This field does not exist in the RFC spec and is not used in the code: drop it.

class/mime/Rfc822Header.class.php

@@ -199,10 +199,6 @@ class Rfc822Header {
                 $value = $this->stripComments($value);
                 $this->mlist('help', $value);
                 break;
-            case 'list-id':
-                $value = $this->stripComments($value);
-                $this->mlist('id', $value);
-                break;
             default:
                 break;
         }