Handle External Links gives you the power to add rel="nofollow" based on header status (i.e. 404), PageRank of destination URL and many more
Handle External Links helps you to control when to set rel="nofollow" on external links in WordPress. You can set nofollow depending on the PageRank, 404 status, whether the URL is blacklisted or not, and many more flexible criterias! Visit getwebinspiration.com for more Plugin details.