Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 12605)
+++ wp-includes/ms-functions.php	(working copy)
@@ -2398,7 +2398,7 @@
 }
 
 function maybe_cancel_post_by_email() {
-	if ( false == defined( 'POST_BY_EMAIL' ) ) {
+	if ( !defined( 'POST_BY_EMAIL' ) || !POST_BY_EMAIL ) {
 		die( __( 'This action has been disabled by the administrator' ) );
 	}
 }