Here is a tragic example of what can go wrong when you send a sensitive document to the wrong party.
The hapless attorney who mis-mailed a document outlining a billion dollar settlement deal may have simply been the victim of his/her email program’s address autocomplete function, resulting in the message being sent to a New York Times reporter with the same surname as the intended recipient. Outlook has that autocomplete function, as do some of the webmail services (e.g., Gmail).
It goes without saying that any “attorney-client privileged…for intended recipient” signature file that may have been included had no effect whatsoever in stopping the Times reporter from scooping the story.
Cheers.