about it?

Gmail's implementation of IMAP is a bit odd, due to their mapping of folders to labels and vice versa. This is particularly noticeable with gmail's preference for "All Mail" in place of "Archives".

https://support.mozilla.org/en-US/kb/thunderbird-and-gmail

I tested different settings, reconstituting the conditions in which the anomaly occurred and found that unchecking the options : "Place a copy" & "Keep message archives" the behavior of the program is normal, predictable.

From my point of view everything is ok now. But if you-developers want more details, I'm at your disposal.

Thanks both for your help. Regards.