Do not send quality-headers if they are set to Normal, which is the default.

class/deliver/Deliver.class.php

@@ -467,9 +467,6 @@ class Deliver {
             case 1:
                 $header[] = 'X-Priority: 1 (Highest)'.$rn;
                 $header[] = 'Importance: High'. $rn; break;
-            case 3:
-                $header[] = 'X-Priority: 3 (Normal)'.$rn;
-                $header[] = 'Importance: Normal'. $rn; break;
             case 5:
                 $header[] = 'X-Priority: 5 (Lowest)'.$rn;
                 $header[] = 'Importance: Low'. $rn; break;