Enclave.. here is one I used and thought I'd mention it. Download Mailbox, an app by Dropbox. https://itunes.apple.com/us/app/mailbox/id576502633?mt=8
There was an option along the lines of "help me get to zero" and this is how I literally cleared out years worth of emails in my Outlook Hotmail.
Its worth maybe using it for that and then deleting it once finished..
Hope this helps. At least it is an app by a very reputable company - Dropbox!