The only plugins I had that have anything to do with email are "Fast Secure Contact Form" and "Postie". I have deleted both of them and then gone back to the Newsletter status page and now I get the error:
FAILED (Undefined index: wp_db_temp_dir)
Read more.- Try to set on main setting a sender address with the same domain of your blog
I now have almost ALL plugins disabled and I still get this error. The only plugins that are active are: Akismet, JetPack, Limit Login Attempts Reloaded, QuickTime Embed, WordPress Database Backup and of course Newsletter