Do not post any material that is protected by copyright unless you are the copyright holder or have the copyright holder's permission. However, you may make fair use of copyrighted material by quoting limited passages of text, provided you credit the author.
I've been completely lax with enforcing this during the campaign, but will be more vigilant in the future. If you post an entire article or email written by someone else, it's apt to be deleted or at least edited. Feel free to refer to other site with links, or use pieces of articles in discussions.