message-wide-reply-confirm-recipients

message-wide-reply-confirm-recipients is a variable defined in `message.el'.
Its value is

nil

Documentation:
Whether to confirm a wide reply to multiple email recipients.
If this variable is nil, don't ask whether to reply to all recipients.
If this variable is non-nil, pose the question "Reply to all
recipients?" before a wide reply to multiple recipients. If the user
answers yes, reply to all recipients as usual. If the user answers
no, only reply back to the author.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.