bulk delete 10465 emails

My 5s is clogged with emails, how do I bulk delete them? I know the trick to select-hold move-deselect to choose all, but it seems to be a bug here as the mail app crash and nothing happens. Sometimes it says "no mails", but the 10463 mails are still there. Tried to flag-unflag etc. They are currently in the "flagged" folder but will not move from there.