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
In your admin panel, click on the 'Settings' menu tab and navigate to 'Handle External Links'. Here you can pick a date that will affect all posts/pages older than this value.
This feature will automatically ping the URLs to check whether the page returns a 404 status or not. If there's a 404 it will set the link to rel="nofollow".
Navigate to the Settings page and select a PageRank from the dropdown. Note: If you select PR 9 it will result in setting nofollow on all your external links because it's practically impossible to have a greater PR.