If you suddenly stopped receiving email notifications every time a comment is left of your self hosted wordpress blog this article might have an easy fix for you. A simple 1 line additional to your .htaccess file got things working again for me.
If you are running an old version of PHP and update Wordpress to 2.6 you may find an error on some of your posts where you’ve inserted Galleries. Simply remove the Gallery to remove the error for a temp fix.