griffgj on "get_var email address query error"

Good idea, however, I still have a problem. I create a file with the following code only:

require('wp-blog-header.php');
$User = get_user_by( 'email', 'teats@griffshp.com' );
echo $User->ID;

When I put this in the root of my WordPress site, it works. When it's in the plugins directory it doesn't. Same file (except I point to the wp-blog-header.php file differently). What's going on? Any ideas?