The message-wide-reply pops up a message buffer that's a wide
reply to the message in the current buffer. A wide reply is a
reply that goes out to all people listed in the To, From
(or Reply-to) and Cc headers.
Message uses the normal methods to determine where wide replies are to go,
but you can change the behavior to suit your needs by fiddling with the
message-wide-reply-to-function. It is used in the same way as
message-reply-to-function (see Reply).
Addresses that match the message-dont-reply-to-names regular
expression will be removed from the Cc header.
If message-wide-reply-confirm-recipients is non-nil you
will be asked to confirm that you want to reply to multiple
recipients. The default is nil.