Why does it download all my mail?
So when you run it for the first time, since no attachments have been downloaded yet, it will download all mail attachments in your email inbox after which it will only scan for mail attachments that arrive from thereon (including read and unread mail).
The default setting will only download messages since last download including unread mails that have not been downloaded by the program.
Subsequent runs by clicking the 'Connect And Download' button will download remaining mail attachments that have not been downloaded yet until it has downloaded all attachments till the present day.