Handle External Links

You can set nofollow depending on PageRank, 404 status, whether URL is blacklisted or not, and many more flexible criterias.
Upload the directory handle-external-links to the wp-content plugins directory..
Activate the plugin through 'Plugins WordPress..
Choose settings by clicking on 'Settings WordPress and navigate to 'Handle External Links...
In your admin panel, click on 'Settings tab and navigate to 'Handle External Links Here you can pick date that will affect all posts pages older than this value...
This feature will automatically ping URLs to check whether the page returns 404 status or not.
If there's 404 it will set the link to rel= nofollow...
Navigate to the Settings page and select PageRank from the dropdown.
Browse the code, check out SVN repository, or subscribe to the development log by RSS...
Option to set nofollow on URLs that doesn't return PageRank value.
Read more