Ticket #35069: 35069.diff

src/wp-includes/pluggable.php

@@ -245 +245 @@
          */
         $atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );
 
+        // A plugin disallowed this email
+        if( empty( $atts ) ) {
+                return false;
+        }
+
         if ( isset( $atts['to'] ) ) {
                 $to = $atts['to'];
         }