For reply e-mail, confirms your intention to reply to all (Reply-All) rather than reply to sender only (plain Reply). If the subject line starts with “Re:” and there are multiple recipients, the sending is confirmed. If you wish, you can set it to confirm sending of all e-mail with multiple recipients, in case you may occasionally remove the “Re:” from the subject.
Canceled sends are not lost; the message simply gets marked with an X to indicate a canceled transmission, and you can modify the recipients and re-send without having to compose it again. Or, if you clicked Cancel in the “Reply-All Confirmer” confirmation dialog by accident, you can just select Resend to proceed with sending the message as-is.