I assume that you are referring to a not junk button in Windows Mail. This should be used for e-mails that have ended up in the junk folder and clicking on it tells Windows Mail to move it to the inbox instead.
For your second question I gather that you wish to reply to e-mails that have been forwarded to you by someone else and reply to the original sender. In this case you need to select the original sender's e-mail address. Getting the senders address depends on how it has been forwarded.