I recently purchased a Expired domain from auction and after I started my new site on it, I am noticing 500+ "not found" errors in Google Webmaster Tools, which are generating from the previous owner's contents.Should I use a redirection plugin to redirect those non-exist posts to any new post(s) of my site? or I should use a 301 redirect? or I should leave them just as it is without taking further action? Please advise.