Can envelops be re-used for sending mails?

An envelope which has previously been used for postal purposes should not be re-used unless it has been suitably reconditioned by completely covering the following information with gummed white slips which must be well stuck down:

  1. old postage stamps;
  2. stamp impressions and registration marks (if any);
  3. addresses (including the address of the sender if no longer relevant);
  4. permit mailing impressions; and
  5. Hongkong Post sorting barcode.