perhaps you are pressing the Ctrl+Enter keys which is the keyboard shortcut to send

No. I was typing a long reply to an email when I suddenly received a saved copy of the email back from the recipient who I know did not send it

Ok so let me get this.

Your typing and you got a saved copy. All good. What version of Thunderbird are you using? What add-ons do you have installed? Where did this mail show up? Is your account IMAP or POP?

Thanks for your help. I uninstalled Thunderbird and reinstalled. Problem seems to have gone. Again thanks for the help